(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "Science For Coalmining Students"

TEXT FLY WITHIN 
THE BOOK ONLY 



00 

160715 >m 



00 



SCIENCE FOR 
COALMINING STUDENTS 



Science for 
Coalmining Students 



BY 

H. MORTON, B.SC.(LOND.), A.INST.P. 

lecturer at the Wigan and District Mining and Technical College; Sometime 

Examiner in Mining Science for the Union oj Lancashire and Cheshire Institutes 

and the Union of Educational Institutions. 

AND 

D. S. MORTON, B.SC. MIN. (LOND.), C.C.M. 



with a Foreword by 

C. McLUCKIE, M.I.MIN.E., Dipl. R.T.C. (GLASGOW). ,,c.c t M. 



LONDON 
THE TECHNICAL PRESS LTD 

LATE OF AVE MARIA LANE, LUDGATE HILL 

GLOUCESTER ROAD, KINGSTON HILL, SURREY 

r 95 2 



First published 1952 



Made and printed in Great Britain by 
William Clowes and Sons, Limited, London and Beccles 



FOREWORD 

bv C. McLucKiE, Esq., M.I.MiN.E., Dipl. R.T.C. (Glasgow), 

C.C.M. 

IN no branch of industry is it more important to have a con- 
stant flow of trained technicians into its ranks than in coal- 
mining. The improvements and developments in mining prac- 
tice which have taken place in the past are all based on scientific 
principles and increase in scientific knowledge is followed by 
new developments. Consequently it is the trained mining 
engineer who will be called upon to apply scientific principles 
in these developments. 

Not only the management but new entrants to the mining 
industry should have a knowledge of the elements of mining 
science to enable them to take an interest in the various phases 
of their work and particularly to increase the standards of 
safety and efficiency within the industry. In this connection an 
understanding of the physical and chemical laws relating to 
mine gases, the causes and" prevention of explosions and the 
physiological effects of noxious gases on human beings is 
essential. Also the electrification of mines whereby the latest 
electrical machinery has been installed, the transmission of 
power and the economical use of electrical energy call for a 
thorough knowledge of electrical science. 

"Science for Coalmining Students'* deals with the basic 
principles underlying these subjects. The book, which is 
interesting, instructive and wide in scope, is the result of many 
years 1 experience in teaching science to mining students on the 
part of one of the authors and several years' practical experience 
underground by the other. Mining students have long felt the 
want of a textbook of science dealing with their own particular 
problems, particularly from the physical point of view. Conse- 
quently the book should prove very useful to candidates taking 
the various examinations for firemen, overmen and under- 
managers and to youths entering the industry. 

C. McLucKiE 
Hindley, 
Nr. Wigan 



ELECTRICITY FOR 
COAL-MINING STUDENTS 



}. STEVENSON, M.C., B.SC., 

and 
W. MILLER, B.SC., Dipl. R.T.C. 

This textbook aims at preparing mining students on Electricity for 
the Mines Department Examination and for Colliery Management 
Certificates of Competency. 

2 39 pages Crown 8 vo Illustrated 



PREFACE 

THE object of this book is to provide a background ot scientific 
knowledge upon which the practice of coalmining is based. The 
various processes which have to be carried out in order to 
transfer coal from the face to the consumer are based on the 
fundamental principles of Hydrostatics, Mechanics, Heat, 
Light, Electricity and Chemistry and in this book these 
principles are allied to their practical applications in mining. 

The book is chiefly based on the syllabuses in Mining Science 
(Physics and Chemistry) for the examinations of the Union of 
Lancashire and Cheshire Institutes, the Union of Educational 
Institutions, the West Riding Education Department and 
similar bodies. But it also includes the greater portion of the 
basic groundwork in science for the more advanced papers in 
Mechanics, Heat Engines and Electricity of these examining 
bodies. The book should therefore be suitable for the Pre- 
liminary Mining Science examinations and should also be useful 
for the higher level examinations for firemen, overmen and 
undermanagers. 

The authors extend their thanks to the past and present staff 
of the Mining Department of the Wigan and District Mining 
and Technical College and to Dr. H. Crregson, A.R.I.C., for 
several useful suggestions. Thanks are also due to the Union of 
Lancashire and Cheshire Institutes and the Union of Educa- 
tional Institutions for permission to include questions from 
their examination papers. 

H. MORTON 
D. S. MORTON 



TABLE OF CONTENTS 

Page 

FOREWORD ....... V 

PREFACE ....... vii 

Chapter 

I UNITS AND MEASUREMENTS .... I 

II HYDROSTATICS, A ...... 7 

III HYDROSTATICS, B . . . . .28 

IV DIFFUSION ....... 37 

V MECHANICS ....... 42 

VI .THERMOMETRY AND EXPANSION . . . dl 

VII QUANTITY OF HEAT ...... 75 

VIII CHANGE OF STATE ...... 86 

IX MECHANICAL EQUIVALENT OF HEAT . . IOT 

X CONDUCTION, CONVECTION AND RADIATION . lo(> 

XI THE ELECTRICAL CIRCUIT AND REPRESENTATION 

OF ELECTRICAL PARTS . . . . .114 

XII MAGNETISM AND ELECTROMAGNKTLSM . . Il8 

xin OHM'S LAW ....... 141 

XIV CELLS AND BATTERIES THE COMPLETE CIRCUIT . 151 

XV HEATING EFFECTS OF CURRENT .... 158 

XVI ILLUMINATION, REFLECTION AND REFRACTION OF 

LIGHT, LENSES ...... 164 

XVII THE CHEMISTRY OF AIR .... 184 

ix 



X TABLE OF CONTENTS 

Chapter Page 

XVIII THE CHEMISTRY OF WATER, HYDROGEN, SULPHUR 

AND CARBON 
COAL DISTILLATION AND PROXIMATE ANALYSIS . 196 

XIX CHEMICAL THEORY, ACIDS, BASES AND ALKALIS, 

HARD AND SOFT WATERS . . . .210 

XX MINK GASES AND EXPLOSIONS .... 224 

XXI EXPLOSIVES ....... 241 

INDEX ........ 245 



CHAPTER I 

UNITS AND MEASUREMENTS 

INTRODUCTION 

Science to a large extent forms the basis of modern mining 
practice and many of its branches have been utilised in the 
various developments and improvements which have taken 
place in mining in recent years. In particular, the applications 
of fluid pressure, the elasticity of gases, the effects of heat on 
matter as regards expansion, change of state, etc., have been 
utilised in mining practice for a considerable number of years. 
So it is with the composition of mine air, the physiological 
effects of its constituent gases and the dangers due to explosions, 
all of which are questions depending on a knowledge of chemical 
science. In recent years, however, the electrification of mines 
whereby the latest electrical machinery has been installed, the 
transmission of power and the cost of electrical energy call for 
a thorough knowledge of electrical science. 

It is with the scientific principles underlying these subjects 
that the following pages are about to deal. 

MEASUREMENT OF LENGTH 

Measurement of quantities such as length and mass is a very 
important part of science. To measure a certain distance, it is 
necessary to decide upon a unit and the distance is expressed 
as a definite number of these units. In the English system the 
unit of length is the yard, with subdivisions the foot and the 
inch. The unit of length in the metric system is the metre and 
the subdivisions of the metre are given below: 

10 millimetres (mm.) I centimetre (cm.) 
10 centimetres (cm.)=i decimetre (dm.) 
10 decimetres (dm.) i metre (m.) 
also i kilometre = I, ooo metres. 

The Vernier 

A standard scale is generally graduated in centimetres and 
tenths of a centimetre or in inches and tenths of an inch. If a 
length is required to one-hundredth of an inch or one-hundredth 



2 SCIENCE FOR COALMINING STUDENTS 

of a centimetre, a vernier scale is used in conjunction with the 
standard scale. In Fig. i, X and Y represent the standard and 
vernier scales respectively. It is arranged that 10 divisions of 
the vernier scale are equal to 9 divisions (e.g. tenths of an inch) 
of the standard scale. Thus one vernier division is equal to -$ 
of a tenth of an inch. 

If the length to be measured is 5-2 inches, together with the 
small element of length HA, the length HA can be found by 
noting where a division line of the vernier scale coincides with 
a division line of the standard scale, viz. A# . 

Now distance B6 =~o in. 3% of i J - in. 






and distance Cc =i~o i n - TO 

and finally distance HA Y O - in. ^o of -^ in. 

= iV of TO" in- 



Thus the length to be measured is 5-2 in. +0-07 in., i.e. 5-27 
inches. 

Verniers are used for obtaining exact measurements in many 
scientific instruments used in mining, such as the vernier 
calipers, the scale of the Fortin barometer and the circular 
scale of the theodolite which measures angles to -fa of a degree 
or one minute of arc. 

VOLUME AND CAPACITY 

Volume 

The units of volume used in the English system are the cubic 
inch and the cubic foot. In the Continental system the units of 
volume are the cubic decimetre (i.e. the litre) and the cubic 
centimetre. 

i cubic decimetre (or i litre) 10x10x10 cub. cm. 

1,000 cub. cm. 

Capacity 

Capacity is the term which is applied to the internal volume 
of a vessel. 

The Measuring Cylinder 

The measuring cylinder (Fig. 2a) is an instrument which is 
used for measuring an exact volume of a liquid. The cylinder is 
generally graduated from the bottom upwards and is provided 
with a lip so that the liquid can be poured easily. When using 
a measuring cylinder, it is very important to have the eye 
level with the lowest part of the liquid surface (or meniscus as 
it is called). Fig. 2b shows how an incorrect reading may be 



UNITS AND MEASUREMENTS 



iG IF IE iD iC iB IA P i Ti\ 



|h ig if le id |c 'b o I 



FIG. I 



MMMMMHI 

-9C 






-80 






-70 






-60 






-50 




7 


-40 




Ir-Z 


-30 




"-- 


-2O 




-10 




t ^^ 


t. FIC.2 




(b) 




B 

FIC.3 




4 SCIENCE FOR COALMINING STUDENTS 

made. If the eye is in the position (i) the reading is too small 
and if in position (2) the reading is too large. Position (3) gives 
the correct reading. 

MASS AND WEIGHT 

All matter, whether solid, liquid or gas, possesses mass. The 
mass of a substance is the quantity of matter contained in the 
substance and the weight of the substance is the force which 
pulls it towards the earth. The unit of mass in the English 
system is the Standard Pound, which is the quantity of matter 
in a piece of platinum, kept by the Board of Trade. In the 
metric system, the unit of mass is the Kilogram, which is the 
quantity of matter contained in a piece of platinum kept by 
the French Government. 

Subdivisions of the pound are well known, while those of the 
kilogram are given in the following table : 

10 milligrams (mgm.)=i centigram (cgm.) 
10 centigrams (cgm.)=i decigram (dgm.) 
10 decigrams (dgm.)=i gram (gm.) 
i kilogram =1,000 grams, 
also i kilogram 2*2 Ib. 

The Common Balance 

The common balance forms part of the equipment of a 
mining laboratory. In the balance shown in Fig. 3 the hollow 
pillar P is fixed to the base board which rests on levelling screws. 
A brass cylinder which supports an agate plane A can be moved 
up and down the hollow pillar by means of the handle B. 
The beam CD rests with its agate knife-edge E on the plane A. 
Knife-edges F and G, fixed to the beam, support hangers from 
which the pans H and K are suspended. When the balance is 
not in use the beam rests on the supports L and M, but when 
the handle is rotated clockwise, the agate plane A is raised and 
the beam swings freely. The free end of the pointer Q which is 
attached to the beam moves along the ivory scale S. The balance 
depends on the lever principle (see p. 50), for when the arms 
are of equal length the weights applied to their ends are also 
equal 

Before weighing an object, the pointer must be opposite the 
zero of the scale when the beam is raised. The screws at C and 
D are adjusted so that this condition is observed. 

A box of weights is shown in Fig. 4. 



UNITS AND MEASUREMENTS 5 

THE SPIRIT LEVEL 

A spirit level is shown in Fig. 5a. The glass tube PQYX, 
which is filled with alcohol except for a small bubble Z, is 
mounted in a metal framework ABCD, the upper surface PQ 
of the tube being arranged to be exactly parallel to the base 
BC. When the base BC is inclined to the horizontal the upper 





FIG. 



A 


M /N 






D 




O 1 If' I 




/ 






Lc ~~ * _ 




^ 






X 


Y 


^ 
^ 
^ 
^ 
^ 
^ 

! 





B 



(a) 



FIC.S. 




surface PQ is also inclined to the horizontal and the bubble 
rises to the highest part of the tube. When the bubble is in the 
centre of the tube, i.e. between the two marks M and N, the 
upper surface PQ is horizontal and consequently the base BC 
is also horizontal. 

The spirit level forms an integral part of many instruments 
used in mine surveying, such as the theodolite, the dumpy 
level and the miner's dial. The theodolite is mounted on a 



6 SCIENCE FOR COALMINING STUDENTS 

horizontal table which rests on three levelling screws, and 
two spirit levels, placed at right angles to each other, are used 
for making this adjustment. In order to level the table the 
screws are adjusted until each bubble is in the centre of the 
tube and the table becomes horizontal in every direction 
(Fig. 5b). 

EXERCISES 

1. Describe a screw gauge and explain how it may be used for finding 
the diameter of a wire. 

2. Give the theory of a vernier which reads to one-hundredth of an 
inch. 



CHAPTER II 

HYDROSTATICS, A 

DENSITY 

The density of a substance may be defined as the weight of 
unit volume of the substance. If 20 c.c. of a sample of coal 
weigh 26 gm., the weight of i c.c. is 1-3 gm. ; that is, the density 
of the coal is 1-3 gm. per c.c. Also in the English system of 
units, if 20 cub. in. of iron weigh 5-2 lb., the weight of i cub. in. 
is 0-26 lb. and the density of iron is 0-26 lb. per cub. in. 

Thus Density-- - 

Volume 

The following table gives the average densities of various 
typesjof coal and shale : 



Substance 


Density 
(gm. per c.c.) 


Lignite 
British bituminous coal 
Anthracite . 
Ordinary shale 
Oil shale . 


1-15 
1-30 

2-65 



A litre of water weighs i kilogram, i.e. 1,000 c.c. of water 
weigh 1,000 grams and the density of water is i gm. per c.c. 

Density is an important physical constant and it is often a 
criterion of the purity of a substance. In mining practice the 
process of coal washing depends on the fact that the impurities 
have greater densities than the coal and, if the impure coal is 
allowed to fall through water in large tanks, the impurities 
descend more rapidly than the coal. In this manner the materials 
form layers of equal density and can be separated, provided 
pulsating motion is given to the contents of the tank to help 
in the separation. 

Another method of separation is to use a liquid of the same 
density as the coal. A suitable liquid is a solution of calcium 
chloride in water. If sufficient calcium chloride is dissolved in 
water a solution of the required density may be prepared. 

7 



8 SCIENCE FOR COALMINING STUDENTS 

When the solution is placed in the separating tank the coal, 
fully immersed, remains suspended near the surface of the 
solution (see p. 23) and the impurities sink to the bottom, 
again provided pulsating motion is given to the contents. 

RELATIVE DENSITY 

The relative density of a substance is the density of the sub- 
stance compared with the density of water. 

Wt. of i c.c. of substance 



Thus R.D. of substance =- 



Wt. of i c.c. of water 



^ .~ _ Weight of substance 

Weight of an equal volume of water 

We may therefore define the relative density of a substance 
as the weight of the substance divided by the weight of an 
equal volume of water. Another name for relative density is 
specific gravity. 

Determination of the Relative Density of a Liquid, e.g. Paraffin 

The relative density of a liquid can be determined by means 
of a relative density or specific gravity bottle (Fig. 6). The 
bottle possesses a ground stopper which is in the form of a 
capillary tube with a very fine bore. If the bottle is filled with 
liquid nearly to the top of the neck and the stopper is inserted, 
the liquid emerges from the top of the tube and any excess 
can be removed. In this way exactly equal volumes of different 
liquids can be obtained. 

To perform an experiment the bottle is first weighed empty 
and then weighed again filled with water. After emptying the 
water and rinsing out with a little paraffin, the bottle is filled 
with paraffin and weighed again. The results may be set out 
as follows : 

Weight of bottle =2575 gm. 

Weight of bottle filled with water =7578 gm. 
Weight of bottle filled with paraffin 64-89 gm. 
Weight of water to fill the bottle =50-03 gm. 
and Weight of paraffin to fill the bottle =39-14 gm. 

Hence R.D. of paraffin= 39 ' 14 gm * 

50-03 gm. 

=078. 



HYDROSTATICS, A 




FIG. 6 



O 



o 



7LB. 




b LB.2OZ. 








FIC.7 



FIG. 8 



JO SCIENCE FOR COALMINING STUDENTS 

ARCHIMEDES' PRINCIPLE 

If a 7-lb. weight is suspended from a spring balance the 
indicator of the balance registers 7 Ib. But if the 7-lb. weight 
is suspended, fully immersed in water, the indicator registers 
about 6 Ib. Evidently there is an apparent loss of weight. 
Moreover the solid fully immersed in water displaces its own 
volume of the water. What is the connection between this 
apparent loss of weight of the solid and the weight of the liquid 
it displaces? 

Ing. 7 shows a 7-lb. weight suspended from a spring balance. 
A large vessel, brimful with water, rests on a household scale 
pan. If the 7-lb. wt. is suspended, fully immersed, in the water 
and the vessel and contents are then removed from the pan, 
the water which has overflowed can be weighed. A set of results 
is shown below: 

(1) Weight of solid in air =7 Ib. 
Apparent wt. of solid in water =6 Ib. 2 oz. 
Apparent loss in weight =14 oz. 
Weight of water displaced =14 oz. 

(2) Weight of solid in air =4 Ib. 
Apparent wt. of solid in water =3 Ib. 8 oz. 
Apparent loss of wt. 8 oz. 
Weight of water displaced =8 oz. 

Evidently the apparent loss of weight of the solid is equal to 
the weight of water displaced. If the experiment is repeated 
with a liquid such as paraffin, a similar conclusion will be 
reached. Thus Archimedes' Principle may be stated as follows : 
// a solid is weighed in air and then in a liquid the apparent loss 
of weight of the solid or the upthrust of the liquid on the solid is 
equal to the weight of the liquid displaced. 

Determination of the Relative Density of a Solid by weighing in 
Air and in Water 

If a solid (e.g. a piece of coal) is suspended by means of a 
piece of thread from one arm of a common balance, the weight 
of the solid in air may be determined. If the solid is then sus- 
pended freely in a beaker containing water which stands on a 
wooden bridge (Fig. 8), the apparent weight of the solid in 
water may also be determined. The relative density of the solid 
may then be calculated in the following manner: 



HYDROSTATICvS, A II 

Weight of solid in air =2572 gm. 

Apparent weight of solid in water -= 5-93 gm. 

Apparent loss of weight of solid in water =19-79 S m - 
Weight of water displaced =1979 gm. 

Weight of an equal volume of water =1979 gm. 

Relative density of solid = 



1979 gm. 
=1-30. 

THRUST AND PRESSURE 

If we consider a vessel with circular holes in its sides to be 
filled with water, the holes have to be closed by plugs to prevent 
the water from running out. The plugs stop the flow of the 
water and the water exerts a force or a thrust on each, whatever 
its position. If the plugs are withdrawn, water runs out of each 
hole as a jet. Each jet is horizontal as it leaves the vessel 
(Fig. 9) but eventually curves down towards the base. This 
shows that the thrust is everywhere at right angles to the 
surface of the vessel. Thus we see that a thrust is a force distri- 
buted over an area and that the thrust of a liquid on a surface 
is always at right angles to the surface. 

Pressure 

The pressure on a surface is the thrust per unit area of the 
surface, and the pressure is uniform when the thrust on each 
unit of area is the same. 

Thus P=?, 

A. 

where F=the thrust in Ib. wt., 

A=the area in sq. in., 
and P=the pressure in Ib. wt. per sq. in. 



Example. A thrust of 300 Ib. is distributed uniformly over an area 
of 20 sq. in. Find the pressure. 

p 

p_= where F 300 Ib.; A -=20 sq. in. 
A 

p _ 300 Ib. 

20 sq. in. 
= 15 Ib. per sq. in. 

Example. The pressure of steam in an engine cylinder is 100 Ib. 



12 SCIENCE FOR COALMINING STUDENTS 

per sq. in. Find the thrust on the piston, the area of which is 5-5 

sq. in. 

p* 
From I* --, we have P 100 \b. per sq. in.; A =5-5 sq. in. 



A 5'5 



TOO!, -i-= . - 

Vsq.in./ 5-5 sq.in. 



sq 
=- 55" 



) 

s. in./ 



PRESSURE DUE TO A COLUMN OF LIQUID 

It has already been seen that a liquid in a vessel exerts a 
thrust on each element of the surface and the thrust is every- 
where at right angles to the surface. In the case of a cylindrical 
vessel, containing a quantity of liquid, the thrust on the base is 
clearly equal to the weight of the liquid column. 
Thus we have : 

^ , , T Weight of column 

Pressure on the base ~ -- --= -- 

Area of base 

p ___ Volume X Density 

Area of base 
= Height x Density 
i.e. P=Arf, 

where h t d and P are the height, density and pressure re- 
spectively. 

Introducing units we have: 



\sq. 
or I 



sq.in./ "^-'~ ~ \cub.i 

Example. A vertical pipe range in a shaft 100 yd. deep is completely 
filled with water. Find the pressure due to the column of water at the 
bottom of the range. 

Density of water ==-62 -5 Ib. per cub. ft. 

Phd, where h is the height in in., d is the density 111 Ib. per cub. in. 
and P is the pressure in Ib. per sq. in. 



- 

Vcub. i 

= 130-2 Ib. per sq. in. 

THE ATMOSPHERE 

Surrounding the earth there is a gaseous covering known 
as the air or the" atmosphere. Apart from any experimental 



HYDROSTATICS, A 




FIC.9 




3O 



IN. 



ric. 10 



ill 



FIG. II 



14 SCIENCE FOR COALMINING STUDENTS 

evidence we are aware of its presence through winds, which 
are really air currents. 

Since air is a form of matter it possesses mass, and the weight 
of a quantity of air is the force pulling the air towards the earth. 
This can be shown by pumping air into a sheet-metal cylinder, 
fitted with a bicycle valve. By weighing the cylinder and 
contents before and after the air is pumped in, an increase in 
weight is observed. 

The Upward Pressure of the Atmosphere 

If a tumbler is filled brimful with water and a sheet of stiff 
paper is held rigidly over the brim, when the tumbler is in- 
verted and the hand removed, the sheet of paper appears to be 
supporting the column of water (Fig. 10). Actually it is the 
upward pressure of the atmosphere on the paper which keeps 
the water in the tumbler. 

The Downward Pressure of the Atmosphere the Mercurial 
Barometer 

Fig. ii shows a tube, J in. bore, 36 in. long and closed at one 
end. The tube is filled with mercury and all the air bubbles are 
removed. If a finger is placed over the open end and the tube is 
inverted and supported with this end under the surface of 
mercury in a small dish, the mercury falls to a definite level, 
leaving a vacuum at the top of the tube. When the height of 
the mercury column above the level of mercury in the dish is 
measured, it is found to be approximately 30 inches or 76 cm. 
The downward pressure of the atmosphere on the mercury 
in the dish is transmitted through the mercury to the bottom 
of the tube. This pressure supports the mercury column. 

The Fortin Barometer 

A Fortin barometer, used on the surface of a mine, is shown 
in Fig. 12. Owing to the variations in the height of the mercury 
column, the level of the mercury in the cistern A must always 
be adjusted to be opposite the zero of the scale S^ The cistern 
possesses a rubber base and a screw S 2 adjusts the surface of 
the mercury to be in contact with the point of an ivory pin P 
which is opposite the zero of the scale. A vernier V, used in 
conjunction with the scale Si, enables readings to J$Q or JQ-Q in. 
to be accurately taken. 



HYDROSTATICS, A 15 

CALCULATION OF THE ATMOSPHERIC PRESSURE 
i cub. ft. of water weighs 62-5 Ib. 

i cub. in. of water weighs Ib. 

1728 

62*^ 

/. i cub. in. of mercury weighs ^ X 13-6 Ib., since the R.D. 

1720 

of mercury is 13*6. 

Thus the atmospheric pressure 

=the pressure due to 30 in. of mercury 

^height x density 

,. x 62-5 , / Ib. \ 
= 30 (in.) X J X 13-6 _--,- 
1728 \cub. in./ 

=147 Ib. per sq. in. 

The Barograph 

In mining practice records of the barometric pressure may be 
kept for future reference and a self-recording barometer or 
barograph which enables this to be done is shown in Fig. 13. 
The instrument consists essentially of an aneroid barometer 
and a revolving drum. The aneroid barometer A is a thin 
hollow corrugated metal box, partially exhausted of air and 
fixed rigidly to a base board. One end of a flat spring S is 
fixed to the top of the box and the other end is connected to 
an upright B. One end of a steel rod CD is also fixed to the 
upper face of the box and the other end of the rod is connected 
through a complicated system of levers (not shown in the dia- 
gram) to the end E of the stylo or lever EFG which is pivoted 
at F. The arrangement transmits and magnifies the motion of 
the top of the box to the end of the stylo which makes con- 
tact with the graph paper attached to the revolving drum H. 
The drum is rotated by clockwork and when there is no change 
in the atmospheric pressure a horizontal line is traced on the 
paper. 

The box A responds to slight changes in the atmospheric 
pressure. When the pressure increases the top of the box is 
forced downwards and the end G of the stylo moves upwards. 
When the atmospheric pressure decreases the spring S raises 
the upper face of the box and the end G of the stylo is depressed. 
Thus the stylo traces out a graph, showing the atmospheric 
pressure at any particular instant. 



l6 SCIENCE FOR COALMINING STUDENTS 






FIG.I2 



F1GJ3 




PRESSURE 
OF CAS 




ATMOSPHERE 
^PRESSURE 




FICJ4 



HYDROSTATICS, A 17 

PRESSURE DUE TO A COLUMN OF AIR 

The pressure due to a column of air, e.g. the air in a mine 
shaft, can be calculated from the formula already established. 

Example. Find the pressure due to a vertical column of air, 900 ft. 
high (a) in Ib, per sq. in. and (b) in inches of mercury, if the mean 
density of the air is 0*079 Ib. per cub. ft. 

(a) P_-M 

=900 (ft.), 0-079 y^) 

-71-1 Ib. per sq. ft. 

7i-r .. 
-- Ib. per sq. in. 

M4 
- 0-494 Ib. per sq. 311. 

(b) 14.7 Ib. per sq. in. ^pressure due to 30 in. of mercury. 

it 10. 0*494 - r 

0-49.4 Ib. per sq. in. = pressure due to 30 x - in. of mercury. 

pressure due to 1-008 in. of mercury. 

From this example it can be seen that the height of the mercury 
column in a barometer increases by approximately I inch for 
every 900 feet down the shaft. 

The Manometer and the Water Gauge 

A manometer is an instrument which is used for measuring 
pressure. Fig. 14 shows a manometer containing a quantity 
of liquid and connected to the gas jet by means of rubber 
tubing. Before the gas is turned on, the liquid is at the same 
level in both branches of the tube. When the tap is opened, the 
liquid level in the right-hand branch rises and that in the left- 
hand branch falls. Considering the equilibrium of the liquid 
we have : 

Pressure | Pressure __ Pressure , Pressure -Atmospheric 
of Gas "^ due to AB ~~ due to CB "*" due to CD ^ Pressure 

But the pressure due to the column AB is equal to the pressure 
due to the column CB. 

TT . Pressure of Gas__ Pressure due , Atmospheric 
in mains ""to column CD Pressure 

Manometers or water gauges are used in the mine for finding the 
difference in pressure between the air in the upcast and down- 
cast shafts. The water gauge is placed on the separation door 
and a tube, fitted to one limb, protrudes through a hole in the 
door. The pressure of the air in the upcast shaft is reduced by 
the fan at the top and the difference of pressure between the 



i8 



SCIENCE FOR COALMINING STUDENTS 



air in the two shafts, often referred to as the ventilation 
pressure, can be represented in inches of water. 
We have: 

\*hd, where h is the difference in level in inches and d 
is the density of water in Ib. per cub. in. 



WOOD 
BASE- 



GLASS 
U TUBE 



ADJUSTABLE 
MEASURING 
SCALE 

COLOURED 
WATER 



SPRING 
CLIP 
SPIRIT LEVEL 




BRASS / 
NOZZLE 



FRONT 
ELEVATION 



SCALE 
ADJUSTMENT 
SCREW 




SIDE 
ELEVATION 



FIG. IS 



1728 

036. 



Hence 



Thus if h =4*5 in. of water, the difference of pressure is equal 
to 4-5x0*036 Ib. per sq. in., i.e. 0-162 Ib. per sq. in. 
A water gauge used in practice is shown in Fig. 15. 



HYDROSTATICS, A 






r>3 





f 


I 


J 


D 
































- 




r 


"^ 


"" 


"i 








r 






1 ^ 

h*\ 


k p f 


_7- 


lr 1-1 


Ir 












^ \ 


F ^ "" 





. 




""" 


-.- 






i y\ 


R"" 


^j-_ 


^7-_ 


JT^~ 


> 









i 


- 





1 





-:| 

i 


5 

KMOT 


I 


fe 


i 


:) -1 
E^ 


p>: 


~~(br 


**"^ *~~ 



d -;?-;. 

i.S 



(a) 



FIG. 17 



ABSOLUTE PRESSURE 

The absolute pressure of a gas is the pressure measured from 
zero or vacuum pressure. In the case of the gas in the mains 
we have: 

Absolute Pressure = Manometer , Atmospheric 
of Gas Pressure Pressure 



20 SCIENCE FOR COALMINING STUDENTS 

Fig. 16 shows a compressed-air receiver where Tj is the inlet 
tap from the pump and T 2 , the exhaust tap. A Bourdon pressure 
gauge G indicates the absolute pressure of the air in the receiver. 
If the tap T 2 is opened, the pressure of the air in the receiver 
falls to atmospheric, viz. 147 Ib. per sq. in., and the gauge G 
indicates this pressure. When the tap T 2 is closed and the tap 
TI is opened to the pump, the pressure of the air in the receiver 
may become 45 Ib. per sq. in. absolute, for example, which is 
the sum of the atmospheric pressure and the excess pressure of 
the air pumped in. 

Compressed air is used as the motive power in coal-cutting 
machines, conveyor-belt motors, drilling machines, etc., but 
the use of ( 4 lectrically driven machinery is increasing. 

The Pressure at a Point in a Liquid is the same in all 
Directions 

The bowl of a thistle funnel A is connected to a manometer 
BCD (Fig. i7a) which contains coloured water by means of a 
piece of rubber tubing. A thin membrane of rubber EF is 
fastened to the rim of the funnel by means of a piece of thin 
copper wire, so that the funnel is airtight. When the funnel is 
immersed in a cylinder containing water, so long as the centre 
O of the membrane is at the same depth d, the pressure indicated 
by the manometer is the same whatever the position of the 
funnel (Fig. lyb). The pressure of the water on EF is trans- 
mitted through the air to the liquid in the manometer and it 
can be seen that the pressure of the water at a point is the 
same in all directions. Thus, considering Fig. iyc, the pressure 
at O along OP is equal and opposite to the pressure along OQ 
and the pressure at O along OR is equal and opposite to the 
pressure along OS, etc. 

The Pressure at a Depth in a Liquid 

Consider a column of liquid ABCDEFGH (Fig. 18) of uniform 
cross-sectional area, with the face ABCD in the surface of the 
liquid and the face EFGH, at a depth h. The column of liquid 
is in equilibrium and the forces acting on it are : 

(1) the horizontal thrusts on the sides which balance one 

another, viz. the thrust P balances the thrust R and 
the thrust Q balances the thrust S ; 

(2) the weight of the column of liquid ; 

(3) the upthrust on the base EFGH ; and 

(4) the downward thrust of the atmosphere on ABCD. 



HYDROSTATICS', A 21 

Hence we have : 

Upward thrust __ Wt. of column , Downward thrust of 
on EFGH ~~ of liquid ""atmosphere on ABCD 

Upthrust on EFGH __ Volume of Column X Density 
Area EFGH Area"EFGH 

Downward thrust of atmosphere on ABCD 
+ AreaTEFGH 



= X Density 4- Downward pressure of 

depth h r J atmosphere, 

since Area EFGH -Area ABCD. 

Hence the difference between the pressures ^ ,, ^ ^ ., 

ATD^T^ Depth x Density 

on EFGH and ABC D r j 

i.e. P=hd. 

PRACTICAL APPLICATIONS 

In some mines flood water has to be arrested so as to prevent 
the flooding of the workings. This is effected by building dams 
of masonry or concrete to stop up the roadway affected. From 
the principles outlined above it can be seen that the pressure 
of the water is everywhere at right angles to the dam and also 
that the pressure increases uniformly with the depth of the 
water column. Thus the strength of the masonry at all parts 
of the dam must be sufficient to withstand the pressures arising 
from the varying heights of water column. 

Another example of the tremendous pressures resulting from 
a considerable head of water is the action of underground 
spring water on the material which is used to fill up disused 
shafts. In Fig. 19, AB represents a disused shaft, filled in with 
material, and CD represents a roadway inclined to the horizontal 
XY and only filled in to a small distance on each side of the 
shaft. Water from underground springs collects in the portion 
CE of the disused roadway and when the pressure at E becomes 
great enough, the material is gradually washed away down 
the roadway FD. The material in the upper portion of the 
shaft sinks and a crater is left near the surface. 

FLOATING SOLIDS 

Fig. aoa shows a vessel, brimful with water and resting on 
a household scale. If a block of wood which has been previously 
weighed on the scale is allowed to float on the water (Fig. 2ob), 
the water displaced overflows into the pan. If the vessel and 



22 SCIENCE FOR COALMINING STUDENTS 



H 



E 

FIG. 18 



v -z-3; y 





B 



FIG. 19 






(b) 

FIG. 2O 



(c) 



HYDROSTATICS, A 23 

contents arc removed, the weight of the displaced water will 
be indicated on the scale (Fig. 2oc). 

A set of results for three different experiments is shown 
below. 

Weight of block 17 oz. 

Weight of water displaced (1)^16^ oz. 

Weight of water displaced (2) 17 oz. 

Weight of water displaced (3) 17^ oz. 

Thus the weight of the floating solid is approximately equal 
to the weight of water displaced. The same is true for paraffin 
or any other liquid in which the block floats, and if there is no 
experimental error we may state that: 

Weight of floating solid Weight of liquid displaced. 

Relative Density of a Floating Rod of Uniform Cross-sectional 

Area 

If we consider a rod of wood of uniform section (Fig. 21 a) 
and L and / are the length of the rod and the length immersed 
respectively, we have : 

Weight of rod = Weight of water displaced 

= Weight of column EFCB of water 
.*. R.D. of rod 

_____ Weight of rod 

Weight of an equal vol. of water 
_ Weight of water displaced, viz. EBCF 



Weight of water equal in volume to rod, viz. ABCD 
_ /xarea of section xwt. of unit vol. of water 

Lxarea of section xwt. of unit vol. of water 
__ / __ Length immersed 

L Total length 

The Hydrometer 

If we consider a solid floating first in a dense liquid and then 
in a liquid of smaller density, the weight of the liquid displaced 
in each case is the same, being equal to the weight of the solid. 
Hence the volume of the denser liquid displaced is less than 
that of the liquid of smaller density and the solid sinks deeper 
in the latter liquid. The hydrometer (Fig. 2ib) is constructed 
on this principle. A bulb A is provided with a tube CD which 
is closed at the end D. A smaller bulb B, containing a quantity 
of mercury, keeps the hydrometer in an upright position. The 



24 SCIENCE FOR COALMINING STUDENTh 

tube CD is graduated from 1,000 to 1,200. When the hydro- 
meter is placed in water it sinks to the mark 1,000, but when 
placed in sulphuric acid of relative density 1-2, it sinks to the 




(W (c) 




FIC.22 



mark 1,200. Tf the instrument sinks to the mark E 1,145 in a 
further liquid, the relative density of this liquid is 1-145. 

The density of sulphuric acid is a criterion as to whether an 
accumulator requires recharging. Acid is withdrawn from the 
accumulator by means of the arrangement shown in Fig. 2ic 
and a small hydrometer registers its density. 



HYDROSTATICS, A 25 

One of the methods for finding the density of coal is to prepare 
a solution of calcium chloride of greater specific gravity than 
the coal. When the sample of coal is placed in the vessel, it 
floats on the surface of the solution. On adding water and 
stirring the solution becomes less dense and the coal just becomes 
immersed with its upper surface in the surface of the liquid. 
In this case the solution and the coal have the same specific 
gravity which can be determined by a suitable hydrometer. 

THE TRANSMISSIBIUTY OF FLUID PRESSURE 

Fig. 22a shows a hollow rubber ball, perforated with small 
holes and filled with water through the opening O. If the thumb 
is placed over the opening and the ball squeezed, the water 
issues from the holes in jets. This shows that the pressure 
applied to the ball is transmitted to the water near the holes. 

The Hydraulic Jack 

The transmissibility of fluid pressure is utilised in the 
hydraulic jacks which are components of the Joy loader, a 
machine used for loading coal or rock on to conveyors. A 
centrifugal oil pump P (Fig. 22b) puts the oil in the tube A 
under pressure. This pressure is transmitted to the oil in the 
cylinder B of the jack and the plunger D is forced upwards 
with a considerable force. 

Example. If the thrust in the tube A, developed by the centrifugal 
pump, is 60 Ib. \vt. and the diameters of the cylinders A and B are 
i in. and 5 in. respectively, find the upward thrust on the plunger D 
(Fig. 22b>: 

60 
Fluid pressure in tube A=- Ib. per sq. in. 

7TX (0-5)2 

Oo 
Fluid pressure in tube B Ib. per sq. in. 

77 X (0'5)2 ^ 

Thrust on plunger T) ~- XTT X (2-5)2 Ib. wt. 

= 1,500 Ib. wt. 



EXERCISES 

1. Define the following terms: (a) specific gravity, (b) density. 
Describe how you would determine the density of air and the specific 
gravity of a piece of coal. (Min. Sc.; U.L.C.I.) 

2. What is meant by the Principle of Archimedes? Sketch, and 
describe, a hydrometer such as may be used to find the specific gravity 
of the acid solution in an electric accumulator or battery. Show clearly 
how the instrument is graduated. (Min. Sc.; U.L.C.T.) 



26 SCIENCE FOR COALMINING STUDENTvS 

3. One end of a U-tube containing water is connected to a gas supply, 
and the difference in level of the liquid in each limb of the tube is 5-45 in. 
Calculate the pressure of the gas supply in Ib. per sq. in. (The weight 
of i cu. ft. of water is 62-5 Ib.) What would be the reading on the gauge 
if it contained a liquid whose density was 50 Ib. per cu. ft. ? 

(Min. Sc.; U.L.C.l.) 

4. Describe the mercury barometer, explain how it works and state 
the purpose for which it is used in mines. (Min. Sc. ; U.L.C.l.) 

5. Describe how you would determine the specific gravity of a piece 
of coal. 

A piece of sandstone weighs 2*75 Ib. in air and 1-75 Ib. when sub- 
merged in water. What is its specific gravity? (Min. Sc.; U.L.C.l.) 

6. Describe fully how you would construct a simple mercury baro- 
meter. Given that the specific gravity of mercury is 13-6, describe how 
the pressure of the atmosphere may be directly determined. 

(Min. Sc.; U.L.C.L) 

7. Describe two simple experiments to show that the atmosphere 
exerts pressure. Explain how, and why, atmospheric pressure may be 
measured by means of a mercury barometer. (Mm. Sc.; U.L.C.L) 

8. (a) What will be the pressure in Ib. per sq. in. at the base of a 
column of water pipes in a mine shaft 630 yd. deep? 

(b) The water gauge reading at a colliery is 6-6 in. What is the 
difference in pressure in Ib. per sq. in. between the air in the fan drift 
and the outside atmosphere ? (i cu. ft. of water weighs 62-5 Ib.) 

(Mm. Sc.; U.L.C.l.) 

9. State Archimedes' Principle. 

A large piece of coal weighs 12 Ib. 8 oz. in air. When suspended in 
water it weighs 2 Ib. 14 oz. Find the specific gravity of the coal. 

(Min. Sc.; U.E.I.) 

10. Explain the action of an aneroid barometer. An aneroid baro- 
meter indicates pressures of 14-7 Ib. per sq. in. and 15-1 Ib. per sq. in. 
at the top and at the bottom of a mine shaft respectively. What is the 
depth of the shaft? (Density of air 0-079 Ib. per cu. ft.) 

(Mm. Sc., ILL.C.I.) 

11. The height of the mercury column in a barometer at the top of 
a shaft, 400 yd. deep, is 29-8 in. What is the height of the mercury 
column in a barometer at the bottom of the shaft ? (Mean density of the 
air in the shaft 0-079 Ib. per cu. ft.; density of mercury 0-49 Ib. 
per cu. in.) (Mm. Sc.; U.L.C.L) 

12. Describe, and explain, the use of (a) a water gauge, and (b) a 
hydrometer. Draw a diagram in each case. (Min. Sc., U.L.C.L) 

13. What is meant by the term ''specific gravity"? A sample of coal 
weighs 120-75 gm. If the apparent weight of the sample when suspended 
in water is 29-47 m - what is its specific gravity? (Min. Sc. ; U.L.C.L) 

14. Describe a water gauge and explain its use. 

A water gauge is fitted on the door leading into a fan drift. The level 
of the water is 5-3 in. higher in the limb connected to the drift than in 
the other. If the atmospheric pressure is 14-70 Ib. per sq. in., what is 
the pressure of the air in the fan drift? (i cu. ft. of water weighs 62-5 Ib.) 

(Min. Sc.; U.L.C.l.) 



HYDROSTATICS, A 2J 

15. The density of air at the top of a mine shall where the barometer 
reads 30 in. of mercury is 0-079 Ib. per cu. ft. What is the density of the 
air at the bottom of the shaft where the barometer reads 30-4 in.? 
Assume the temperature to be the same in each case. 

(Min. Sc.; U.L.C.I.) 

1 6. Describe the construction of a mercury barometer. If the height 
of the mercury is 29-5 in., what is the pressure of the atmosphere? 
(Mercury weighs 0-49 Ib. per cu. in.) (Mm. Sc.; U.L.C.I.) 

17. Define (a) density, and (b) specific gravity. 

A specific gravity bottle weighs 20-76 gm. empty. When the bottle 
is filled with water it weighs 50-72 gm. and when filled with mercury 
it weighs 428-20 gm. What is the specific gravity of mercury? 

(Min. Sc.; U.L.C.I.) 

18. A barometer indicates a pressure of 29-5 m. of mercury at the 
top of a mine shaft and 31-1 in. at the bottom. What is the depth of 
the shaft? (Density of mercury ^-0-49 Ib. per cu. in, density of air in the 
shaft =0-079 Ib. per cu. ft.) (Mm. Sc.; U.L.C.I.) 

19. Describe a water gauge and explain its use. The difference in 
level between the water surfaces in a water gauge is 6*4 in. What differ- 
ence of pressure in pounds per square foot does this reading represent? 
(i cu. ft. of water weighs 62-5 Ib.) (Min. Sc.; U.E.I.) 

20. What is meant by the terms (a) density, and (b) specific gravity ? 
What is the volume of 2-5 tons of water if the density of water is 

62-5 Ib. per cu. ft? (Min. Sc.; U.E.I.) 



CHAPTER III 

HYDROSTATICS, B 

BOYLE'S LAW 

The Investigation of the Relationship between the Volume and the 
Pressure of a given Mass of Air at Constant Temperature 

Fig. 23 shows an apparatus which is used for finding the 
relationship between the volume and the pressure of a given 
mass of air at constant temperature. The apparatus consists of 
a glass tube AB of f in. bore and mounted on a wooden stand. 
The tube is closed at the end A and the end B is connected by 
pressure tubing C to the reservoir D which is fixed to a wooden 
slider E. This slider moves along a slot in the stand and is kept 
in position by a wing nut at the back. The reservoir, pressure 
tubing and a portion of the tube AB contain mercury and a 
quantity of air is trapped in the tube AB. The pressure of this 
air is given by : 

P-B+A, 

where B = the barometric pressure in inches of mercury, 

A=the excess pressure in inches of mercury, 
and P the absolute pressure of the enclosed air in inches of 
mercury. 

To perform an experiment, the length of the air column AX 
and the excess pressure XY are measured by means of the 
scale F, attached to the apparatus. The barometric height is 
observed and the absolute pressure of the air in AB is calculated. 
When the reservoir is moved into another position the length 
of the air column and the excess pressure are again read and 
so on. A set of results is shown below (barometric height B 
29-8 inches of mercury) : 



Length 


AX (/inches 


Excess Pressure 


Absolute Pressure 


r>/ 


of 


mercury) 


(h inches of mercury) 


P=B + /* inches 


IV 




18-5 


5'0 


34-8 


643-8 




16-3 


9-6 


39-4 


642-2 




14-4 


14-8 


44 -(> 


(>42-2 




12-b 


^'3 


5i'i 


643-9 




10-9 


29-2 


59'0 


643-1 



HYDROSTATICS, B 2Q 

These results show that the product PI is approximately con- 
stant. But the volume V of trapped air is proportional to the 
length /. 

Hence PV is a constant, 

or V varies inversely as P. 

The temperature of the enclosed air remains practically con- 
stant during the experiment. 

The results of the above experiment verify Boyle's Law, 
which may be stated as follows: The volume of a given mass of 
a gas varies inversely as its pressure if the temperature remains 
constant. 

Example. If 1,000 cub. ft. of air at a pressure of 15 Ih. per sq. in. 
are compressed to a volume of 200 cub. ft. what is the final pressure of 
the air, assuming constant temperature? 

From J 'V =C f where C is a constant, 

we have P--I5 lb. per sq. m. 

V 1,000 cub. ft. 
15x1,000 -C 

i.e. PV -15 XT, ooo. 

Now put V 200 cub. ft. 

Px 200 --=1,000x15. 
1,000x15 
200 

"75 lb- P or sc l- in - 

Example. A compressed-air receiver contains 200 cub. ft. of air at a 
pressure of 60 lb. per sq. in. Air is allowed to escape until the pressure 
becomes 40 lb. per sq. in. What volume will the air which escapes 
occupy at atmospheric pressure, viz. 15 lb. per sq. in. ? Assume tempera- 
ture constant. 

Initial pressure x Initial volume Final pressure x Final volume. 

60 ( - ) x 200 (cub. ft.) -40 ( -} x V. 
\sq. in./ \sq.m.' 

. r 200x60 , ,, 
\ cub. ft. 
40 

--300 cub. ft. 

Thus 100 cub. ft. of air at 40 lb. per sq. in. pressure escape. 
'Again : 
Initial pressure x Initial volume^ Final pressure x Final volume. 



40 
T 



X 100 (cub. ft.) -15 (-^- ) x V. 
\sq. in./ 



sq. in./ \sq. 

_, 40x100 , ., 

V^=- ---- cub. ft. 

15 
=266-7 cub. ft. 



30 SCIENCE FOR COALMINING STUDENTS 

Practical Applications of Boyle's Law and Barometric 
Readings in Mining 

The reading of the barometer at frequent intervals is very 
important in mining practice. Colliery officials are enabled to 
ascertain the condition of underground workings with regard to 
the emission of gases from the strata, the gobs and the cavities. 
When the atmospheric pressure is decreasing, the pressure 
of these gases in the cavities, gobs, etc., falls with a consequent 
increase in volume. The gases spread out into the workings and 
the ventilation may require adjustment to counteract this 
danger. If, on the other hand, the pressure of the atmosphere 
is increasing, the gases an* kept confined to the strata, cavities, 
etc., and danger from escaping gases is at a minimum. 

THE WATER BAROMETER 

Example, What is the height ot a column of water which will exert 
the same pressure as a column of mercury 30 inches high? R.D. of 
mercury- -13*6. 

Let h height of the water column in inches. Pressure of the 
atmosphe re h x density of water, and pressure of the atmosphere -- 
30 y density of mercury. 

h density of water = 30 y density of mercury. 

, .. x density of mercury 

7*=30 (m.)X - ^ -- ~ y - 

density of water 

-30 (in.) x 13-6. 
408 in. 
=34 ft. 
Thus the height of the water barometer is 34 feet. 

PUMPS 

The Lift Pump 

Fig. 24a shows a model of a lift pump. A is the pipe, the 
lower end of which is immersed in water and B is the barrel 
in which the piston P moves. The piston P is connected by 
means of a rod to the end G of the handle GH which is pivoted 
at F. A ball valve Vj rests in a socket at the top of the pipe and 
a valve V 2 closes a circular hole in the piston. 

Suppose the piston is at the bottom of the barrel. When the 
piston is raised, a partial vacuum is created in the barrel and 
the pressure of the atmosphere closes the valve V 2 . Air and 
water rise up the pipe A, through the valve V t and into the 
barrel. When the piston is depressed the pressure above the 
valve Vi becomes greater than that below and the valve closes. 
Also the valve V 2 opens, allowing water to rise through the 



HYDROSTATICS, B 



\ W\\\ XXN vl I EyvS 




FIG. 23 




FIG. 24 



32 SCIENCE FOR COALMINING STUDENTS 

piston. A succession of upward and downward strokes delivers 
the water from the spout S. 

Tn practice, owing to leakage of air past the piston, water 
cannot be raised 34 feet as would be expected from the previous 
theory. The height in actual practice is no more than 28 feet. 

The Force Pump 

In this type of pump (Fig. 24b) water is raised into the barrel 
during the upward stroke of the piston. Then, on the downward 
stroke, the valve V t closes, preventing the water from running 
back to the pipe. The water is forced along the horizontal tube, 
through the valve V 2 and up the delivery pipe A. During the 
next upward stroke of the piston, more water passes up the 
pipe and through the valve V 1; but the valve V 2 closes, pre- 
venting the water in the delivery tube from running back 
into the barrel. The air in the vessel B acts as a cushion by pre- 
venting sudden changes of pressure and in this way the water 
is delivered continuously. With this pump water can be forced 
to any height consistent with the force applied to the pump 
handle. 

The Reciprocating Pump 

A reciprocating pump used in mining operations is shown 
in Fig. 25. As the piston P moves to the left, the valve A is 
closed and the valve B opens, allowing water from the pipe 
F to enter the cylinder W to the right of the piston. During 
this operation the water already on the left of the piston 
is forced through the valve C and up the delivery pipe E, 
the valve D being kept closed by the pressure of the water 
above it. 

As the piston moves to the right, the valve B closes and the 
valve A opens, allowing water to enter the cylinder to the left 
of the piston. At the same time the water already to the right 
of P is forced past the valve D and up the delivery pipe E, 
the valve C being kept closed by the pressure of the water 
above it. 

THE SIPHON 

A simple siphon tube is shown in Fig. 26a. A tube DC, 
bent twice at right angles, is placed with one end in a cylinder 
containing water. By applying the mouth to the end C, water 
can be drawn through the tube. The water continues to flow 
out of the tube until the level of the water in the cylinder 
is opposite the end C. Then'the water ceases to flow. 



HYDROSTATICS, B 33 

To explain this siphon effect it is necessary to remember that 
the pressure along the horizontal line XY is everywhere the 
same and equal to the downward pressure of the atmosphere 
on the water surface in the cylinder. Thus the pressures inside 
the tube at A and B are each equal to the atmospheric pressure. 
But the downward pressure of the water at C is greater than 
the pressure at B, i.e. greater than the atmospheric pressure. 
Thus at C we have the upward pressure of the atmosphere and 
the downward pressure of the water which is greater than 
atmospheric pressure. Consequently the water runs out of the 
tube. 

The siphon principle is applied in practice in a variety of 
processes, chiefly in the emptying of tanks which are in inaccess- 
ible places. It is also used for emptying sump holes containing 
water in mining operations. Fig. 26b shows a siphon tube ABC 
with a non-return valve at A, a priming tap at B and a dis- 
charge tap at C. With A and C closed, the tube is filled with 
water through the priming tap B. This tap is now closed and 
the tap C is opened. Water is thus siphoned over from the 
sump hole to the lower level. The figure shows a special per- 
forated entrance tube D which prevents solid matter from 
entering the tube. 

THE CENTRIFVUAL FAN 

If a small solid is attached to a piece of thin string and the 
free end of the string is held in the hand, when the solid is 
whirled round in a circle, the tension of the string becomes 
considerable. If the speed of the solid is gradually increased, 
the tension of the string increases, the string ultimately breaks 
and the solid flies off at a tangent. The force acting from the 
centre of the circle along the string which breaks or tends to 
break it, is known as the centrifugal force. 

This is one of the principles involved in the action of the 
centrifugal fan shown in Fig. 27. W is the impeller wheel, fitted 
with blades. When the wheel is rotated by an electric motor 
at the side of the casing C, air is thrown off the blades tangenti- 
ally and forced through the opening O. Air is drawn through 
the eye E to neutralise the vacuum which tends to be created. 

Fans are used extensively in mining for ventilation purposes. 
Air is drawn up one shaft by a fan, placed on the surface and 
connected to the shaft by means of a drift, and a strong current 
of air descends the other shaft. In this manner, by forced 
convection (see p. 112), a continuous supply of fresh air is 
circulated through the workings. 



34 SCIENCE FOR COALMINING STUDENTS 



ATMOSPHERE 
PRESSURE 




B-Y 




yc 

AATMOS. 
PRESSURE 



(a) 



FIC.26 



L 



SIDE ELEVATION 




L 



J 



FRONT ELEVATION 




PLAN 




FIG.28 



FIG.27 



HYDROSTATICS, B 35 

THE ANEMOMETER 

The anemometer (Fig. 28) is an instrument used for measur- 
ing the velocity of air currents in underground roadways. 
The instrument consists of a set of vanes G, mounted on a hub 
and surrounded by a guard ring which protects the vanes 
from damage. The movement of the vanes is transmitted by 
means of shafting and gears to pointers which move along 
three circular scales in a metallic housing. The housing is 
provided with a glass cover and is rigidly fixed to a metallic 
base by four supports. 

The movement of the pointers is controlled by a lever L 
which permits rotation of the vanes with or without the move- 
ment of the pointers. 

The instrument actually measures the length of the air 
column (in feet) passing through the vanes and the three scales 
already mentioned indicate measurements in feet, hundreds of 
feet, and thousands of feet respectively. 

When used in conjunction with a stop watch the time taken 
for a certain column of air to flow may be ascertained and 
subsequently the velocity of air flow may be calculated. If 
the measurement is taken exactly over one minute the anemo- 
meter reading then gives the velocity of air flow in feet per 
minute directly. 

Measurement of Quantity of Air flowing per Minute 

If the cross-sectional area of the roadway transverse to the 
direction of the air flow is measured at the same place as the 
anemometer reading is taken, then the quantity of air flowing 
past this position per unit time can easily be calculated. 

Example. The velocity of air flow at a point in an underground 
roadway is 3,000 ft. per minute. If the cross-sectional area of the 
roadway is 30 sq. ft., find the quantity of air flowing through the road- 
way per minute. 

Quantity of air flowing ^3,000 ft./min. X3O sq. ft. 
90,000 cub. ft. per min. 

EXERCISES 

1. Explain how a simple ram or force pump operates. What factors 
determine the depth from which a pump will draw water to its own 
level? (Min. Sc.; U.L.C.I.) 

2. How does the ventilation of the workings of a mine exemplify 
(a) Boyle's Law, and (b) Graham's Law? (Min. Sc. ; U.L.C.T.) 

3. In connection with the ventilation of a mine, explain the use and 



36 SC1ENCK FOK COALMINING STTDENTS 

principles of action of the anemometer and the water gauge. What is 
recorded by the anemometer? (Mm. Sc. ; U.L.C.l.) 

4. Describe, in detail, how air currents in mines are measured and 
how the quantity of air passing through an airway is calculated. 

(Mm. Sc.; U.L.CT.) 

5. State Boyle's Law. What volume of air originally at 60 Ib. per 
sq. in. absolute would occupy 1,000 cub. ft. when expanded to a pressure 
of 14-4 Ib. per sq, in, absolute? (3\1in. Sc.; U.L.C.l.) 

6. State Boyle's Law. A compressed air receiver contains 55 cub. ft. 
of air at an absolute pressure of 80 Ib. per sq. in. Assuming the tempera- 
ture to remain constant, what volume would this quantity of air 
occupy at atmospheric pressure, which is 14-7 Ib. per sq. in. ' 

(Min. Sc.; U.L.C.L) 

7. Describe, and explain, the action of a pump of the reciprocating 
type. Draw a diagram to illustrate your answer. If the pressure of the 
atmosphere is i \ m j Ib. per sq. in., what is the theoretical depth from 
which the pump can draw its water? (i cub. ft. of water weighs 62-5 Ib.) 

(Min. Sc.; U.L.C.L) 

8. State Boyle's Law. A compressed air receiver has a volume of 
200 cub. ft. and contains air at an absolute pressure of 30 Ib. per sq. in. 
If 600 cub. ft. of air at atmospheric pressure (viz. 15 Ib. per sq. in.) arc 
pumped in, what is now the pressure of the air in the receiver' Assume 
the temperature to remain constant. (Min. Sc. ; U.L.C.L) 

9. Give a diagram of an anemometer. For what purpose is it used in 
mining ? 

Find the quantity of air in cub. ft. per minute which flows past a 
fixed station in an underground roadway of rectangular section, 12 ft. 
by 9 ft., if the velocity of the air when measured at this station is 
500 ft. per minute. (Mm, Sc.; U.E.I.) 

10. Give a diagram of a fan and explain its use in mining, 

(Min. Sc.; U.K.I.) 
n. A quantity of gas is maintained at constant temperature. What 

is the effect on the volume of the gas of (a) a reduction of pressure and 

(b) an increase of pressure? 
How may the composition of mine air be affected by a reduction of 

the atmospheric pressure' (Min. Sc.; U.E.I.) 



CHAPTER IV 

DIFFUSION 

DIFFUSION IN LIQUIDS 

Fig. 29 shows a flat cork on the surface of a solution of 
copper sulphate in a gas jar. When water is gently poured upon 
the cork, the water forms as a distinct layer on the solution 
with a definite line of demarcation between them and the cork 
floats on the surface of the water. The function of the cork is to 
allow the water to be poured upon the solution without risk 
of mixing. When the cork is removed and the jar is left for a 
few weeks it will be found that the line of demarcation has 
disappeared and the liquids have mixed. A number of heavy 
particles of copper sulphate solution have risen in the jar and 
a number of lighter particles of water have descended to take 
their places. This process is known as diffusion. 

DIFFUSION OF GASES 

Diffusion also takes place when two gases are placed in 
contact with each other. Although the heavier gas may be 
below the lighter one, molecules of the heavy gas diffuse up- 
wards and molecules of the lighter gas diffuse downwards. 

If a porous pot A is fitted with a rubber stopper, through 
which one end of a glass U-tube is inserted (Fig. 3oa), and a 
quantity of coloured water is poured into the U-tube, after a 
time the levels in the two branches become the same. If an 
inverted beaker D is placed over the pot and coal gas is led 
from the jet into the beaker, the water level descends to B in 
one branch and ascends to C in the other. This shows that the 
molecules of coal gas pass through the pores of the pot at a 
greater rate than the molecules of air pass from the pot into 
the beaker. Thus the molecules of coal gas, a lighter gas than air, 
diffuse through the pores more quickly than the molecules of air. 

Fig. 3ob shows a similar apparatus with the porous pot A 
enclosed in an upright beaker containing carbon dioxide. 
The water level ascends to B in the left-hand branch and 
descends to C in the right-hand branch. This shows that mole- 
cules of air diffuse through the pores of the pot at a greater 
rate than molecules of carbon dioxide. 



3 



SCIENCE VOK COALMINING STUDENTS 



FIG.29 



D 




COAL 
CAS 



B 



(a) 




FIG.3O 



GRAHAM'S LAW OF DIFFUSION 

Graham's Law of Diffusion states that gases diffuse at rates 
which are inversely proportional to the square roots of their 
densities. Expressing this law by means of a formula, we have: 

Rate of diffusion of gas^i_ / V/densityj)f gas 2 
Rate of diffusion of gas 2 \7density~of "gas~i 

Taking methane and air as an example, since the densities 
of methane and air are in the ratio 0-555 to i, we have: 

Rate of diffusion of methane V~ i / x 



Rate of diffusion of air 



Vo-555 



Again, taking the case of carbon dioxide and air, the den- 
sities of carbon dioxide and air are in the ratio of 1-528 to i. 



DIFFUvSION 39 

Hence 

Rate of diffusion of carbon dioxide _V~ i __ 
Rate of diffusion of air ~~ Vi*528~~~ 
From (i) the rate of diffusion of methane 

= i -34 X rate of diffusion of air 
and from (2) the rate of diffusion of carbon dioxide 

o-8r xrate of diffusion of air 
Hence 

Rateof diffusion of methane __ 1 34 

= 5 



dioxide 



-AIR CURRENT 



^AIR MOLECULES DIFFUSION AND VENTILATION 

BEFORE DIFFUSION .... _. IN PROGRESS 

PIG. 31 



PRACTICAL APPLICATIONS IN MINING 

The phenomenon of diffusion is important from the point of 
view of the miner. Methane, which is a constituent of firedamp, 
is lighter than air and collects in the wastes and gobs near 
the roof and in the rise workings. The process of diffusion 
causes the methane, although lighter than air, to diffuse down- 
wards into the roadway, but the ventilation of the mine keeps 
up a steady flow of fresh air and gradually accumulations of 
methane are removed (Fig. 31). 

This, however, is a very slow process and removal of accumu- 
lations of the gas is made by direct ventilation, i.e. by directing 
the ventilating current into the gas. Brattice cloth (Fig. 32) 
is placed in the lower portion of the roadway and the ventilating 
current is confined to the upper space. 

Carbon dioxide, which is a constituent of blackdamp, collects 
near the ground of the workings and it has to be removed for 
safety purposes. The process of diffusion causes the heavy 
carbon dioxide molecules to rise into the ventilating air current 



4O SCIENCE FOR COALMINING STUDENTS 

and the accumulations of carbon dioxide are gradually 
removed (Fig. 33). 

Again this is a very slow process and for rapid removal of 




o oU o 

O flrf) * 

o < o 





o 








o 





z u * 








> s 








O 





ft i 


Z 








< 





c 


z 


o Oi 


1 


Z 





Q 


</> 


o ' 


D 





u. 


o o o 


5 


o 




oof 





o 



o " 




o 




o o 




o o 


z 


o o 


o 


o o 


^ 


* 


u. 

1A. 


o o 


o 


o o 


UJ 


o o i 


QC 

o 





u. 
UJ 





CD 


o o 














O 










blackdamp the ventilating current is directed into the gas by 
means of brattice cloth. 

Colliery officials are required to ensure that the ventilation 



DIFFUSION 41 

of the mine is such that the statutory limits in relation to safety 
in mines are not exceeded. When the general body of the air 
in a working place contains i^ per cent, of methane all elec- 
tricity is cut off from cables and electrical appliances. If the 
general body of the air contains 2\ per cent, of methane all 
men are withdrawn. In the case of carbon dioxide the statutory 
limit is i per cent. 

EXERCISES 

1. (Jive definitions of (a) Boyle's Law, (b) (iiaham\s La\s, Kxplam 
fully how each has an application in the ventilation of mines. 

(Min. Sc.; U.L.C.I.) 

2. Describe two experiments to illustrate the diffusion of gases. 
Explain why diffusion takes place, Why is the diffusion of gases of 
special interest to the miner? 

Describe, with sketches, two experiments to show that gases mix 
by diffusion. (Min. Sc. f U.L.C.T.) 

3. Describe an experiment which shows that firedamp diffuses at a 
greater rate than air. Give an example of the diffusion of gases, taken 
from the ventilation of the workings of a mine. (Min. Sc ; U.E.I.) 

4. State Graham's Law of Diffusion. Describe an experiment which 
shows that different gases diffuse at different rates. Explain how the 
diffusion of gases affects the composition of mine air. 

(Mm. Sc.; U.L.C.I.) 



CHAPTER V 

MECHANICS 

FORCE 

It is common knowledge that a body at rest remains at rest 
unless acted upon by a force such as a pull or a push of sufficient 
magnitude. Also in the case of a body moving with a constant 
speed in a straight line, a force has to be applied to increase or 
decrease the speed. Again a force must be applied to change the 
direction of motion of a body even though the speed of the body 
may be unchanged. 

Representation of a Force 

If a string AB is fastened to a hook in the wall and pulled in 
the direction AB (Fig. 34a) with a force equal to 10 Ib. wt., 
the force on the hook can be represented by a straight line CD, 
5 in. long (scale I in. =2 Ib. wt.), where CD is parallel to AB. 
The arrow shows that the force is a pull which acts in the 
direction C to D. If a rod EF is hinged to the wall at a point E 
(Fig. 34b) and a push of 10 Ib. wt. is applied along FE, this 
force can be represented by a straight line GH, 5 in. long (scale 
i in. 2 Ib. wt.) where GH is parallel to FE. 

It will be noticed that either a pull along EF or a push along 
FE can be applied in the rod EF (Fig. 34b), whereas in the case 
of the string AB (Fig. 34a) only a pull can be applied in the 
direction AB. 

Thus a force can be represented by a straight line (a) in 
magnitude by the length of the line drawn to scale, (b) in direc- 
tion by the direction of the line and (c) in sense by an arrow. 

Resultant of Two Forces, acting at a Point in the same 
Straight Line 

If the string (Fig. 34a) is pulled by two persons, one with a 
force of 10 Ib. wt. and the other with a force of 15 Ib. wt., the 
combined effect of these two forces is a force of 25 Ib. wt. 
Again, in the case of a cage ascending a shaft, if the upward 
pull in the winding rope is 2,000 Ib. wt. and the weight of the 
cage, contents, etc., acting vertically downwards, is 1,900 Ib. 
wt., then the combined effect of the two forces is an upward 

42 



MECHANICS 



43 



5 IN. 




FIG. 34 



IN. 




10 





FIG. 36 



44 SCIENCE FOR COALMINING STUDENTS 

force of 100 Ib. wt. which produces the upward motion of the 
cage. The combined effect of the two forces in each of the above 
cases, is known as the resultant. Thus the resultant of two forces, 
acting at a point, in the same straight line and in the same 
direction, is the sum of the two forces. The resultant of two 
forces, acting at a point, in the same straight line and in 
opposite directions, is the difference of the two separate forces 
and the point of application moves in the direction of the larger 
force. 

When two forces, acting at a point, are inclined at an angle 
to each other, the resultant can be determined by the following 
law. 

PARAT.T.KLOGKAM OF FORCES 

If two forces, acting at a point, are represented in magnitude 
and direction by the adjacent sides of a parallelogram, the 
resultant of these two forces can be represented in magnitude 
and direction by the diagonal of the parallelogram passing 
through this point. 

Example. Two forces, one of 8 Ib. wt. and the other of 6 Ib. wt., act at 
a point and make an angle of 60 with each other. Find the magnitude 
of the resultant force and the angle which the resultant makes with the 
larger force. 

Suppose O (Fig. 35) represents the point of application of the forces. 
Let the force of 8 Ib. wt. act along OA and the force of 6 Ib. wt., along 
OH, the angle BOA being 60. 

If the scale, i in. 4 Ib. wt., ib chosen and Oa and Ob are marked off 
to equal 2 in. and 1-5 in. respectively, when the parallelogram OaCb is 
completed, the diagonal OC is found to be 3-05 in. long. 

Hence the resultant =3 -05 x 4 Ib. wt. 

= 12-2 Ib. Wt. 

Also /_COa= 25. 



TRIANGLE OF FORCES 

Fig. 36 shows two spring balances P and Q, attached to the 
pegs A and B, which are fixed to a stand. The hooks of the 
balances are connected by a suitable length of string and an- 
other piece of string which supports a known weight W is 
connected to the point O. The system is now balanced with 
the point O at rest or in equilibrium. A drawing-board to which 
a sheet of paper is attached is supported just behind the strings. 
If a point a is marked on the paper and the lines ab and be are 
drawn parallel to OA and OB respectively, to represent the 
forces in the spring balances P and Q to scale, when the points 
c and a are joined, the line ca is vertical and parallel to the 
string supporting the weight W. 



MECHANICS 45 

If the string supporting W is moved into another position, 
similar results are obtained. A table of results is shown below 
(scale i in. --I lb.). 



p (lb.) 


QOb) 


W (lb.) 


ab (in.) 


be (in ) 


f# (in.) 


3'5 


2\5 


4 


3-5 


2'5 


3'9 


3'0 


3'i 


4 


3-0 


3'J 


4*T 


2-9 


2-6 


4 


2-9 


2 -ft 


4-0 



In each of the above cases the forces P and Q are represented 
in magnitude and direction by the straight lines ab and be and 
in each case the remaining side ca represents the remaining 
force W in magnitude and direction. We are thus led to the 
Triangle Law of Forces, which may be stated as follows: 
// three forces, acting at a point, are in equilibrium they can be 
represented in magnitude and direction by the sides of a closed 
triangle, taken in order. 

Let the forces P, Q and R, acting at O, be in equilibrium 
(Fig. 37). If the sides ab y be and ca of the triangle abc are parallel 
to the directions of the forces P, Q and R respectively and the 
side ab represents the force P in magnitude, then the remaining 
sides be and ca represent the forces Q and R respectively in 
magnitude. Thus we see that if the magnitude of one force and 
the directions of the three forces are known, then the magni- 
tudes of the two remaining forces can be calculated. 

Equilibrium and Equilibrant 

If only the forces P and Q act on a particle at O (Fig. 37) the 
particle will move along the direction of the resultant. If, 
however, a force R is applied so as to keep the particle at rest 
or in equilibrium, this force R is the equilibrant of the two 
forces P and Q. Si-nilarly the force Q is the equilibrant of the 
forces P and R and the force P is the equilibrant of the forces 
R and Q. 

Example. A string AC supports a load of 50 lb. wt. Another string is 
attached to a point B of the first string and is pulled in a horizontal 
direction BD, so that AB makes an angle of 30 with the vertical 
(Fig. 38). Determine the forces in BA and BD. 

If we draw PQ vertical and equal to 5 in. (scale i in. ^=10 lb. wt.) 
and we also draw QR and PR parallel to BD and AB respectively, then 
QR=2-8 in. and RP=57 in. 

Hence the force in BD ==2-8 x 10 lb. wt. 
=28 lb. wt. 

and the force in BA =57 x 10 lb. wt. 
=57 lb. wt. 



46 SCIENCE FOR COALMINING STUDENTS 





FIG. 37 




5 IN. 




5-7 IN. 



Q. 2-8 IN. R 



FIC.38 



2-5 TONS 




Q 



FIC.39 



MECHANICS 47 

Example. The framework ABC (Fig. 39) forms part of the structure 
of a colliery winding head gear. A vertical load of 2-5 tons acts on the 
point A. Find the forces in the members BA and CA. 

If we draw PQ vertical 5 in. long (scale i in. -=J ton) and QR and 
PR parallel to CA and AB respectively, then QR=43 in. 

Hence the force in CA, acting from C to A, 
=4-3 xo-5 tons 

2-15 tons. 
Also RP = 2-5 in. 

Hence the force in BA, acting from B to A 
^ 2-5x0-5 tons 

1-25 ton. 

MOMENTS 

Consider a spanner AB (Fig. 4oa). If a force of 10 Ib. wt is 
applied at A at right angles to its length and at 6 in. from the 
centre C of the nut, the turning effect is greater than if the 
10 Ib. wt. were applied at Aj, where AiC is 4 in. Again, if a force 
of 20 Ib. wt. be applied at A at right angles to its length, the 
turning effect is greater than when a force of 10 Ib. wt. is 
applied. This turning effect, or moment as we shall term it, 
depends on (a) the force applied along AF and on (b) the 
distance of the force from the point about which the turning is 
taking place. 

The moment of a force about a point is sometimes called the 
torque and is defined as the product of the force and the perpen- 
dicular distance of the force from the point. Thus we have: 

Moment or Torque = Force x Perpendicular Distance 

When a force of 10 Ib. wt. is applied along AF, the moment is 
10 Ib. x6 in., i.e. 60 Ib. in. 

It is important to remember that in calculating moments, 
it is the perpendicular distance which is taken. This is illustrated 
by the following example. In Fig. 4ob AB represents the con- 
necting-rod of an engine and OA the crank. If F is the force 
along the connecting rod, the turning moment on the flywheel 
is given by FxOC, where OC is the perpendicular distance 
between the crankshaft and the connecting-rod. 

Clockwise and Counter-clockwise Moments 

Suppose the point about which moments are taken is the 
centre O of the face of a clock. Then the moment of the force F 
about the point is a clockwise moment, since the turning is 
in the same direction as the motion of the hands of a clock 
(Fig. 4ia). If the turning is counter-clockwise, the moment of 



48 SCIENCE FOR COALMINING STUDENTS 




(a) 




FIG. 40 




Y 

FIG. 42 



the force F about the point is called a counter-clockwise 
moment (Fig. 4ib). 

Fig. 42 shows two spring balances P and Q which support a 
uniform rod in a horizontal position, the supporting strings 
being vertical. The rod is in equilibrium under the action of two 



MECHANICS 49 

upward forces whose values are indicated on the spring bal- 
ances and three downward forces. (N.B. Since the rod can be 
balanced at its centre C, the weight of the rod may be considered 
to act vertically downwards through this point.) If a point 
O is chosen on the rod and the distance of each force from this 
point is measured, the moment of each force about O may be 
calculated and the sum of the clockwise moments may be 
compared with the sum of the counter-clockwise moments. 
If the weights are moved into new positions or a different point 
O is chosen, a set of results as shown in the following table 
can be obtained. 



Sum of Clockwise 
Moments (Ib. in.) 


Sum of Counter-clockwise 
Moments (Ib. in.) 


(4'5X5)-|- (0-5x2) + (5x8) 
= 63-5 


(4x2-1) + (4x13-5) 
=62-4 


(4-5X7-5) + (5X5-5) 
= 61-3 


(4X4-6) + (0-5x0-5) + (4x10-8) 
=61-9 



PRINCIPLE OF MOMENTS 

The results in the previous table verify the Principle of 
Moments which states that if a number of forces, acting in the 
same plane, keep a body in equilibrium, the sum of the clockwise 
moments about any point in the plane is equal to the sum of the 
counter-clockwise moments about the same point. 

Upward and Downward Forces 

With the apparatus shown in Fig. 42 the sum of the upward 
forces (i.e. the sum of the readings given by the balances) 
may be compared with the sum of the downward forces (i.e. the 
sum of the two hanging weights and the weight of the rod). 
When the weights are moved into different positions and the 
observations are repeated, a table of results similar to the 
following may be obtained. 



Sum of Upward 
Forces (Ib. wt.) 


Sum of Downward 
Forces (Ib. wt.) 


6-25 + 5-25 = ii-5 


4-f7+o-5 = ii'5 


4-2 +7-3 = n-5 


4+7-fO'5 = ii'5 



50 SCIENCE FOR COALMINING STUDENTS 

These results show that if a bar is at rest under a system of vertical 
forces, the sum of the upward forces is equal to the sum of the 
downward forces. 

Example. A uniform beam AB, 20 feet long and weighing 100 lb., 
rests horizontally on two supports, one at the end A and the other at 
the end B. Vertical loads of 120 lb. and 180 lb. are applied at distances 
of 8 ft. and 12 ft. respectively from the end A. Find the upward forces at 
the supports (Fig. 43). 

Let X upward force at A. 

and Y ^upward force at B. 

Taking moments about A, we have: 

Sum of clockwise moments Sum of counter-clockwise moments. 

120x8 lb. ft. + iooxiolb. ft. + 180x12 lb. ft.=2oY lb. ft. 
[N.JLJ. A uniform rod can be balanced at its centre. Hence the weight 
of the rod may be considered to act vertically downwards through its 
centre.] 



Y=2o61b. 

But X+Y -=1204-100 + 180. 

X + 206 120 + 100 + 180. 
X 400 206. 



LEVERS 

A lever is a rigid bar which is capable of turning about a 
pivot or a fulcrum. By means of a lever a load may be raised 
or a resistance overcome by applying an effort at a more 
convenient point. In Fig. 44 several types of lever are shown. 
In each case, taking moments about the fulcrum F, we have: 



where W=the resistance or load 

and P=the effort. 

The Bent Lever 

In some cases the lines of action of the resistance and the 
effort are inclined at an angle to each other. The lever used in 
this case is shown in Fig. 45, where W is the resistance acting 
at right angles to OB and P is the effort, applied at right angles 
to OA. Taking moments about the fulcrum O, we have: 

P.OA-W.OB. 

Example. In the lever shown in Fig. 45, the lengths of the arms OA 
and OB are 10 in. and 6 in. respectively. What effort applied at A, at 
right angles to the arm OA, is required to overcome a resistance of 
1,000 lb. applied at B, at right angles to OB? 



MECHANICS 



IX , 


Y 


1 1 




H" '*"" 




* 






\ 1 

> 


f 


) 


/ 










'I8O LB, 




I2OLB. } 


/ 




J* 


IOO LB. 





FIG.43 



*- a > 



,P Tw 



FIG. 44 



b > 



W 



W 



FIG. 45 



O 
*~s 

O 



V 



52 SCIENCE FOR COALMINING STUDENTS 

Let P-the effort. 

Taking moments about O, we have: 

Pxio (1b. in.) =1,000x6 (Ib. in.) 



10 



PRACTICAL APPLICATIONS 

Fig. 46 shows a crowbar AB which is used for raising an edge 
of a large boulder from the ground. The fulcrum F is a small 
piece of stone. The force exerted by the boulder on the end B 
represents the load W while the effort P is applied at the 
end A. 

Fig. 47 shows a lever safety valve, used with a steam-engine 
boiler. The valve V, attached to the point A, is held down by the 
load W which is placed at B. The force exerted by the steam 
may be considered to be the effort P which is required to raise 
the load W and since F is the fulcrum we have: 



Fig. 48 shows a portion of the lever system used for applying 
the brake to a haulage wheel or winding engine drum. The 
lever A which carries the brake-shoe B is pivoted at C. A 
horizontal rod D is pinned at one end to the upper end of the 
lever A, and at the other end to the toggle E, which is pivoted 
at the upper end of the lever F. The brake-shoe G is fixed to 
this lever. A horizontal effort P is applied along the rods D 
and H from left to right and the brake-shoes B and G are 
pressed tightly against the rim of the wheel. Consi4ering the 
lever A, the force W exerted by the rim of the wheel on the 
brake-shoe B corresponds to the load and we have : 



The frictional resistance (see p. 55) between the wheel and the 
brake-shoes brings the wheel to rest. 

COUPLES 

Suppose a force P acts at each end of a lever AB, pivoted at 
its centre O, the forces being at right angles to the lever (Fig. 
49a). When each force produces a moment of the same sense 
about O, the sum of the moments 

=P.AO+P.BO 
=P . AB. 



MECHANICS 



53 




FIG.46 






a 




P 

FIG.47 



w 



1 




FIG. 48 



54 SCIENCE FOR COALMINING STUDENTS 

The two forces in the above case constitute a couple and the 
moment of the couple =one force x perpendicular distance 
between the forces. 

Example. A force of 10 Ib. wt. is applied tangentially at each end of 
a diameter of a circular tap of a compressed-air receiver (Fig. 49b). If 
the diameter of the tap is 2-5 in., find the moment of the couple applied 
to the tap. 

Radius of tap= in. 

Moment of couple 10 Ib. x 1-25 in.-f 10 Ib. x 1-25 in. 
= 12-5 Ib. in. -f 12-5 Ib. in. 
-^25 Ib. in. 




B 



FRICTION 

If a block of wood rests on a table, a horizontal force must be 
applied to the block in order to move it along the table. Al- 
though the surfaces in contact may appear to be smooth, when 
examined by a microscope they are found to possess irregu- 
larities or prominences. When the surfaces are in contact, these 
prominences interlock and a horizontal force must be applied 
to the block to cause the prominences of its surface to ride 
over or wear away those of the other surface. If the surfaces 
are pressed together, this horizontal force must be increased. 



MECHANICS 



55 



The resistance to be overcome in order for one surface to 
slide over the other surface is known as the frictional force. 

Fig. 50 shows a block of wood resting on a horizontal surface. 
If a horizontal force P is applied to the block, not sufficient to 
move it, a frictional force F which is equal to the force P acts 
between the block and the surface in the opposite direction. If 
the force P is gradually increased, the frictional force F also 
increases, still remaining equal to the force P, until the block 
just begins to move. The value of the frictional force in this 
case is known as the limiting frictional force. 

The Relation between the Frictional Force and the Load 

Fig 5ia shows the board AB on a table with a wooden slider 
S resting on the board. One end of a piece of string is attached 
to a hook in the slider and, after passing over the pulley P, 
the string supports a slotted weight holder. If a i Ib. weight is 
placed on the slider and weights (tenths and hundredths of a 
Ib.) are added to the string until the slider just begins to move, 
the torce required to just overcome the frictional force F may be 
determined. In the same way when loads of 2 Ib., 3 Ib., etc., 
are placed on the slider, the force required to just overcome 
the frictional force may again be determined. A set of results 
is shown below : 

Weight of slider =o*2 Ib. 



Load (W) 


Fractional 


F 


Ib. 


Force (F) Ib. 


W 


1-2 


0-25 


O'2I 


2'2 


o-45 


0-20 


3'2 


0-66 


0-21 


4-2 


0-81 


0-19 


5'2 


1*04 


0-20 



From the graph (Fig. 5ib) and also from the last column of the 
table, it will be seen that 

p 

is approximately constant. 
W 

This constant is known as the coefficient of friction and is 
generally denoted by the symbol /u, where 

= 
F ~~W 



SCIENCE FOR COALMINING STUDENTS 



P 



FIG. SO 



/w\ 



A| F' er- 


I 


_^ FRICTION AL FORCE (IBS.) 
~0 8 6 


W 


< 














^ 








^ 


X 








x 


,/ 








,/ 


J 








X 












) 


3. 3,4,5 fa 



(b) 

FIG. 51 



MECHANICS 57 

r *e. - * t T? - ^ i \ Frictional Force (F) 
i.e. Coefficient of ruction (u) = --- = ^ ' 

Load (W) 

This result is really a special case of the more general result : 

F 



where R is the perpendicular force between the two surfaces 

(Fig- 52). 

Example. A brake is applied to the rim of an endless rope haulage 
surge wheel and the perpendicular force between the brake block and 
the wheel is 150 Ib. wt. If the coefficient of friction, between the block 
and the rim is 0-35, find the frictional force tending to stop the wheel. 



F 

From fi=^ we have: 
rv 



K-^-150 Ib. wt. and fi -^0-35. 

p 

Hence 0-35---- 

*5 

F -=150x0-35 Ib. 
-=52-5 Ib. 

WORK 

Work is done when the point of application of a force moves. 
When a body of mass one pound is raised through a vertical 
distance of one foot, a force of i Ib. wt. acts vertically upwards 
against the weight of the body and its point of application 
moves through a vertical distance of i ft. In this case the work 
done is the foot-pound (ft.-lb.). Thus we may define the unit 
of work in the English system as the work done by a force of 
i Ib. wt. in moving its point of application through a distance 
of i ft. in the direction of the force. 

Suppose a force of F Ib. wt. moves its point of application 
through a distance of d feet. 

Then work done by a force of i Ib. wt. through i ft. =i ft.-lb. 
work done by a force of F Ib. wt. through i ft. F ft.-lb. 
work done by a force of F Ib. wt. through d feet =Fd ft.-lb. 

Denoting the work done by W, we have : 



Example. Find the work done in raising a load of 25 Ib. wt. through a 
vertical distance of 11-5 ft. 
Work done =25 Ib. X 11-5 ft. 
=287-5 ft.-lb. 



58 SCIENCE FOR COALMINING STUDENTS 

Example. A cage weighs 2,000 Ib. and the winding rope weighs i J Ib. 
per foot. Find the work done in raising the cage from the bottom of a 
shaft 500 ft. deep to the surface. 

Work done in raising cage = 2,000 Ib. X5oo ft. 

= 1,000,000 ft.-lb. 
Work done in raising winding rope 

Weight x average distance raised 

500 < T i Ib. x ~ ft. 

2 

187,500 ft.-lb. 

Total work done = 1,000,000 ft.-lb. + 187,500 ft.-lb. 
^-1,187,500 ft.-lb. 





FIG. 53 



Work done by a Torque 

Suppose O represents the centre of a nut which is being turned 
by a spanner (Fig. 53). If a force F be applied at the end A 
of the spanner and the spanner is turned from the position 
OA to the position OB, then the work done by the force F 

=Fxarc AB, 

assuming that the force always acts at right angles to the 
spanner. 



MECHANICS 59 



/. Work done =F x A X OA 
OA 

=Fd x angle BOA expressed in radians 
= Torque X angle oi twist in radians 

i.e. W=T<, 

where T is the torque, <f> the angle of twist and W the work done. 

Example. A mean force of 50 Ib. wt. is applied at the end of a spanner 
which is turning a screw. The distance between the force and the centre 
of the nut is 1-5 ft. Find the work done in turning the spanner through 

30. 

Torque ^T=5O x 1-5 Ib. ft. 
=75 Ib. ft. 

Angle of twist in radians ^ ~- 



0-524 radians. 
.'. Work done -W-T^ 

= 75x0-524 ft.-lb. 
-39-3 ft.-lb. 

POWER 

Power is the rate of doing work. If 20 ft.-lb. of work are per- 
formed in two seconds, the power or rate of working is 10 ft.- 
lb. per second. 

Horse Power 

The unit of power is the Horse Power which is equal to a 
rate of working of 33,000 ft.-lb. per minute or 550 ft.-lb. per 
second. 

Example. A pump raises 1,000 gallons of water from a sump through 
a vertical distance of 20 ft. in 25 minutes. At what horse power does 
the pump work? (i gallon of water weighs 10 Ib.) 

Weight of water raised =1,000 X 10 Ib. 

Work done in 25 minutes = 1,000 x 10 x 20 ft.-lb. 

XTT i j j 1,000 X 10 X 20 ,, -, 

.*. Work done per second = - - - -- ft.-lb. 

25 x 60 

" -I33'3 ft.-lb. 
/. Horse Power = 



550 
=0-24. 



EXERCISES 



i. Explain what is meant by the term ''friction." A haulage engine 
draws a load of 25 tons along a level road. Find the pull needed if the 
friction is 0-05 of the load. (Min. Sc.; U.L.C.I.) 



60 SCIENCE FOR COALMINING STUDENTS 

2. Define force, work, and power. State the units in which each is 
measured. The pull in a haulage rope is 10 cwt. and the rope speed is 
6 m.p.h. Calculate the H.P. transmitted through the rope. 

(Min. Sc.; U.L.C.I.) 

3. A uniform bar AB is TO ft. long and weighs 120 Ib. It carries a load 
of 330 Ib. at a point 8 ft. from A. The bar is hinged at A, and a rope 
inclined at 45 to AB is attached to B and keeps the bar horizontal. 
What is the pull in the rope? (Min. Sc. ; U.L.C.I.) 

4. What is the moment of a force ? 

Describe two methods of using a crowbar to overturn a heavy stone. 

(Min. Sc.; U.L.C.I.) 

5. Explain (a) the effects of a force on any body; (b) how a force 
may be measured ; (c) how a force may be represented. 

A load of 200 Ib. is supported by ropes from two points x and y 
TO ft. apart on a horizontal girder. The load hangs 8 ft. vertically below 
a point on the girder 3 ft. along xy. Determine the pull in each rope. 

(Min. Sc.; U.L.C.I.) 

6. The lever safety valve on a boiler is 2^ in. in diameter and weighs 
4 Ib. It is attached to the lever at a point 3 in. from the fulcrum. The 
lever is 33 in. long, weighs 9 Ib. and its centre of gravity is at a distance 
of 12 in. from the fulcrum. What weight must be suspended at the end 
of the lever if the valve is to open when the steam pressure is 150 Ib. 
per sq. in.? (Min. Sc.; U.L.C.I.) 

7. How much work is done in punching a hole through a wrought- 
i ron plate i in. thick if the mean force on the punch is 80 tons? 

(Min. Sc.; U.L.C.I.) 



CHAPTER VI 

THERMOMETRY AND EXPANSION 

EXPANSION 

All substances, with one or two exceptions, whether solid, 
liquid or gas, expand when heated and contract when cooled. 
This expansion and contraction is illustrated by the following 
experiments. 

In Fig. 54a AB represents an iron rod and CD an iron frame. 
Each is mounted on a wooden handle. The rod exactly fits into 
the frame and the end of the rod fits into the circular hole E. 
When the rod is heated, however, it no longer fits into the 
frame and the end of the rod can no longer be inserted in the 
hole E. The rod has expanded lengthways and also sideways. But 
when the rod cools to air temperature, it will again fit the frame. 

To show the expansion of a liquid, the apparatus shown in 
Fig. 54b is used. A flask is provided with a tight-fitting rubber 
stopper through which a glass tube passes. The flask and a 
portion of the tube contain coloured water. When the flask is 
placed in a water bath which is heated by a bunsen burner, the 
liquid falls from A to B and then finally rises to C. During the 
initial heating the inside of the flask expands more than the 
liquid, causing the liquid level to fall from A to B. But finally 
the liquid expands more than the vessel and this is shown by 
the rise of the liquid from B to C. 

To show the expansion of a gas (e.g. air) we use the apparatus 
shown in Fig. 54c. If the stopper and glass tube fit airtight 
and the hands are placed over the flask, bubbles of air appear 
in the water. This shows that the air has expanded due to the 
heat from the hands. When the flask cools the water rises in the 
tube, showing that the air contracts when cooled. 

HEAT AND TEMPERATURE 

Just as water flows from a high level to a low level, so heat 
flows from a point of high temperature to a point of low 
temperature. Thus temperature 'is analogous to water level and 
flow of heat corresponds to flow of water. Heat is therefore a 
quantity which can be measured in heat units, while tempera- 
ture is the degree of heat of a body. 



62 SCIENCE FOR COALMINING STUDENTS 

THE THERMOMETER 

The thermometer is an instrument for measuring tempera- 
ture. It consists of a graduated tube of narrow bore, with a 
bulb at one end, the other end being closed. Mercury is con- 
tained in the bulb and when the temperature increases the 
mercury expands and rises up the tube (Fig. 55). 

The Fixed Points of a Thermometer 

Before a scale can be engraved on the stem of an ungraduated 
thermometer the fixed points must be marked. These fixed 
points are (a) the Freezing Point of pure water and (b) the 
Boiling Point of pure water under normal atmospheric pressure, 
viz. 76 cm. of mercury. 

To mark the freezing point the bulb is allowed to remain 
in a mixture of ice and water (free from impurities) and the 
stem is marked opposite the mercury level (Fig. s6a). To mark 
the boiling point, the bulb is placed in the steam from boiling 
water in a hypsometer (Fig. 5&b) and the stem is again marked 
opposite the mercury level. 

Scales of Temperature 

The freezing point is marked o Centigrade (o C.) or 32 
Fahrenheit (32 F.) and the boiling point is marked 100 Centi- 
grade (100 C.) or 212 Fahrenheit (212 F.). 

Fig- 57 shows two identical thermometers, except one is 
marked on the Centigrade scale and the other on the Fahren- 
heit scale. 

If F is the Fahrenheit reading corresponding to the Centigrade 
reading C, we have: 

XB YD 



i.e. 



AB ED 
C-o F 32 



loo o 21232 

^=F=3? 

ioo 180 

Example. Convert 20 C. into the Fahrenheit scale. 

In the formula = _Zl?, substitute C=2o. 
ioo 180 

TT F 32 20 

Hence ?r^- = 

i 80 ioo 

F= x 20+32 
ioo 

=68 F. 



THERMOMETRY AND EXPANSION 



nA 



DlD 



(a) 



E. 




/IX 





FI0.55 





(b) F1G.54 ( C > 




t 



FIC.56 




(b) 



64 SCIENCE FOR COALMINING STUDENTS 

Example. Convert 20 F. into the Centigrade scale. 
In the above formula substitute F=2o. 
2032 C 



TT 

Hence - 

180 100 



TOO , 
_(,o 3 

7 C. 



EXPANSION OF SOLIDS 

It is a common statement that one solid expands more than 
another. What is implied is that a rod of one material expands 
more than a rod of another material of the same length and 
for the same temperature increase. We are thus led to the idea 
of coefficient of linear expansion, which may be defined as the 
increase in length of a solid per unit length per degree rise in 
temperature. 

Example. An iron rod 100 cm. long is heated from 15 C. to 100 C. 
Find the increase in length, if the coefficient of linear expansion of iron 
is O-OOOOTI per dcg. C. 

Increase in temperature (100 15) C 85 C. 

Expansion of i cm. for i C. =0-000011 cm. 
J expansion of 100 cm. for i C. 100x0*000011 cm. 
Expansion of 100 cm. for 85 C. 100 x O-OOOOTI x85 cm. 

0-0935 cm. 

Let L initial length of a solid, 
/temperature increase, 
# = coefficient of linear expansion, 
Ar=-the expansion. 

Then expansion of i unit of length for i degree a, 

expansion of L units of length for i degree = La, 
and expansion of L units of length for t degrees La/, 

i.e. x -La/. 
Also Final length Initial length -{-Expansion. 

Units 

Now i cm. of iron expands o-oooon cm. for i C. rise in 
temperature or: 

Coefficient of linear,, Q-OOOOII cm. 
expansion of iron i cm. x i deg. C. 

__0'OOOOII 

~ideg.C. 
=0-000011 per deg. C. 



THERMOMETRY AND EXPANSION 65 

Also i ft. of iron expands o-oooon ft. for i C. rise in tempera- 
ture, or: 

Coefficient of linear^ o-ooooii ft. 
expansion i f t . "x i deg. C . 

=0-000011 
i deg. C". 
= o-oooon per deg. C. 

From these considerations it can be seen that the value of the 
coefficient of linear expansion is the same number whatever 
unit of length is employed. 

Now let us consider the Fahrenheit scale of temperature. 
We already know that one Fahrenheit degree is equal to f of a 
Centigrade degree. 

Thus i ft. of iron for i C. rise expands o-oooon ft. 

i ft. of iron for i F. rise expands f xo-oooon ft. 

i.e. 0-0000061 ft. 

Hence the value of the coefficient expressed in the Fahrenheit 
scale is 0-0000061 per deg. F. The value of the coefficient is 
therefore dependent on the scale of temperature used. 

Example. A line of cast-iron pipes is 150 feet long at 20 C. How much 
does the pipe-line expand when bteani at 100 C. is passed through it? 
Coefficient of linear expansion of cast iron -0-000012 per deg. C. 
Expansion of T ft. for i C. rise 0-000012 ft. 

,, 150 ft. for T C. ,, -^150x0-000012 ft. 
,, 150 ft. for 80 C. ,, --150x0-000012 x8o ft. 

-=0-144 ft. 
-=1-73 in. 

Example. The measured length between two points by means of a 
surveyor's chain at 60 F. is 120-55 yd. Find the true length if the chain 
is correct at 32 F. Coefficient of linear expansion of steel =0-0000067 
per deg. F. (see p. 68). 

Expansion of chain Length xCoefft. of linear expansion x Tempera- 
ture change. 

^120-55 > 0-0000067 X28 yd. 
--0-02 yd. 

True length =^ 1 20-55 yd.-j 0-02 yd. 
^=120-57 yd. 

Determination of the Coefficient of Linear Expansion of a Metal 

Tube 

The metal tube XY which is provided with an inlet and an 
outlet for steam is supported vertically in a wooden stand 
and held in position by means of two brackets A and B (Fig. 58). 

5 



66 SCIENCE FOR COALMINING STUDENTS 



IOOC 



oc. 



B 



E 2l2F.(B.Pt.) 



C F 

FIG. 57 



D 



(a) 



STEAM 




M 



C 
X 
ES A 




FIG. 58 





D 



B 



FIG. 59 



THERMOMETRY AND EXPANSION 67 

The lower end of the tube rests on a steel point in a small 
cavity sunk in the base. A thermometer T fits through a cork 
which is inserted in a circular aperture Z in the tube. The 
expansion of the tube is measured by a micrometer screw M, 
fixed by a bracket C to the wooden stand. 

To perform an experiment the length of the tube is first 
measured and the tube is then placed in its position on the 
stand. The micrometer screw is adjusted so that its point 
touches the upper end of the tube. After noting the readings of 
the micrometer and the thermometer, the screw is raised to 
allow for the expansion of the tube. 

When steam has passed through the tube for about ten 
minutes, the micrometer screw is again adjusted and its reading 
is noted along with the reading of the thermometer T. The 
length of the tube having been measured, the coefficient of 
linear expansion of the material is calculated in the following 
manner. 

Initial length of tube 99*5 cm. 
Initial micrometer reading 3-rb^ mm - 
Final micrometer reading =-- 4-nro mm - 
Initial temperature 20 C. 

Final temperature 100 C. 

Expansion iVo mm - 

=0-091 cm. 

Hence the coefficient of linear expansion of the material 
__ 0-091 cm. 

99-5 cm. x8o degrees C. 
=0-000011 per deg. C. 

THE USES OF EXPANSION AND CONTRACTION 

In mining engineering, expansion and contraction play a 
very important part in many constructional processes. In order 
to fit the crank on a flywheel shaft, the circular hole in the 
crank is made slightly too small. The crank is heated until it 
just fits on the shaft and when it cools a tight fit is obtained. 
This method of fitting is known as a shrink fit. 

In some cases expansion fitting is employed. The internal 
member, e.g. the crankpin (Fig. sga) is made slightly larger 
than the circular hole in the crank and the pin is cooled to a 
low temperature such as the temperature of liquid air (182 C.) . 
The pin contracts, and when inserted in the hole, it expands 
and a tight fit is obtained. 



68 SCIENCE FOR COALMINING STUDENTS 

Errors due to Expansion in Measuring Instruments 

A surveyor's chain consists of links which have a definite 
length at some specified temperature. When measuring distances 
with a chain at a temperature higher than this specified tempera- 
ture, the links have expanded and the measured length is less 
than the true length. 

vSuppose AB (Fig. 59!)) is the length to be measured and CD 
the measuring chain. Suppose the length CD of the chain to 
contract to its length at the specified temperature, i.e. to QDj. 
Then the error is the distance DjE, which is the expansion of 
the scale. 

Allowance for Expansion 

In many appliances allowance has to be made for expansion 
due to increase in temperature. Thus hot-water pipes are fitted 
with telescopic joints to allow for expansion when hot water 
passes through them (Fig. 59c). The pipe A expands into the 
pipe B and buckling is prevented. 

In steam pipes used in boilers, loop joints are used to take up 
the expansion (Fig. 5gd). 

The above examples are just a few of the practical applica- 
tions of expansion and contraction of solids. Other examples 
will no doubt suggest themselves to the student. 

EXPANSION OF GASES 

We have already seen that change of pressure and change of 
temperature each produce a change in the volume of a gas. 
Hence in finding the amount of expansion due to increase in 
temperature we must eliminate any effect due to change of 
pressure, by keeping the pressure constant. 

We may thus define the coefficient of expansion of a gas at 
constant pressure as the increase in volume per unit volume at 
o C., per degree Centigrade rise in temperature, under the condition 
of constant pressure. 

This coefficient is 2-73- per deg. C. We may now enunciate 
Charles' Law, which states that a given mass of any gas expands 
by 2^3 of its volume at o C. for each degree Centigrade rise in 
temperature, provided the pressure remains constant. 

Suppose V =the volume of a given mass of gas at o C. 

V!=the volume at if C., 
and V 2 =the volume at t 2 C. 

Then V c.c, at o C, become V + c.c. at i C., 



THERMOMETRY AND EXPANSION 69 

and V c.c. at o C. become V + - c.c. at tf C., 

273 
assuming constant pressure. 

V 



Similarly v 2 = 

Dividing (i) by (2) we have: 



273 

Yi=?7i+A ...... (i) 

V 2 2 73 +/ 2 ^ 

Suppose we choose the temperature 273 C. as the level from 
which we measure our temperatures. This temperature level, 
viz. 273 C., is known as the absolute zero of temperature. 
Thus we have : 

Absolute Temperature Centigrade Temperature +273, 
or T! Absolute =^+273. 

Writing 'L\ for 273+^1 and T 2 for ^2+273, equation (3) be- 
comes : 

V 1=Y_2 

Tj T 2 

V . . 

i.e. a constant. 

Thus the volume of a given mass of a gas varies directly as its 
absolute temperature provided the pressure remains constant. 

Experimental Verification 

The above relationship can be verified experimentally by 
means of the apparatus shown in Fig. 6oa. A tube BCE consists 
of two branches, one leading to the bulb B, fitted with a tap T, 
and the other open to the atmosphere. A branch tube D passes 
through a rubber stopper in the base of a large glass vessel 
and the lower end of the tube D is connected by pressure tubing 
to a reservoir R. The volume of the bulb is accurately known 
and the left-hand branch tube leading from it is calibrated in 
cubic centimetres. Mercury is poured into the tube and the 
vessel is filled with water. 

To perform an experiment the tap T is opened and the 



7 o 



SCIENCE FOR COALMINING STUDENTS 



reservoir R is adjusted so that the levels of the mercury columns 
are just below the bulb. The tap T is then closed, the volume 
of air in the bulb and tube is read and the temperature is noted. 
Steam is passed through a delivery tube into the water until the 
temperature rises about 10 C. The mercury is adjusted to be 
at the same level in both branches of the tube so as to keep the 
pressure constant, and the volume of air and the temperature 
are again noted. This procedure is repeated until a temperature 




B 




-273 



FIG.60 



of 90 C. is reached. The results are tabulated and a graph 
connecting volume of air and temperature is drawn (Fig. 6ob) . 
The graph is the straight line AB and when this line is produced 
it cuts the axis of temperature at the point C (273 C.). 



Thus 



273+* 



=a constant, 



i.e. 



Volume 



Absolute Temperature 



=a constant at constant pressure. 



THERMOMETRY AND EXPANSION ^^ 

Example. A given quantity of gas occupies a volume of 100 cubic feet 
at 17 C. The gas is heated at constant pressure to 77 C. Find the 
volume of the gas at this temperature. 

Since the volume is proportional to the absolute temperature, we 
may write: 

V '=-Y? 

fi 'T 2 ' 

where Vj volume at TI Absolute 

and \ r 2=volume at T 2 Absolute. 

Substitute Vj-^ioo cub. ft. 

TI =^273 + 17 Absolute 
T 2 ^273 +77 Absolute. 
100 _ V 2 

2 73-M7~ 2 73+77 



273 + 17 

^50 

i>_ x 100 
290 

~ 120-7 cub. ft. 

Relationship between the Volume, Pressure and Absolute 
Temperature of a given Mass of a Gas 

Let Vi=the volume of the given mass of gas at Tj Absolute 

and pressure P t 
and V 2 =the volume of the given mass of gas at T 2 Absolute 

and pressure P 2 . 

The changes of pressure and temperature both affect the 
volume. 

Let the change of pressure have its full effect at constant 
temperature. 

Then P 1 V 1 =P 2 *>, where v==some intermediate volume. 

Now let the temperature have its full effect at constant 
pressure. 

Then 



v __V 2 



_ 

li AI 
Eliminating v from the two equations, we have 



T! T 2 

Initial Pressure x Initial Volume 

__ 

Initial Absolute Temperature 

__Final Pressure x Final Volume 
Final Absolute Temperature 



72 SCIENCE FOR COALMINING STUDENTS 

A bsolute Zero of Temperature on Fahrenheit Scale 
Coefficient of expansion of a gas per deg. C. 

=-- - X per deg. F. 
9 



, ^ 

- -- per deer. F. 
491-4 v 

Hence the absolute zero of temperature is 32 F. 491-4 F., 
i.e. -459-4 F. 

Thus Fahrenheit Absolute Temperature 

~459'4+Fahrenheit Temperature. 

Example. A receiver contains 1,200 cub. ft. of compressed air at a 
temperature of 85 F. and 75 Ib. per sq. in. absolute pressure. What 
volume would this quantity of air occupy at 60 F. and 15 Ib. per sq. in. 
absolute pressure? 



we have Vj =1,200 cub. ft. 

P 1= =75 Ib. per sq. in. 
TI 459-4 +85 Absolute 
P 2 = i5 Ib. per sq. in. 
T 2 =459-44-60 Absolute. 

75 x x.200^ 15 V 2 
459-4+85 459-4+60 

V 2 = 1,200 x^ X ^^ cub. ft. 

15 544'4 
=5,724 cub. ft. 

CHANGE OF DENSITY OF A GAS DUE TO CHANGES OF PRESSURE 
AND TEMPERATURE 

We have already established the relationship 
P 1 V 1 __P 2 V 2 
T! " T 2 ' 
for a given mass of gas. 

Let di =density of a gas at pressure P] and absolute tempera- 

ture T! 
and d 2 =clensity o f the gas at pressure P 2 and absolute tempera- 

ture T 2 

Then for mass m of the gas we have : 
m=Vidi and w= 



THERMOMETRY AND EXPANSION 73 

Hence the above equation becomes : 



Example. The density of air at a pressure of 14-7 Ib. per sq. in. and 
o C. is 0-0807 Ib. per cub. ft. Find the density of air at the bottom 
of a mine shaft where the pressure is 15-2 Ib. per sq. in. and the tempera- 
ture 20 C. 



we have Fj 14-7 Ib. per sq. in. 

TI =273 Abs. 
d\ 0-0807 Ib. per cub. ft. 
T 2 -273 +20 Absolute. 
P 2 15-2 Ib. per sq. in. 
347 __ *5- 2 
0-0807 - v 2 73 ^2 y 2 93 



: X 0-0807 x 273 ., , t . 

' __.. 'r 3 lb. per cub. ft. 

293 X 14*7 



--0-0777 1- P er 



EXERCISES 

1. Explain the meaning of temperature, and describe how ordinary 
temperatures are usually measured. (Min. Sc.; U. L.C.I.) 

2. Explain how the coefficient of expansion of a gas at constant 
pressure may be found. Given that the coefficient of expansion of air 
at constant pressure is 1/273 per degree Centigrade, show that the volume 
of a given mass of air is proportional to the Absolute temperature. 

(Min. Sc., U.L.C.I.) 

3. The temperature of a range of steam pipes, 45 yd. long, varies 
from 30 F. to 350 F. What will be the increase in length if the co- 
efficient of expansion is 0-0000065 per F. ? Explain how this expansion 
may be provided for to avoid breakage of the pipes. 

(Min. Sc.; U.L.C.I.) 

4. How does the density of a gas vary with the temperature and 
with the pressure? If the density of air at 32 F. and at a barometric 
pressure of 30 in. is 0-08 Ib. per cub. ft., what will be the density at a 
temperature of 83 F. and a barometric pressure of 31-5 in.? 

(Min. Sc.; U.L.C.T.) 

5. At a temperature of 40 F. and a pressure of 15 Ib. per sq. in. a 
mass of air has a volume of 10-63 cu b. ft. If the air is compressed to 
60 Ib. per sq. in. and is then at a temperature of 100 F., what is its 
volume? (Min. Sc.; U.L.C.I.) 

6. An air receiver has a capacity of 40 cub. ft. The gauge pressure of 
the air in the receiver is 15 Ib. per sq. in., and the temperature 60 F. 
If air is admitted to the receiver until the gauge pressure rises to 80 Ib. 
per sq. in. and the temperature to 100 F., calculate the quantity of 
air at N.T.P. that has been added. (Min. Sc.; U.L.C.I.) 



74 SCIENCE FOR COALMINING STUDENTS 

7. An air compressor draws in 5,000 cub. ft. of air per min. at a 
temperature of 60 F. and a pressure of 15 Ib. per sq. in. absolute. If 
the discharge pressure is 100 Ib. per sq. in. (gauge), and the tempera- 
ture is 140 F., calculate the volume of compressed air discharged. If 
before entering the pipe-line the temperature has fallen to 75 F., 
calculate the volume passing. (Min. Sc.; U.L.C.I.) 

8. How may expansion be provided for in a range of steam pipes? 
If the range is 40 ft. long and the temperature vanes between 40 F. 
and 400 F., what expansion will occur? Coefficient of expansion of 
iron is 0-0000066 per degree F. (Min. Sc.; U.L.C.I.) 

9. The density of air at a temperature of 32 F. and under a pressure 
of 14-7 Ib. per sq. in. is 0-079 Ib. per cub. ft. What is the density of the 
air at the bottom of a mine shaft where the pressure is 15*4 Ib. per 
sq. in. and the temperature 65 F. ? (Min. Sc.; U.L.C.T.) 

10. A line of cast-iron pipes carries steam to the bottom of a shaft 
500 yd. deep. The temperature of the steam is 327 F. When the steam 
is cut oft the temperature of the pipes falls to 72 F. Calculate the con- 
traction of the line of pipes, if the coefficient of linear expansion of cast 
iron is 0-0000062 per degree F. (Min. Sc, ; U.L.C.i.) 

u. A pipe range of mild steel is 63-5 ft. long at a temperature of 
55 F. What will be the expansion when feed water from an economiser 
is pumped through it, at a temperature of 260 F. ? The coefficient of 
linear expansion of mild steel is 0-0000066 per degree F. 

(Min. Sc.; U.L.C.I.) 



CHAPTER VII 

QUANTITY OF HEAT 

HEAT UNITS 

When considering sources of heat such as the furnace, the 
coal fire, the stove, etc., it is often said that one source gives 
out more heat than another. The fact that we refer to heat in 
such terms as much heat or little heat leads us to the idea that 
heat is a quantity. Now all quantities can be measured in some 
unit and the simplest unit in the case of heat is the calorie, 
which may be defined as the quantity of heat which must be given 
to one gram of water to raise its temperature through one degree 
Centigrade. 

The calorie is too small a unit for many practical purposes 
and larger units, viz. the Pound Calorie and the British Thermal 
Unit are used. The Pound Calorie is the quantity of heat which 
must be imparted to one pound of water to raise its temperature by 
1 C., while the British Thermal Unit (B.Th.U.) is the quantity 
of heat which must be given to one pound of water to raise its 
temperature by i F. 

The Pound Calorie is often referred to as the Pound Degree 
Centigrade Unit or the Centigrade Heat Unit (C.H.U.). 

A still larger heat unit is the Therm, which is equivalent to 
100,000 B.Th.U. 

Example. How much heat must be supplied to 20 grams of water to 
raise its temperature from 15 C. to 39 C.? 

Heat gained by i gm. of water for i C. rise = i cal. 
Heat ,, ,, 20 gm. ,, ,, ,, i C. ,, =20 cal. 
Heat ,, ,, 20 gm. ,, ,, ,, 24 C. 20x24 cal. 

480 cal. 

Example. How much heat must be supplied to 20 gallons of water to 
raise the temperature of the water from 60 F. to 90 F.? (i gallon of 
water weighs 10 Ib.) 

Weight of water = 20 x 10 Ib. 

200 Ib. 

Heat gained by i Ib. of water for 1 F. rise = i B.Th.U. 
Heat ,, ,, 2oolb. ,, for i F. rise = 200 B.Th.U. 
Heat 200 Ib. for 30 F. rise =200x30 B.Th.U. 

=6,000 B.Th.U. 
75 



76 SCIENCE FOR COALMINING STUDENTS 

The examples given above show that : 

Quantity of heat gained _ Weight of Change in tern- 
or lost ~~ water x perature 

i.e. VLmt, 

where m is the weight, t the temperature change, and H the 
heat gained or lost. 

SPECIFIC HEAT 

Two identical test tubes are chosen. Into one, 50 grams of 
water are poured, and into the other, 50 grams of paraffin. A 
Centigrade thermometer is placed in each tube and both tubes 
are immersed in a water bath, heated by a bunsen burner, at 
equal distances from the burner. 

When the bath is heated, it will be seen that the temperature 
of the paraffin rises more rapidly than that of the water. Each 
liquid receives heat at the same rate and the rise in temperature 
of the paraffin in a given time is greater than that of the water. 
Hence the quantity of heat to raise the temperature of the 
paraffin through i C. is less than the quantity of heat to raise 
the temperature of the water i C. 

So it is with other substances. The quantities of heat required 
to raise the temperatures of equal weights of various substances 
through i C. are different. Taking water as the standard of 
reference, we may define the specific heat of a substance as the 
quantity of heat which must be imparted to unit weight of 
the substance to raise its temperature by one degree com- 
pared with the quantity of heat which must be imparted to 
unit weight of water to raise its temperature by one degree. 
Thus: 

Specific heat of a substance 

__ Quantity of heat to raise i gm. of substance i C. 
Quantity of heat to raise i gm. of water i C. 

__ Quantity of heat to raise i gm. of substance i C. 
i calorie 

Also the specific heat of a substance 

__ Quantity of heat to raise i Ib. of substance i F. 
_.< i B.Th.U. 

In each case the units of both numerator and denominator 
are the same and the value of the specific heat is simply a ratio 



QUANTITY OF HEAT 77 

which is the same whatever the unit of mass or the scale of 
temperature may be. 

c <; ^ 4. , o-ii B.Th.U. 
i.e. Specific heat of iron= _ ___- 

i J3. 1 h.U. 

=0*11 

.~ , , e . o-ii calorie 
or Specific heat of iron - --. - 

i calorie 

o-ii 

The value of a specific heat is sometimes expressed as calories 
per gram per degree Centigrade or as British Thermal Units per 
pound per degree Fahrenheit. 

c -a -U j. ' I][ calorie 

i.e. Specific heat of iron - 

i gram X i deg. C. 

Now a calorie is a gram degree Centigrade unit. 

rT .^ t , , . o-ii gm. X deg. C. 

Hence Specific heat of iron ^ 

i gm. x i deg. C. 

^o-n (a ratio as before). 

c .. , , , . o-n B.Th.U. 

or Specific heat of iron . - 

i Ib. X i deg. t . 

__0'ii Ib. xdeg. F. 

i Ib. deg. F. 
o-ii (a ratio). 

Example. How much heat must be given to 100 grams of iron in 
order to raise its temperature through 20 C. ? Sp. ht. of iron o-r i cal. 
per gm. per deg. C. 

Quantity of heat gained by J gm. of water for i C. = i cal. 
Quantity of heat gained by i gm. of iron for i C. o-i i cal. 
Quantity of heat gained by i oo gm. of iron f or i C. T oo x o- 1 j cal. 
Quantity of heat gained by 100 gm. of iron for 20 C. -^100 x o-i i 

X20 cal. 
-220 cal. 



HEAT GAINED BY m GRAMS OF A SUBSTANCE OF SPECIFIC HEAT 
5 FOR A RISE IN TEMPERATURE OF t C. 

Heat gained by i gm. of substance for i C. rise s calories 
Heat gained by m gm. of substance for i C. rise=ws calories 
Heat gained by m gm. of substance for t C. rise =mst calories 

i.e. Heat gained = Weight x Specific heat x Temperature rise. 



78 SCIENCE FOR COALMINING STUDENTS 

HEAT LOST BY m LB. OF A SUBSTANCE OF SPECIFIC HEAT s FOR 
A TEMPERATURE FALL OF f F. 

Heat lost by i Ib, of substance for i F. fall =s B.Th.U. 
Heat lost by m Ib. of substance for i F. fall ws B.Th.U. 
Heat lost by m Ib. of substance for 1 F. fall =mst B.Th.U. 

From the two cases considered above we have in appropriate 
units : 

Heat gained or ^Weight of sub- Specific Temperature 
lost stance heat change 

or in symbols H=mst 

Thermal Capacity 

The thermal capacity of a substance is the quantity of heat 
which must be given to the substance to raise its temperature by 
one degree. 

From H=mst, we have: 

H 

Thermal capacity =ms 
t 

I cal \ 

Thermal capacity =w (gm.) xs( - J~- - ) 

J Vb \gm.xdeg. C./ 

/ cal. \ 

=ms 

\deg. C./ 

i.e. Thermal capacity = weight xsp. ht. (cal. per deg. C.) 

The thermal capacity of 20 grams of copper, of specific heat 
0-094 cal. per gm. per deg. C., is 20x^094 cal. per deg. C., i.e. 
1-88 cal. per deg. C. Also the thermal capacity of 40 Ib. of iron, 
of specific heat O'li B.Th.U. per Ib. per deg. F., is 40x0-11 
B.Th.U. per deg. F., i.e. 4-4 B.Th.U. per deg. F. 

Water Equivalent 

The water equivalent of a substance is the weight of water which 
would gain the same quantity of heat as the substance for the same 
temperature rise. 

Let m = weight of substance of specific heat s, 

tf=the temperature increase, 
z0=the water equivalent. 
Then: 

Heat gained by water = Heat gained by substance. 



i.e. 



QUANTITY OF HEAT 79 

i.e, wt 



=m (gm.)xs(- C -Y^ - | 
Vb ' \gm.xdeg.C./ 

=WW (d 



Ideg. 
=ms (gm.) 

Example. A steel tank weighing 100 Ib. contains 500 Ib. of water at 
15 C. How much heat must be supplied to the tank and contents to 
raise the temperature to 40 C.? Specific heat of steel 0-12 C.H.U. per 
Ib. per deg. C. 

Water equivalent of tank ioo x 0-12 Ib. 

Heat gained by tank- 100 x 0-12 x (40 15) C.H.U. 

-300 C.H.U. 
Heat gained by Waterloo x (40- 15) C.H.U. 

=-12,500 C.H.U. 

Total heat gained = 12,500 C.H.U. +300 C.H.U. 
-12,800 C.H.U. 



METHOD OF MIXTURES 

When a quantity of hot water at a temperature / t is mixed 
with a quantity of cold water at a temperature t 2 , the mixture 
attains some intermediate temperature t$. The temperature of 
the hot water falls from ^ to 3 and that of the cold water 
rises from t 2 to / 3 . 

If the heat losses to the surroundings and to the vessel are 
negligible we have : 

Heat lost by hot water = Heat gained by cold water. 

If a hot solid is transferred to a quantity of cold liquid con- 
tained in a vessel and if we take into account the heat gained 
by the vessel, we have: 

Heat lost by = Heat gained by .Heat gained by 
hot solid ~"~ cold liquid "*" cold vessel 

again assuming that the heat losses to the surroundings during 
the mixing are negligible. 
. A few examples will illustrate the use of the above formulae. 

Example. A tank contains 100 Ib. of water at 60 F. What weight of 
water at a temperature of 180 F. must be run into the tank in order to 



80 SCIENCE FOR COALMINING STUDENTS 

raise the temperature of the water to 100 F., assuming no heat losses 
to the tank and surroundings. 

Let m weight of hot water at 180 F. 

Heat lost by hot water -^m (180 100) B.Tli.U. 
=8ow B.Th.U. 

Heat gamed by cold water 100 X (100 --f>o) B.Th.U. 

4,000 B.Th.U. 

Hence Sow B.Th.U. 4,000 B.Th.U. 

4,000 1t 

m~~ Ib. 

80 

=50 Ib. 

Example. An iron ball weighing 0-5 Jb. is transferred from a furnace 
into an iron vessel weighing 1-2 Ib. and containing 2-5 Ib. of water at 
15 C. The temperature of the vessel and contents rises to 35 C. If 
the specific heat of iron is o-i C.H.U. per Ib. per deg. C. and assuming 
that no heat, is lost to the surroundings, calculate the temperature of 
the furnace. 

Let /the temperature of the furnace. 

Heat lost by iron bal 1-- weight xsp. ht. xtemp. fall. 

=o-5xo-ix(/- 35) C.H.U. 
Heat gamed by water 2-5 x (35- 15) C.H.U. 
Heat gained by iron vessel = 1-2 xo-r x (35 15) C.H.U. 
Hence: 

0-5 KO-I x(/-35)=2-5X(35-J5) + T-2Xo-i x(35-i5)- 
i.e. 0-05 x(/- 35) -=50 + ^-4 

/ 35 - 5 ^ 
0-05 

^-1,048 
/ -1,048 +35 
-1,083 C. 



Determination of the Specific Heat of a Solid by Method of 

Mixtures 

Fig. 61 shows a double calorimeter, which consists of a small 
copper vessel surrounded by a large vessel, the space between 
being packed with cotton wool. This arrangement reduces the 
heat losses from the inner calorimeter to the surroundings to a 
minimum. 

To perform an experiment the inner calorimeter is weighed 
empty and then containing a quantity of water. A piece of 
aluminium which has been previously weighed is heated tb 
100 C., by suspending it for a few minutes from a piece of 
cotton thread in a beaker containing boiling water. The tempera- 



QUANTITY OF HEAT 8l 

ture of the boiling water and the initial temperature of the 
water in the calorimeter are noted. The piece of aluminium is 
then transferred to the calorimeter and, after stirring, the 
final temperature of the mixture is noted. A set of results is 
shown below : 

Weight of calorimeter ^=40*1 gm. 

Weight of calorimeter + water 141-8 gm. 

Weight of aluminium =32*5 gm. 
Initial temperature of water in calorimeter =^15 C. 

Initial temperature of aluminium = 100 C. 

Final temperature of mixture =20 C. 

Specific heat of copper ~o-i 

Let ,\'= Specific heat of aluminium. 

Heat lost by solid, i.e. aluminium 

= 32'5 X.vx (10020) cal. 

Heat gained by water 

=ioi'7x(2O 15) cal. 

Heat gained by calorimeter 

^40-1 xo-i x(2o 15) cal. 

Hence 32-5 X.rX (10020) 

=-101-7 x (20 15) -1-40-1 xo-i x (20-15), 
i.e. 2,600 x =508-5 +20-05 

-528-55 



2,600 
=0-20 cal. per gm. per deg. C. 

N.B. A small amount of hot water is transferred from the beaker 
to the weighed calorimeter, but this does not materially affect 
the result. 

CALORIFIC VALUE 

It is a well-known fact that fuels such as coal, paraffin, town 
gas, etc., give out heat when burned. Also one fuel may give 
out more heat than an equal weight of another fuel. This leads 
to the idea of the calorific value of a fuel, which may be defined 
as the quantity of heat given out by the complete combustion of unit 
weight of the fuel. Thus the calorific value of a given sample of 
coal may be stated as 13,500 B.Th.U. per Ib. 

6 



82 vSCIENCE FOR COALMINING STUDENTS 

The following table gives the calorific values of various types 
of bituminous coals mined in the Wigan area. 



Type 


Calorific Value 
(B.Th.Uperlb.) 


Where found 


Non-coking 
Slightly coking . 
Medium coking 
Strong coking . 


13,85014,400 
14,400-14,670 
14,670-15,030 
15,030-15,660 


Upper seams 
Scams such as Florida 
Wigan 6 ft. and mines below 
Arley 



In the case of town gas, the calorific value may be defined 
as the quantity of heat given out by the complete combustion 
of i cubic foot of the gas at o C. and 14-7 Ib. per sq. in. absolute 
pressure. Gas is generally manufactured so that its calorific 
value is 480 B.Th.U. per cubic foot and this is the usual stand- 
ard of quality. 

THE BOMB CALORIMETER 

An apparatus used for the determination of the calorific value 
of a fuel, such as coal, is the bomb calorimeter (Fig. 62). The 
steel vessel A, known as the bomb, is provided with a steel 
cap B which can be screwed on to it. The cap is provided with 
a steel tube through which oxygen can be admitted from the 
branch tube D by turning the screw C. The supply of oxygen 
ensures complete combustion of the fuel. A crucible G, con- 
taining a weighed quantity of the fuel, is suspended by wires 
which are attached to the terminals E and F. These wires 
serve as leads to the ignition wire I which passes through the 
fuel. : 

The bomb rests on supports in a calorimeter H which con- 
tains a quantity of water and the water is stirred by the stirrer 
J. The calorimeter is enclosed in an outer vessel, the space 
between them being packed with cotton wool to minimise the 
heat losses to the surroundings. 

To perform an experiment, a weighed quantity of the fuel is 
placed in the crucible and the steel cap is then screwed on the 
bomb. A quantity of oxygen is admitted and the circuit con- 
taining the ignition wire is completed (see Ch. XI). The electric 
current raises the wire to incandescence and the fuel is ignited. 

The initial temperature of the calorimeter and contents having 
been noted, the water is stirred until the heat produced by 
the combustion of the fuel raises the temperature to a final 
steady value. Since the water equivalent of the calorimeter 



QUANTITY OF HEAT 




FIG. 61 




FIG.62 



84 SCIENCE FOR COALMINING STUDENTS 

and contents is known, the calorific value of the fuel may be 
determined as follows : 

Weight of fuel =1-2 gm. 

Weight of water in calorimeter 2,340 gm. 
Water equivalent of calorimeter =320 gm. 
Initial temperature of water ^17*5 C. 
Final temperature of water -^20-7 C. 
Heat gained by water =2,340 x 3-2 cal. 

Heat gained by calorimeter =320 x 3-2 cal. 
Total heat gained -^2,660x3-2 cal. 

Heat produced by the burn- g^ j 

mg of 1-2 gm. of fuel ' 

2 660 X ^"2 

Calorific value of the fuel cal. per gm. 

1-2 K & 

^7,093 cal. per gm. 



EXERCISES 

1. Explain clearly, the meaning of (a) the unit of heat, (b) Specific 
Heat, (c) Latent Heat. How does the specific heat of a liquid compare 
with that of a solid? Give one example of how the high specific heat 
of a substance has a practical value. (Min. Sc.; U. L.C.I.) 

2. Define the following terms: Calorie; British Thermal Unit; 
Specific Heat. Describe fully, how the specific heat of a solid substance 
may be determined. (Min. Sc. ; U. L.C.I.) 

3. A metal tank weighs 50 Ib. and contains 9 gallons of oil with a 
specific gravity of 0-78. The temperature of the oil rises from 50 F. to 
80 F. in 3 min. What quantity of heat has been added per .rnin. ? The 
specific heat of the metal is 0-12, and that of the oil 0-52. 

(Min. Sc.; U.L.C.I.) 

4. Define (a) the Centigrade heat unit, and (b) specific heat. Steam 
from an engine is condensed by water flowing through a condenser. If 
500,000 B.Th.U. are taken from the steam per hour and the inlet and 
outlet temperatures of the circulating water are 60 F. and 105 F. 
respectively, find the weight of water flowing through the condenser per 
minute. (Min. Sc.; U.L.C.I.) 

5. In an experiment to determine the specific heat of a metal, 155 gm. 
of the substance was heated to 100 C. and then placed in 100 gm. of 
water in a calorimeter whose water equivalent was 8-5 gm. The initial 
temperature of the water was 18 C., and the final temperature was 
21-5 C. Calculate the specific heat of the metal. (Min. Sc. ; U.L.C.I.) 

6. A piece of copper of weight 2-5 Ib. is allowed to remain in a current 
of hot gases in a flue for a considerable time. It is then transferred to a 
copper vessel weighing 3 Ib. and containing 20 Ib. of water at 15 C. 
and the temperature rises to 20 C. Assuming the specific heat of copper 



QUANTITY OF HEAT 85 

to be 0-093 C.H.U. per Ib. per deg. C., determine the temperature of 
the flue gases. 

7. The heat produced by the burning of 3-5 gm. of coal is passed into 
a vessel of water equivalent 200 gm. and containing 5,200 gm. of water 
at 17 C. The temperature of the water rises to 21-5 C. Find how much 
heat is generated by the burning of i gm. of coal. 



CHAPTER VIII 



CHANGE OF STATE 

LATENT HEAT 

If a beaker containing a mixture of ice and water is placed 
on a tripod, fitted with a sheet of gauze, and the beaker and 
contents are heated by means of a bunscn burner, it will be 
found that the temperature remains at o (\ so long as any ice 
is left unmelted and provided that the mixture is well stirred. 
If the heating is continued when all the ice is melted, the tem- 
perature of the water increases until boiling point is reached 
and the temperature again remains constant. If, during the 
experiment, the temperature is noted every minute, a set of 
results like the following is obtained. 



Temp. ( C.) 








o 


o 


M 


26 


39 


5i 


62 


75 


, 


100 

1 I 


IOO 


IOO 
13 


IOO 


IOO 


Time (Minutes) 





i 


2 


3 


4 


5 


6 


7 


8 


9 


10 


12 


*4 


15 



In the graph shown in Fig. 63, AB represents the change of 
state from ice to water, BC the rise in temperature of the water 
with time, and CD the change of state from water to steam. 
The point B represents the instant when all the ice has melted 
and the point C represents the instant when boiling commences. 
The heat supplied during the operation AB is termed latent 
heat of fusion while the heat supplied during the operation CD 
is known as latent heat of vaporisation. During a change of 
state it will be seen that the temperature remains constant. 
Thus latent heat may be defined as the heat which must be im- 
parted to or withdrawn from a substance to change its state at 
constant temperature. The line BC represents the rise in tempera- 
ture of the water with time and the heat supplied in this opera- 
tion is called sensible heat, which may be defined as the heat 
which must be given to or withdrawn from a substance to change 
its temperature without change of state. 

Melting Point 

The melting point of a substance is the temperature at which 
the substance changes from the solid to the liquid state. 

86 



CHANGE OF STATE 87 

Freezing Point 

The freezing point of a substance is the temperature at which 
the substance changes from the liquid to the solid state. 

Melting point and freezing point are the same temperature 
for any particular substance, the difference being that at 
melting point heat is supplied to the substance whereas at 
freezing point heat is withdrawn from the substance. 

Thus: Solid at Melting Latent Heat of >Lj id 
Point fusion ^ 

_, Liquid at Freezing Latent Heat of Q r , 
anci T ^ . . ..-^ . ^oOiio, 

Point t usion 

Boiling Point 

At this stage we may consider the boiling point of a liquid as 
the temperature at which a liquid boils, that is the temperature 
at which bubbles of vapour appear at all points of the liquid. 

Condensation Point 

The condensation point of a vapour is the temperature at 
which the vapour changes into its liquid state. 

Thus: Liquid at Boiling Latent Heat of^ Qas QJ . f 

Point Vaporisation r 

o i Gas or vapour at Con- Latent Heat of T - - -, 
and , , f ^ . , Tr ... -^Liquid, 

densation Point Vaporisation n 

LATENT HEATS OF FUSION AND VAPORISATION 

The Latent Heat of Fusion of a substance is the quantity of 
heat which must be imparted to unit weight of the substance to 
completely change it from the solid to the liquid state at con- 
stant temperature. 

The latent heat of fusion of ice is 80 calories per gram, 80 
Centigrade Heat Units per Ib. or f X 80, i.e. 144 British Thermal 
Units per Ib. 

The Latent Heat of Vaporisation of a liquid is the quantity of 
heat which must be given to unit weight of the liquid to com- 
pletely change it into the gaseous state at constant temperature, 
i.e. boiling point. 

The latent heat of vaporisation of water or the latent heat 
of steam is 540 calories per gram, 540 Centigrade Heat Units 
per Ib. or f X 540, i.e. 972 British Thermal Units per Ib. 



88 SCIENCE FOR COALMINING STUDENTS 

Example. Feed water at 60 F. is supplied to a boiler. How much 
heat is required to convert 2,000 Ib. of this water into steam at 212 F. p 

Sensible heat gained by water 2,000 X (212 --60) B.Th.U. 

304,000 B.Th.U. 
Latent heat gained 2,000 x 97 2 B.Th.U. 

1,944,000 B.Th.U. 

Full quantity of heat gained 304, OOO-J-T, 9 \ j,ooo B.Th.U. 

--2,248,000 B.Th.U. 

THE MIXING OF STEAM AND WATER 

When steam is passed into cold water contained in a vessel, 
a final common temperature is attained. Thus if no heat is lost 
to the surroundings during the mixing we have : 

Heat lost by Heat lost Heat gained Heat 

steam in + by water by cold + gained by 
condensing formed water vessel 

Example. A copper tank weighs 20 Ib. and contains 150 Ib. of water at 
15 C. How much steam generated at joo C. must be passed into the 
water in order to raise the temperature of the vessel and water to 25 C. ? 
Latent heat of steam =540 C.H.IT, per Ib.; specific heat of copper ^0-093 
C.H.U. per Ib. per deg. C. 

Let iv = weight of steam in Ibs. 

Heat lost by steam in condensing =540^ C.H.U. 

Heat lost by water formed = u> (100 25) C.H.U. 
Heat gained by cold water 150 X (25 15) C.H.U. 

Heat gained by vessel 20x0-093 x (25 15) C.H.U. 
-f 75?' 150 X 104-20 x 0-093 X 10 
61 *w = 1,500 -f- 1 8-6 

1,518-6 * 

w = ~? 

615 
=2-47 Ib. of steam. 



Determination of the Latent Heat of Steam by Method of 
Mixtures 

The experiment is carried out in the following manner. A 
small copper calorimeter is weighed empty and then about 
two-thirds filled with water. The calorimeter is placed in an 
outer vessel and the space between the two vessels is packed 
with cotton wool, which reduces the heat losses to the sur- 
roundings during the passing of the steam to a minimum. 

Fig. 64 shows the steam-generating plant with the steam- 
drying tube B, connected to the boiler A. The steam is allowed 



CHANGE OF STATE 



8 9 

to pass for a few minutes before it is used in order to allow the 
temperature of the apparatus to reach 100 C. There is then no 
tendency for the steam to condense in the delivery tubes. The 



JlOO 
d 

UJ 

?80 

uu 

Q 

D 

^60 

E 
40 

i 

20 
A 






D 

\ 




















> 


/ 
























j 


/ 


f 
























/ 


/ 
























; 


/ 




















f 


, f 


1 + 


/ 


( 























TIME -MINUTES 
FIG. 63 




FIG.64 

temperature of the water in the calorimeter is noted and the 
end of the delivery tube is immersed in the water until a 
temperature of about 50 C. is reached. This final temperature 



fjO SCIENCE FOR COALMINING STUDENTS 

and the weight of the calorimeter and contents are noted. A 
set of results is shown below. 

Weight of calorimeter 43*56 gm. 

Weight of calorimeter | water =148-43 gm. 

Weight of calorimeter -+ water +-steam 154-71 gm. 
Initial temperature of water = 15 C. 

Final temperature of mixture 49 *- 

Temperature of steam -=TOO C. 

Specific heat of copper ^--o-i cal. per gm. per 

deg. C, 

Let L =- - latent heat of steam . 

Heat lost by steam in condensing =6-28L cal. 
Heat lost by water formed =6-28 x (10049) cal. 

Heat gained by water in calorimeter ^104-87 x (4915) cal. 
Heat gained by calorimeter 43.56x0-1 X 

(49-15) cal. 

/. 6-28L+6-28x(ioo 49) = 104-87 X (49 15) +43-56x0-1 X 

(49-15) 



L=54O cal. per gm. 

EVAPORATION 

If a dish containing water is left for some time, the water 
ultimately disappears and changes into water vapour. The 
molecules of the water are in continual motion and some of 
them break through the surface and escape into the air above, 
becoming water vapour. This process continues until all the 
water disappears. 

Thus there are two processes by which a liquid changes into 
a vapour, (i) by actual boiling, or ebullition as it is sometimes 
called, and (2) by evaporation. 

Boiling takes place at one temperature only, for any particu- 
lar atmospheric pressure, whereas evaporation takes place at 
all temperatures. In both cases latent heat of vaporisation must 
be imparted to the liquid to effect the change of state. In the 
case of boiling, the heat is derived from external sources, 
whereas in evaporation the heat is taken from the liquid itself, 
thus causing the liquid to cool. 

The rate of evaporation depends on the area of the surface of 
the liquid. The greater the surface area, the greater is the rate 



CHANGE OF STATE QI 

of evaporation. The quantity of vapour already present in the 
space above the liquid has also an effect on the rate of evapora- 
tion. Thus the drier the air above a water surface the greater 
is the rate of evaporation. But, if the air is saturated no 
evaporation takes place. It is therefore obvious that air currents 
increase the rate of evaporation, for if the air is motionless 
saturation conditions will soon be attained. Air currents re- 
move the saturated air and supply fresh dry air to the surface. 
This allows evaporation to continue. 

REFRIGERATION 

Refrigeration is the term applied to the artificial freezing of 
a liquid, generally the freezing of water. The following experi- 
ment illustrates refrigeration. 

A few drops of water are poured on a wooden block A and a 
copper vessel B containing ether is placed on the water (Fig. 65). 
If air is blown through the ether by means of a piece of tubing 
C, connected to the bellows, the vessel will be frozen on to the 
block of wood. 

Ether is a very volatile liquid, that is, a liquid which readily 
changes into a vapour. The process of blowing air through the 
ether brings more air into contact with the liquid and increases 
the rate of evaporation. Heat is required for vaporisation and 
some of this heat is taken from the water underneath the vessel. 
The water cools to o C. and then freezes. 

VAPOUR PRESSURE 

When a mercurial barometer is set up as in Fig. 66a, the 
space above the mercury column is a vacuum. If a drop of 
ether is inserted through the bottom of the tube by means of 
a bent pipette, the ether rises to the space above, vaporises and 
the column of mercury is depressed. If all the ether vaporises, 
the space above the mercury column contains unsaturated 
ether vapour (Fig. 66b). If the addition of the ether is continued, 
a stage is reached when a thin film of liquid ether remains on 
the top of the mercury column and at this stage the ether 
vapour is saturated (Fig. 66c). 

The saturation vapour pressure of the ether is given by 

P=B-H, 

where B=the barometric height in cm. 

H=the height of the mercury column in cm. 
and P=the saturation vapour pressure of the ether in cm. 
of mercury at the temperature of the experiment. 



92 SCIENCE FOR COALMINING STUDENTS 

Saturated and Unsaturated Vapours 

The above experiment illustrates the difference between a 
saturated and an unsaturated vapour. In the case of a saturated 



B 



WATER 




FIG.65 



AIR 

FROM 

BELLOWS 

-ETHER 



t 



i nilA 

(a) (W (c) 

FIC.66 



DROP 

OF 
WATER 




Fl ' 67 /L\ STEAM 

(b) 



vapour there is excess of liquid in contact with the vapour, 
whereas in the case of an unsaturated vapour there is no excess 
of liquid. Again, in the unsaturated vapour state (Fig. 66b), as 



CHANGE OF STATE 93 

more and more liquid is added, the mercury column is depressed 
more and more ; but when the vapour becomes saturated further 
addition of liquid has little effect on the height of the mercury 
column. This shows that a vapour in the saturated state exerts 
its maximum vapour pressure. 

Boiling Point of a Liquid 

We are now in a position to examine more closely what is 
meant by the boiling point of a liquid. Fig. 6ya shows a U-tube 
ABC, the shorter branch of which is closed at A and the longer 
branch open to the atmosphere. The tube contains a quantity 
of mercury and a drop of water. By inverting the tube a few 
times, the air in the branch AB can be withdrawn and the 
mercury rises to fill this branch, leaving the drop of water 
trapped at the end A. If the U-tube is now placed in a steam 
jacket D and steam is passed round the tube for a few minutes, 
the mercury in both branches attains the same level. A small 
drop of water is left in contact with the saturated water vapour 
in AB (Fig. 6;b). 



Thus: Satoati V P r ^ ure =,Atmaspheric Pressure. 
of water at 100 C. 

But the water from which the steam is produced is boiling at 
100 C. 

Hence: Sat " ration V*P ^"^Atmospheric Pressure. 
of water at Boiling Point r 

We are thus led to the conclusion that water boils at the 
temperature for which its saturation vapour pressure is equal 
to the atmospheric pressure. But the pressure on the surface of 
water or any other liquid may be varied by pumping air into 
or withdrawing air from the boiling flask and, in each case, the 
liquid boils at the temperature for which its saturation vapour 
pressure is equal to the pressure on its surface. 

Determination of the Boiling Point of Water under Different 

Pressures 

The apparatus used for determining the boiling point of water 
at different pressures is shown in Fig. 68a. The boiling flask A 
which is closed by a stopper contains a quantity of water and 
rests on a tripod. A thermometer T and one end of the delivery 
tube B pass through the stopper and the delivery tube is sur- 
rounded by a Liebig's condenser L through which cold water 
from the tap circulates. The other end of the delivery tube is 



94 SCIENCE FOR COALMINING STUDENTS 

connected by pressure tubing to one branch of a glass T-piece 
Tj. The other two branches of the T-piece T t are connected 
through pressure tubing, one to a large Winchester D and the 



F C 




0U 

t 

70 
60 
SO 

{ 

40 

** 






> 








/ 








/ 






/ 






t 


/ 






r O 8O 9O JOO v 



. v 

^BOILING POINT(DEG.C.) 
(b) 

FIG. 68 

other to a branch of a second T-piece T 2 . The other branches of 
this T-piece are connected through pressure tubing, one to the 
manometer M which contains mercury and the other to the 



CHANGE OF STATE 95 

glass tube FG. A piece of pressure tubing, closed by a clip H is 
attached to the end G of the tube FG. 

It is essential that all the joints of the apparatus should be 
airtight. If not, they should be sealed with vacuum grease. 
The function of the Winchester D is to supply a large volume of 
air to the apparatus so as to prevent fluctuations of pressure. 

In performing an experiment, the clip H is released and a 
vacuum pump is attached to the tube FG. The pressure of the 
air inside the apparatus is reduced to about 30 cm. of mercury 
and the water in the flask is heated until boiling commences. 
The thermometer T is read along with the difference in the 
mercury levels of the manometer. A little air is then admitted 
into the apparatus by removing the pump, slightly releasing 
the clip H and then closing it again. The boiling point and the 
manometer reading are again noted. This process is repeated 
until the pressure of the air in the apparatus becomes equal to 
that of the atmosphere. 

A compression pump is now applied to the end of the tube 
FG and the observations of boiling point and pressure are again 
repeated. A table of results is shown below and a graph con- 
necting boiling point and pressure is shown in Fig. 68b. 



Barometric 
Pressure 13 


Excess 
Pressure 


Total 
Pressure 


Temperature, 


(cm. of mercury) 


k (cm.) 


B + h (cm ) 


B. Ft C. 


76-0 


o 


76-0 


IOO 


76-0 


10-3 


05-7 


96 


70-0 


T9'3 


56-7 


92 


76-0 


--31-5 


44'5 


85 


76-0 


-40-5 


35'5 


80 



The Steam in an Engine Boiler 

When water is heated in an engine boiler, steam is given off 
and in the confined space above the water its pressure increases. 
Thus the pressure on the surface of the water increases with a 
consequent increase in the boiling point. This process continues 
until the supply pressure is reached, when the steam enters the 
engine cylinder. 

MOISTURE IN THE ATMOSPHERE. DEW POINT 

If a glass tumbler containing cold water is brought into a 
warm room it will be noticed that water collects on the outside 
of the tumbler. The cold surface cools the air near it and water 
is deposited on the surface. 



g6 SCIENCE FOR COALMINING STUDENTS 

Evidently the atmosphere contains a certain quantity of 
invisible water vapour and during cooling water is deposited 
when a certain temperature is reached. This temperature is 
known as the dew point and at this temperature the air is 
saturated with water vapour. Thus we may define dew point 
as the temperature at which the quantity of water vapour actually 
present in a given volume of air is sufficient to saturate this 
volume and condensation commences. 

The following table gives the weight of water vapour in one 
cubic foot of a saturated atmosphere at various temperatures. 



Temperatiue 



40 F 
ho" F. 
80 F. 



Wt. of water vapour 
per cub. ft. of air 

0-00031 Ib 
0-00041 11). 
0-00084 Ib. 
0-00157 Ib. 



The above table shows that the weight of water vapour to 
saturate, one cubic foot of air increases with the temperature. 

RELATIVE HUMIDITY 

We are now in a position to consider the quantity of water 
vapour actually present in a given volume of air compared 
with the quantity of water vapour required to saturate this 
volume of air, This ratio is known as the relative humidity of 
the atmosphere. 

Thus: Relative Humidity (R.H.) 

^Weight of water vapour actually present per cu^. ft. of air 
"Weight of water vapour to saturate I cub. ft. of air at the 
same temperature. 

The Wei- and Dry-bulb Hygrometer 

The wet- and dry-bulb hygrometer (Pig. 69) consists of two 
mercurial thermometers A and B. The thermometer A has its 
bulb uncovered and registers the temperature of the air. The 
thermometer B has its bulb covered with a strip of muslin 
which also dips into a small vessel C, containing water. Water 
rises continuously up the muslin and keeps the bulb of the 
thermometer wet. Since evaporation takes place and the heat re- 
quired for this evaporation is taken from the bulb the tempera- 
ture of the wet bulb is generally lower than that of the dry 
bulb. When, however, the air is saturated with water vapour, 



CHANGE OF STATE 



97 



no evaporation takes place and the temperatures of the bulbs 
are the same. 

To find the relative humidity of the atmosphere the readings 
of the wet and dry bulbs are noted and the relative humidity 
is read off directly from the hygrometer table shown below. 



Tempera- 
ture of 


Difference of Temperature between Wet and Dry Bulbs 
Relative Humidity (per cent.) 


Dry Bulb 





1 


2 


3 


4 


5 


6 


7 


8 


9 U 


10 


11 


12 


32 F. 


IOO 


87 


7 6 


<>5 


56 


48 


4' 


35 


30 


*7 


^3 


19 


16 


36 F. 


100 


<)I 


82 


74 


66 


59 


53 


47 


42 


3 


34 


30 


27 


40 V, 


TOO 


92 


8 4 


76 


70 


63 


58 


5^ 


47 


43 


38 


34 


3* 


44 F. 


IOO 


92 


84 


7* 


72 


65 


()0 


55 


50 


4 


41 


38 


34 


48" F. 


IOO 


92 


5 


79 


73 


67 


02 


57 


5* 


48 


44 


40 


30 


5*" V* 


IOO 


93 


86 


80 


74 


69 


6 3 


59 


54 


5 


46 


43 


39 


50 r F. 


100 


93 


7 


81 


75 


70 


<>5 


Oo 


56 


52 


48 


45 


4i 


60 F 


IOO 


93 


88 


82 


7 b 


7i 


6(1 


62 


58 


54 


50 


46 


43 


04 F. 


IOO 


94 


88 


82 


77 


72 


08 


<>3 


59 


55 


5-2 


48 


45 


08 F. 


IOO 


94 


88 


3 


78 


73 


69 


65 


Oo 


56 


53 


5 


46 


70 F. 


IOO 


94 


89 


3 


78 


74 


09 


05 


61 


57 


54 


50 


47 


80 F. 


IOO 


95 


90 


85 


80 


76 


72 


67 


64 


60 


57 


53 


50 



A whirling hygrometer used in mining is shown in Fig. 70. 
The instrument is held in the hand and whirled about the 
handle. In this way the bulbs are in contact with a greater 
volume of air and a more representative result is obtained. 

IZxample. In a mine, the air entering the downcast shaft shows a 
dry-bulb reading of 40 F. and a wet-bulb reading of 34 F. The air 
leaving the upcast shaft shows a dry-bulb reading of 60 F. and a wet- 
bulb reading of 57 F. If 80,000 cub. ft. of air is circulated per minute, 
what weight of water is taken up by the circulating air per hour? 

From tables R.H. of air entering downcast shaft ^0-58 
Weight of water vapour to saturate i cub. ft. of air at 40 F. 0-00041 Ib. 

Wt. of water vapour actual!}' present per cub. ft. =0-00041 x --- Ib. 

TOO 

--0-000238 Ib. 

From tables R.H. of air leaving upcast shaft =0-82. 
Weight of water vapour to saturate i cub. ft. of air at 60 F.-- 0-00084 Ib. 

.'. Wt. of water vapour actually present per cub. ft. ^=0-00084 x Ib. 

IOO 

0-000689 Ib. 
.'. Wt. of water taken up by i cub. ft. of air --0-000689 Ib. 

0-000238 Ib. 
=0-000451 Ib. 
Wt. of water taken up by 80,000 cub, ft. of air =80,000 x 0-000451 Ib. 

= 36-08 Ib. 

/. Wt. of water taken up per hour =60 x 36-08 Ib. 

=2,165 Ib. 



9 8 



SCIENCE FOR COALMINING STUDENTS 



l40F.f 




FIG.69 




FIC.7O 

HYGROMETRY AND MINE AIR 

The condition of the atmosphere with regard to the amount 
of moisture it contains affects the health of the miner. The 
human body gives off moisture, organic acids, etc., in the form 
of perspiration and it is partly due to this that the temperature 
of the body is prevented from rising. 

When the difference between the readings of the wet and dry 
bulbs is considerable the atmosphere is moderately dry, and 



CHANGE OF STATE 99 

evaporation of the miner's perspiration proceeds at a good rate. 
But when the difference in the readings is small, the atmosphere 
is near its saturation point and in this case it is difficult for 
evaporation and removal of perspiration to take place. Thus 
the cooling of the body which always accompanies evaporation 
is retarded, the blood becomes overheated and the miner suffers 
from heat stroke. 

There are other important aspects of the relative humidity 
of mine air. When the air is dry there is a greater possibility 
of coal dust explosions. In wet mines, however, where there is 
excess of moisture, the danger of coal dust explosions does not 
exist. 

One of the indications of gob fires is the "sweating" of the 
strata. This is due to the condensation of the water vapour, 
produced during the burning of the coal, on the floor, roof 
and sides of the roadways. The relative humidity of the air in 
the return airway in this case is considerable and is an 
indication of " sweating/' Under these conditions the manage- 
ment has to take immediate action. 

EXERCISES 

1. What is meant by the terms (a) Specific Heat, and (b) Latent Heat? 
What weight of steam at 212 F. must be passed into 3,000 Ib. of water 
at a temperature of 60 F. and contained in a steel tank weighing 200 Ib., 
so that the temperature of the water and tank rises to 90 F. ? The 
specific heat of steel is o-ir B.Th.U. per Ib. per deg. F. and the latent 
heat of steam at 212 F. is 970 B.Th.U. per Ib. 

(Min. Sc.; U.L.C.I.) 

2. Define relative humidity. Describe the wet- and dry-bulb hygro- 
meter, and explain how you would use it for finding the relative humid- 
ity of the air at the bottom of a mine shaft. Why, in general, is the 
reading of the wet bulb lower than that of the dry bulb ? 

(Min. Sc., U.L.C.J.) 

3. Explain the terms "Relative Humidity," "Dew Point" and 
"Vapour Pressure." How may the relative humidity of mine air be 
determined? " (Min. Sc. ; ILL C I.) 

4. Why are the readings of a hygrometer of interest to the miner? 
At one coal face the wet- and dry-bulb readings are 65 F. and 70 F. 

respectively. At another face, the readings are 68 F. and 70 F. respec- 
tively. At which face does the air contain the more moisture and why ? 

(Min. Sc.; U.E.I.) 

5. What is meant by (a) "unit of heat," and (b) "specific heat"? 
What amount of heat will be required to convert 250 Ib. of water at 

60 F. into steam at 212 F.? The latent heat of steam is 966 B.Th.U. 
perlb. (Min. Sc.; U.L.C.T.) 

6. What is meant by ' * Relative Humidity ' ' ? Describe how the humid- 
ity of mine air varies in the passage of the air from the surface through 



100 SCIENCE FOR COALMINING STUDENTS 

the downcast shaft, the intake airways, the working places, the return 
airways, and finally through the upcast to the surface. Explain fully 
how these variations in humidity are caused. (Min. Sc.; U. L.C.I.) 

7. A tank contains TOO Ib. of water at 60 F. Steam at atmospheric 
pressure is fed into the tank until the temperature rises to 100 F. 
Jf the tank has a water equivalent of 20 Ib., calculate the quantity of 
steam which has been added (a) if the steam is dry, and (b) if the steam 
has a dryness fraction of 0*88. The latent heat of steam at atmospheric 
pressure is 970 B.Th.U. per Ib. (Min. Sc.; U.L.C.T.) 

8. Under what circumstances will the reading of the wet-bulb 
thermometer of a hygrometer be lower than the reading of the dry-bulb 
thermometer? Explain, fully, why the readings of a hygrometer are 
of interest to the miner. (Min. Sc.; U.L.C.I.) 

9. Describe the construe tion and use of a wet- and dry-bulb hygro- 
meter. What difference in reading would you expect to find if a hygro- 
meter were carried from the bottom of the downcast shaft to the 
bottom of the upcast shaft? Explain the cause of the difference. 

(Min. Sc.; U.L.C.I.) 

10. Define (a) British Thermal Unit, and (b) Latent Heat of Vaporisa- 
tion. 

Steam at 212 F. passes through a condenser at the rate of 250 Ib. 
an hour, and the condensed water leaves the condenser at a temperature 
of 80 F. How much heat is withdrawn from the steam by the con- 
denser per minute? Latent heat of steam at 212 F. 970 B.Th.U. per 
Ib. (Min. Sc.; U.L.C.I.) 

ji. Define (a) Specific Heat, and (b) Latent Heat of Vaporisation. 

A steel boiler, weighing 200 Ib., contains 1,000 Ib. of water at 60 F. 
How much heat, assuming no losses due to radiation, must be imparted 
to the boiler and its contents to convert 250 Ib. of the water into steam 
at 212 F.? Latent heat of steam at 212 F.=97o B.Th.U. per Ib. 
Specific heat of steel -o-rr B.Th.U. per Ib. per deg. F. 

(Min. Sc.; U.L.C.I.) 



CHAPTER IX 

MECHANICAL EQUIVALENT OF HEAT 

ENERGY 

We have already seen (Ch. VII) that fuels such as coal, paraffin, 
petrol, etc., when burned give out heat. In the case of the 
combustion of the coal in an engine furnace, the heat produced 
raises the temperature of the water in the boiler to boiling 
point and converts some of it into steam. The steam under 
pressure enters the cylinder and on expansion performs the 
work required to drive the piston. The motion of the piston is 
transmitted to the connecting-rod and thence to the flywheel. 
Thus heat is transformed into work and then into energy of 
motion, which is known as kinetic energy. 

Again, in the petrol engine which is known as an internal- 
combustion engine because the petrol vapour is ignited in the 
cylinder, the heat produced by the ignition of the petrol vapour 
produces sudden expansion of the gaseous products of com- 
bustion. The piston is driven along the cylinder and its energy 
of motion is transmitted to the connecting-rod, flywheel, etc. 
As in the steam engine heat is transformed into work and 
then into kinetic energy. 

Kinetic Energy 

Kinetic energy is the energy a body possesses by virtue of 
its motion, and it is measured in the same units as work, that 
is, in foot-pounds in the English system or in ergs in the Centi- 
metre Gram system of units. 

Potential Energy 

In the case of a reservoir on the top of a hill, if the water is 
allowed to descend to a lower level it gains energy of motion or 
kinetic energy, which may be transmitted to a water wheel. 
Before the water is released it possesses energy due to its posi- 
tion, which is known as potential energy. Potential energy 
is stored-up energy and only requires releasing in order to be 
transformed into kinetic energy. Thus we see that heat, work 
and kinetic and potential energies are all forms of energy 



IO2 



SCIENCE FOR COALMINING STUDENTS 



and any one form may be converted into other forms of 
energy. 

PRINCIPLE OF THE CONSERVATION OF ENERGY 

Energy can neither be created nor destroyed but it can be 
transformed from one form into another. The quantity of heat 
produced by the combustion of the coal in the engine furnace 
depends on the quantity of energy in the coal, and the work done 
by the expansion of the steam in the engine cylinder depends 




]R 



H 



FIG. 71 

on the quantity of heat lost by the steam. Also the work ex- 
pended against the force of friction in a machine appears in 
the form of heat, and the quantity of heat produced depends 
on the amount of work expended. Actually a definite relation- 
ship exists between the work expended and the quantity of 
heat which appears. This relationship will now be discussed. 

MECHANICAL EQUIVALENT OF HEAT 

Heat and work are mutually convertible. When a definite 
amount of work is expended an equivalent amount of heat 
appears, and when a definite amount of heat is expended an 
equivalent amount of work appears. 



MECHANICAL EQUIVALENT OF HEAT IO3 

If an amount of work W is done and during the performance 
of this work a quantity of heat equal to H is produced, 

W 

then Tf~ a constant. 

rl 

If we call the value of this constant J, which is known as 
Joule's Equivalent, we have: 



Now introducing units, 



and since i C.H.U.-f B.Th.U. 



H (B.Th.U.) 



=778 ft.-lb. per B.Th.U. 
Thus we may say that : 

i Centigrade Heat Unit 1,400 ft.-lb. 
and i British Thermal Unit =778 ft.-lb. 

Determination of the Mechanical Equivalent of Heat by 
Callender's Apparatus 

A brass drum A with a circular opening B is mounted on a 
spindle which rests on bearings fixed to a stand (Fig. 71). The 
drum can be rotated by means of the handle H and the number 
of revolutions is given by the revolution counter R. A silk 
ribbon is wrapped round the drum. One end of the ribbon is 
connected through a spring balance S to a horizontal cross- 
piece attached to the stand. The other end of the silk band 
supports a set of slotted weights W. As the drum is rotated the 
ribbon is kept taut and the spring balance indicates a reading. 
A special thermometer T, reading in tenths of a Centigrade 
degree, is inserted in the opening B. 

To perform the experiment, a quantity of water of known 
weight is inserted in the drum, and the temperature of the water 
is noted. The drum is rotated a known number of times and 
the temperature of the water is again noted. The reading of the 
spring balance is also noted at intervals and the average 
reading is determined. 



104 SCIENCE FOR COALMINING STUDENTS 

A typical set of results is shown below: 

Diameter of drum =3 in. 

Mean spring balance reading =O'26 Ib. 
Suspended weight 1-20 Ib. 

Weight of drum =o*55 lt>- 

Specific heat of brass =0-09 

Weight of water =0-22 Ib. 

Number of revolutions --=1,000 

Initial temperature of water 16'3 C. 
Final temperature of water -=18-2 C. 
Frictional force on drum =1-20 0-26 Ib. 

=0-94 Ib. 

Work done per revolution =0*94 x-& XTT ft.-lb. 
Work done in 1,000 revolutions 1,000x0-94 Xt^r ft.-lb. 
Water equivalent of drum = 0-55 X 0-09 Ib. 
Heat gained by drum and water =(0-22 +0-55 X 0-09) X 

1-9 C.H.U. 

TT r I,OOOXO-94XO-257r ,, /-TTTT 

Hence: ] - <-?- ~ ft.-lb. per C.H.U. 

J (0-22 +0-09 x 0-55) x 1-9 

^1,442 ft.-lb. pcrC.HU. 

Example. What amount of work is equivalent to the heat required 
to raise the temperature of 1,000 Ib. of \\ater from 60 F. to 90 F.? 
J -778 ft.-lb. per B.Th.U. 

Heat required = 1,000x30 B.Th.U. 

^--30,000 B.Th.U. 

Work equivalent =30, 000x778 ft.-lb. 
23,340,000 ft.-lb. 

Example. The weight of a flywheel is 1,200 Ib. and the diameter of 
its shaft is 3 in.. If the coefficient of friction between the shaft and its 
bearings is 0*05 and the shaft makes 300 revolutions per minute, calcu- 
late the heat produced in the bearings per hour. (51,400 ft.-lb. per 
C.H.U.) 

From M ^~, 

where F - -frictional force, W - - load, and jj, - the coefficient of friction, 

F 

we have : 0-05 = 

1,200 

F = 1,200x0-05 

=-60 Ib. wt. 
Work done per revolution =TT X -& (ft.) X6o (Ib. wt.) 

==47- 1 ft.-lb. 
Work done per hour 47-1 x 300x60 ft.-lb. 

TT A. j j 47* 1 x 300x60 ~ u TT 

Heat produced ?- C.H.U. 

1,400 

=-605-6 C.H.U. 



MECHANICAL EQUIVALENT OF HEAT 105 

EXERCISES 

1. What amount of work is equivalent to the heat required to raise 
the temperature of 500 lb. of water from o C. to TOO C.? (J = 1,400 
ft.-lb. per C.H.IT.) 

2. A vessel, of water equivalent 3 lb., contains 40 lb. of water at 
60 F. The water is churned by a paddle wheel working at 4 h.p. Assum- 
ing no heat Josses, find the temperature of the water after 10 minutes. 

3. The perpendicular force between a shaft, diameter 2 in., and its 
bearings is 200 lb. wt. and the coefficient of friction is O-T. If the shaft 
makes 300 revolutions per minute, calculate the heat produced per 
hour, (j --1,400 ft.-lb. per C.H.U.) 



CHAPTER X 

CONDUCTION, CONVECTION AND RADIATION 

TRANSFER OF HEAT 

Heat may be transferred from one point to another by con- 
duction, convection and radiation. 

When a poker is held in the fire, heat travels along it from the 
fire to the hand, that is from the hot end to the cold end, and 
there is a gradual fall in temperature along the poker. 

If we consider the poker to consist of a line of particles 
A, B, C, D, E, etc., where A is the hot end, the heat passes along 
the particles in the direction of A to E. The particle A transfers 
some of its heat to the particle B, B transfers some of its heat to 
C and so on and the temperatures of the particles A, B, C, D, E, 
etc., are in descending order of magnitude. This mode of trans- 
fer of heat is known as conduction, which takes place mainly 
in solids. 

When a beaker containing water is placed on a tripod and 
heated by means of a bunsen burner, the water just above the 
flame expands, becomes less dense and rises to the surface. 
Heat is carried by the particles of water in a stream (Fig. 72) 
and a cold water current descends from the surface of the water 
to the bottom of the beaker to keep up the circulation. This 
mode of transfer of heat is known as convection and takes place 
in liquids and gases. 

If a person stands directly in the sun's rays and then in the 
shadow cast by a wall, the effect of radiant heat can be under- 
stood. Thus in Fig. 73a the point A is directly under the influence 
of the sun's rays, but in Fig. 73b the point A is shielded. From 
this we can see that radiant heat travels in straight lines as in 
the case of light. Moreover, the material through which radiant 
heat passes is not heated. In fact, radiant heat can pass through 
a vacuum as in the case of the space between the sun and the 
earth and the mode of transfer is known as radiation. 

CONDUCTION OF HEAT 

Substances which allow heat to pass through them very 
readily are known as good thermal conductors, whereas those 
substances which allow heat to pass through them with 



CONDUCTION, CONVECTION AND RADIATION 107 




FIG.72 





(b) 



FIC.73 




FIG. 74 





FIG. 75 




108 SCIENCE FOR COALMINING STUDENTS 

difficulty are known as poor conductors or thermal insulators. 
Metals are good thermal conductors and, in general non- 
metals such as wood, paper, vulcanite, bakelite, etc., are poor 
thermal conductors. 

Fig. 74 shows a compound bar, one part of which is made of 
wood and the other part of brass. A sheet of paper is wrapped 
round the dividing line and the bar is held over a bunsen flame 
so that the joint is in the flame. The paper over the wood is 
burned while that over the brass is unaffected. In the case of 
the brass which is a good thermal conductor the heat is con- 
ducted away from the paper rapidly, whereas in the case of the 
wood, a poor thermal conductor, the heat remains in the paper, 
raises it to its ignition temperature (see p. 226) and the paper 
burns. 

The Wire Gauze Experiment 

It a sheet of iron gauze is held about two inches above a 
bunsen burner and, with the gas turned on, a light is applied 
below the gauze, the flame extends to the gauze and no further 
(Fig. 75). If the light is applied above the gauze, the flame does 
not extend below. The iron gauze, being a good thermal con- 
ductor, conducts the heat away from the flame and the gas on 
the other side of the gauze does not reach its ignition tempera- 
ture. 

PRACTICAL APPLICATIONS 

Both good and poor thermal conductors have their uses in 
industry. The wire gauze experiment described above has its 
main practical application in the miner's flame safety lamp. 

FLAME SAFETY LAMP 

The outer portions of the lamp (Fig. 76) may be divided into 
three sections, the bonnet A, the frame B and the fuel tank 01 
base C. The frame is screwed into the base and is locked either 
by means of a lead rivet or a magnetic lock. The bonnet is 
screwed into the frame ring L and, when the lamp is assembled, 
is held in a locked position by a locking spindle. The frame 
consists of a base and a top ring which are rigidly connected by 
five fixed standards D and the locking spindle is held in position 
by the frame. 

The internal assembly of the lamp is as follows. An asbestos 
ring E is placed on the frame base and the lamp glass F is 
mounted on it. This glass is prevented from any lateral move- 
ment by a circular metal flange which forms part of the frame 



CONDUCTION, CONVECTION AND RADIATION 109 




MAGNIFIED 
SECT ION AT X 




FIG. 76 



FLAME SAFETY LAMP(RELIGHTER TYPE) 
RELIGHTER SYSTEM OMITTED 



BONNET A 
FRAME B 
FUEL TANK C 
STANDARD D 
ASBESTOS RING-E 
GLASS F 
INNER GAUZE-G 
OUTER GAUZE-H 



BRASS RING I 
BOTTOM FEE D J 
CONTROL RING 
LOCK-K 
FRAME RING-L 
WICK TUBE M 
CONTROLLER 
WICK N 
WICKTUBE-Q 



base. Another asbestos ring is placed on the top of the glass and 
the inner gauze G which is generally of 20 or 28 mesh is set in 
position on it. The outer gauze H of similar mesh is then fitted 
over the inner one. This outer gauze is provided with a flange 



IIO SCIENCE FOR COALMINING STUDENTS 

which extends round the glass and prevents lateral displace- 
ment between gauzes, asbestos ring and glass. 

The internal brass ring I which is riveted to the bonnet is 
also provided with a flange which presses down on the gauzes 
when the bonnet is screwed into the frame ring. The asbestos 
rings are provided so as to obtain airtight and flame-proof 
joints between the glass and the metallic surfaces of the inner 
gauze and the frame base. 

The air supply enters the lamp through a series of perforations 
in the frame ring. This supply, however, may be cut off by means 
of a control ring J which has similar perforations. The move- 
ment of the ring is such that in one position the two series 
of perforations coincide and in another position there is no 
coincidence. When the air supply passes through the frame 
ring the lamp is said to be in "bottom feed/ 1 If this supply is 
cut off the air enters the perforations near the top of the bonnet 
and the lamp is in "top feed." In this case the lamp may be 
usefully employed in the detection of methane near the roof. 
The control ring is moved by a catch fitted through the frame 
ring, the two positions "open" and "shut" being marked O 
andS. 

THE LAGGING OF PIPES 

Felt, asbestos, etc., being poor conductors of heat, are used 
for wrapping round steam pipes to keep them hot. In this 
manner, very little heat is allowed to pass from the pipes to 
the surrounding air. The temperature of the surface of the non- 
conducting material in contact with the pipes is practically the 
same as that of steam, whereas the outer surface of the material 
is not much above air temperature. 

CONVECTION OF HEAT 

Fig. 77a shows an apparatus which is used for illustrating 
convection currents in water. A flask A is provided with a rubber 
stopper which is fitted with the glass tubes BC and DE. These 
tubes have their exits in a glass vessel F. The flask is filled with 
coloured water and the vessel F along with the tubes BC and 
DE contain pure water. When the water in the flask is heated, 
a stream of coloured water rises up the tube DE and enters the 
vessel F, colouring the water at the top. It is evident that in 
order to keep the flask A filled, cold water must descend the 
tube CB which reaches to the bottom of the flask. 

A practical application of convection currents in liquids is 
the hot-water system of a building (Fig. 77b) or the hot-water 



CONDUCTION, CONVECTION AND RADIATION III 




(a) 




FIG. 77 



(b) 



t 



B 



I 



FIG. 78 




FIG. 79 



112 SCIENCE FOR COALMINING STUDENTS 

supply for pit-head baths. Hot water from the boiler A passes 
up the pipe BC, through the radiator R to the top of the 
cistern 1). Another pipe EF leads from the top of the cistern 
to a tank G which contains cold water. A branch pipe HI leads 
to the bath, sink, etc. To keep up the circulation, cold water 
descends the pipe JK and enters the bottom of the cistern. Cold 
water also descends the pipe LM which leads to the bottom 
of the boiler. 

Convection Currents in Air 

A box A, possessing a glass front, is fitted with two tubes 
B and C which serve as chimneys (Fig. 78). If smouldering paper 
is placed directly under the tube C and a lighted candle is 
placed underneath the tube B, the air above the candle is 
heated, becomes less dense and rises up the tube, carrying 
with it particles of charred paper. To keep up the circulation, a 
cold air current descends the tube C. 

The Ventilation of a Coal Mine 

The above principle used to be generally employed in the 
ventilation of a mine. A fire was kept burning at the bottom 
of the upcast shaft. The heated air ascended this shaft and a 
cold air current descended the downcast shaft to keep up the 
circulation. In modern mines, however, forced convection is 
used. A centrifugal fan is placed near the top of the upcast 
shaft (Fig. 79) and the air in this shaft is drawn by the fan 
into the atmosphere. An air current descends the downcast 
shaft to keep up the circulation. 

EXERCISES 

1. Sketch, and describe, either (a) a flame safety lamp, or (b) an 
electric safety lamp. Which type of lamp do you prefer and why ? 

(Min. Sc.; U.L.C.I.) 

2. Describe experiments to illustrate the principle of the flame safety 
lamp. (Mm. Sc.; U.L.C.I.) 

3. Explain what you understand by conduction, convection and 
radiation of heat. Give an example of each from mining practice. 

(Min. Sc.; U.L.C.I.) 

4. What is meant by conduction of heat? Describe an experiment 
which illustrates the principle of the flame safety lamp. Under what 
circumstances is a cap formed and what use can be made of it? 

(Min. Sc.; U.E.I.) 

5. Describe one experiment 111 each case to show convection currents 
(a) in a liquid, (b) in air. Give examples of convection in mining. 

(Min. Sc.; U.L.C.I.) 



CONDUCTION, CONVECTION AND RADIATION 113 

6. Explain the difference between conduction and convection of heat. 
Describe an example of the utilisation of conduction of heat in mining 
practice. (Min. Sc.; U.L.C.I.) 

7. Explain why an ignition of gas inside a flame safety lamp does 
not readily ignite gas outside the lamp. What is meant by "flame-proof 
switch-gear"? How is such apparatus made flame-proof? 

(Min. Sc.; U.L.C.I.) 



CHAPTER XI 

THE ELECTRIC CIRCUIT AND 
REPRESENTATION OF ELECTRICAL PARTS 

INTRODUCTION 

Every mining student is familiar with some of the electrical 
devices which are to be found both on the surface and under- 
ground, such as accumulators, terminals, bells, switches, etc. 
They have also some idea of the use of the ammeter and volt- 
meter, having seen these instruments in mining laboratories, 
power stations, etc. But before dealing in detail with the prin- 
ciples underlying electrical appliances it is necessary to explain 
a few terms which are in common use in electrical practice 
and also to show how electrical appliances are represented in 
diagrams. 

CONDUCTORS AND INSULATORS 

All substances may be classified, as far as the passage of an 
electric current is concerned, into (a) conductors and (b) insula- 
tors. Substances which allow electricity to pass through them 
with little resistance to the current are known as conductors, 
whereas substances which allow electricity to pass through 
them with difficulty or with a great resistance to the current 
are known as insulators. Substances which are good thermal 
conductors are also good electrical conductors and Vice versa. 
Thus all metals, including the liquid mercury, are good con- 
ductors of electricity. Ebonite, wood, rubber, bakelite, etc., 
are electrical insulators. 

Screw Terminals 

A terminal (Fig. 8oa) serves as an inlet or an outlet for an 
electric current. 

REPRESENTATION OF VARIOUS ELECTRICAL PARTS BY DIAGRAMS 

The Accumulator 

The accumulator possesses two terminals. An electric current 
leaves the positive terminal and after passing through the 
various resistances, etc., re-enters the cell at the negative 
terminal. An accumulator is shown in diagrams as in Fig. Sob. 

114 



THE ELECTRIC CIRCUIT 





(a) 






3D 



FIG. 8O 



h- 



(b) 




[A 

(c) 



(e) 




FIG.8I 



(a) 



(b) 

FIG.82 



B 



wyw 




FIG.83 



Il6 SCIENCE FOR COALMINING STUDENTS 

The Ammeter 

The ammeter is an instrument designed for measuring the 
strength of an electric current. The instrument possesses a 
scale marked o i amperes,* o 10 amperes, etc., according 
to the strength of the current it is intended to measure. Fig. 8oc 
shows how an ammeter is represented diagrammatically. 

The Plug Key 

The plug key (Fig. Sod) supplies a ready means of completing 
or breaking an electric circuit. Two slabs of brass A and B, 
fitted with screw terminals X and Y, are mounted on an ebonite 
base D. The brass plug C which is provided with an ebonite 
handle connects the plates A and B arid an electric current 
can flow from one terminal to the other. Fig. 8oe shows how a 
plug key is represented diagrammatically. 

Fixed and Variable Resistances 

A long piece of wire offers more resistance to an electric 
current than a short piece of the same material and diameter. 
Thus with a variable resistance which consists of a long coil of 
wire, various lengths of wire may be included in an electric 
circuit. 'Jhe resistance of the circuit is thereby varied with a 
consequent variation in the current strength. Fig. 81 shows a 
variable resistance. Insulated wire (i.e. wire covered with some 
insulating material such as cotton thread, shellac, etc.) is 
wound round a porcelain tube and the ends of the wire are 
iixed to the terminals A and B. The steel rod CD, along which 
the slider S moves, is insulated from the supports P ^nd Q and 
carries a terminal E at one end. If the current enters at A it 
traverses the coil as far as the slider S and then passes along the 
rod to the terminal E. 

When the current enters at A and leaves at B, it traverses the 
whole length of the coil. The resistance is now constant and the 
appliance becomes a fixed resistance. Figs. 8aa and 8ab show 
how variable and fixed resistances are represented in circuit 
diagrams. 

The Complete Circuit 

Fig. 83 shows a complete electric circuit. The current leaves 
the accumulator, passes through the plug key, through the 
variable resistance as far as B and then through the ammeter 
A, to the negative terminal of the accumulatpr. 

* The ampere is the unit of current strength. 



THE ELECTRIC CIRCUIT 117 

By including more of the variable resistance in the circuit, 
i.e. by moving the slider to the right, the total resistance round 
the circuit is increased and the current thereby decreased. 
This is shown by the ammeter A. Vice versa, by moving the 
slider to the left, the resistance is decreased and the current 
increased. 



CHAPTER XII 

MAGNETISM AND ELECTROMAGNETISM 

MAGNETISM 

Magnetism is that property of a substance which is utilised 
in the mariner's compass or the miner's dial. Both these instru- 
ments depend on the principle that a compass needle which 
possesses a north pole at one end and a south pole at the other 
sets itself along a definite direction which is known as the 
magnetic meridian. The north pole of the needle points to the 
North Magnetic Pole of the earth, and the south pole of the 
needle to the South Magnetic Pole. The geographical meridian at 
a point on the earth's surface is the true North and South line 
through this point and the direction of the magnetic meridian 
at this point makes an angle of 17 West with the geographical 
meridian (Fig. 84). The angle between the geographical and 
magnetic meridians is known as the angle of declination. 

By means of the miner's dial, which in effect is a freely sus- 
pended compass needle, the direction of the magnetic meridian 
underground and consequently the true North and South line 
can be determined. 

Attraction and Repulsion 

A bar magnet also possesses a north pole at one*end and a 
south pole at the other and when freely suspended shows the 
same directional property as the compass needle. If a bar 
magnet A is suspended in a paper stirrup and the north pole 
of another bar magnet B is brought near the north pole of the 
suspended magnet, this latter pole is repelled. If the south pole 
of the magnet B is brought near the north pole of the magnet A, 
this latter pole is attracted. Thus a north pole repels a north 
pole free to move and a south pole attracts a north pole. 
Similarly a south pole attracts a north pole and repels a south 
pole. In a more general way we may say that like poles repel 
each other and unlike poles attract each other. 

If a thin rod of unmagnetised steel is suspended in the stirrup 
and either pole of a bar magnet is brought near each end of the 
rod in turn, attraction takes place in each case. 

118 



MAGNETISM AND E LECTROM AGNETISM 
MAC.N 





MAG.S 



FIG. 84 



FIG.B6 



FIG.85 




120 SCIENCE FOR COALMINING STUDENTS 

MAGNETIC FIELDS 

If one end of a bar magnet is immersed in a dish containing 
iron filings, when the magnet is withdrawn a cluster of filings 
clings to the magnet. This attractive force, however, is not 
confined to the surface of the magnet. At points removed from 
the surface, the attractive force is operating. A compass needle, 
placed a few inches from a pole of a bar magnet, sets itself in a 
definite direction. The space around the magnet, at any point of 
which the magnet exerts a force, is known as the sphere of influence 
or the field of the magnet. 

If a bar magnet is placed on a sheet of paper and a small 
compass needle is placed near its north pole, the position n 
of the end of the compass needle remote from this pole 
can be marked. If the compass needle is now moved until its 
south pole is opposite the mark n and the process is repeated, 
a line of dots is obtained on the paper. It will be found that the 
line stretches from the north pole of the magnet to the south 
pole. This line is known as a line of force. Other lines of force 
can be plotted until the magnetic field is completed (Fig. 85). 

A line of force of a magnet is the path traced out by a single 
north pole, free to move, and stretches from the north pole of 
the magnet to the south pole. Fig. 86a shows the lines of force 
due to a single bar magnet. The lines describe circuitous paths 
from the north to the south poles. Fig. 86b shows the lines of 
force due to two bar magnets in line, with the north and south 
poles opposite each other, and Fig 86c shows the lines of force 
due to a horse-shoe magnet. 

MAGNETIC EFFECTS OF AN ELECTRIC CURRENT 

Fig. 87 shows a long coil R consisting of about 100* turns of 
cotton-covered copper wire of S.W.G. 20. The electric circuit is 
arranged with an accumulator B in series with the coil R, a 
variable resistance VR and a plug key K. A soft-iron rod is 
placed in the coil and a small dish containing iron filings is 
supported underneath the exposed end of the rod. When the 
key is inserted and the variable resistance is adjusted so as to 
obtain a suitable current, iron filings collect in a cluster on the 
end of the rod. When the circuit is broken by withdrawing the 
plug key the filings fall off the rod. 

The experiment evidently shows that a bar of soft iron can 
be magnetised by means of a coil of wire conveying a current. 
It also shows that the soft iron only remains magnetised so 
long as the electric current flows. The magnetism -acquired is 
known as temporary magnetism. 



MAGNETISM AND ELECTROMAGNETISM 121 



'B 



:R 






V.R. 

AAA/i 



FIG. 87 





FIG.88 




FIG. 89 



S N 

F1C.9O 



122 SCIENCE FOR COALMINING STUDENTS 

If the experiment is repeated with a steel knitting needle and 
the current is passed for a considerable time, a cluster of filings 
again collects on the exposed end of the needle. The filings also 
remain on the rod when the current is discontinued. 

In this case the magnetism acquired is known as permanent 
magnetism. Thus temporary magnetism only exists as long as 
the magnetising current continues to flow, but permanent 
magnetism remains after the magnetising current is discon- 
tinued. 

Polarity of a Long Coil of Wire conveying an Electric 
Current 

An accumulator, a long coil of wire, a variable resistance 
and a plug key are arranged as in Fig. 87, but the soft-iron 
rod is dispensed with. If the wiring of the circuit is examined, 
the direction of the current, clockwise or counter-clockwise 
when viewed from one end, may be ascertained. If the plug 
key is inserted and the current is increased a compass needle, 
placed near this end, is deflected according to the following rule. 
If the current flows in a clockwise direction, the north pole of 
the needle is attracted, whereas if the current is counter-clock- 
wise, the south pole is attracted. Thus on viewing one end of 
the coil directly, if the current is clockwise, this end is a south 
pole, but if the current is counter-clockwise the end of the coil 
is a north pole (Fig. 88). 

A coil conveying a current has therefore a magnetic field 
associated with it. The lines of force leave the north pole and, 
after circuitous paths, re-enter the south pole (Fig. 89). 

The Electromagnet 

Fig. 90 shows an electromagnet, shaped like a horse-shoe. 
The soft iron is wound with insulated wire in such a manner 
that, when the current flows, one end of the coil becomes a 
north pole and the other end a south pole. Lines of force leave 
the north pole and enter the south pole and these lines consti- 
tute the field of the magnet. Since the core is made of soft iron, 
the magnetism acquired is temporary magnetism. 

Magnetic Materials 

If thin rods of various materials such as iron, steel, nickel, 
aluminium, tin, glass, etc., are placed in the coil of wire shown in 
Fig. 87, it will be found that iron, steel and nickel can be mag- 
netised, whereas tin, aluminium, glass, etc., are unaffected. 



MAGNETISM AND ELECTROMAGNETISM 123 

Substances such as iron, steel and nickel which can be mag- 
netised are known as magnetic materials and substances which 
cannot be magnetised are known as non-magnetic materials. 

The Electric Bell 

The wiring system of an electric bell is shown in Fig 91. 
The current enters at the terminal A, passes through the coils 
of the electromagnet E to the steel spring D, which is attached 
to a bracket C. The current then passes along the soft-iron 
rod F, through the leaf G which consists of a thin strip of steel 
and through the contact screw H to the terminal B. The 
current magnetises the soft-iron core of the electromagnet, the 
soft-iron rod F is attracted and the hammer I, attached to the 
rod F, strikes the bell J. In this operation the circuit is broken 
at the contact H, the soft-iron core loses its magnetism and the 
spring D restores the rod F and the leaf G into contact with 
the screw H. The operation is then repeated and continued as 
long as the current continues to flow. 

Electric equipment, including special types of electric bells 
which are free from dangerous sparking at make and break, 
are used extensively in mines where signals have to be given 
in haulage and other operations, and a knowledge of the con- 
struction, use and repair of this equipment is essential in 
modern mining practice. 

The Telephone 

The principle of the electromagnet is also utilised in the 
telephone, which consists of a microphone A and a receiver 
B (Fig. 92). The main features of the microphone are an ebonite 
cylinder C, closed by a thin carbon diaphragm D and a carbon 
plate E which is fitted with a terminal X. Another terminal Y 
is attached to the diaphragm D and the cylinder contains loosely 
fitting carbon granules. Leads connect the terminals X and Y 
through a battery of cells, to the coils of the permanent magnet 
M of the receiver and the winding of the coils is such as to 
increase its magnetisation when the current flows. 

A thin diaphragm of soft iron F is fitted in the ebonite 
casing near the poles of the magnet. The sound waves from the 
speaker set the diaphragm D in vibration and this alters the 
spacing of the carbon granules, producing an alteration in the 
resistance and a consequent alteration in the current through it. 
The variation of the current produces a variation in the strength 
of the permanent magnet M and the soft-iron diaphragm F is 



124 SCIENCE FOR COALMINING STUDENTS 



.CARBON PLATE 



CARBON 
DIAPHRAG 





EBONITE- 



PERMANENT 
MAGNET-" 



d4?l 



M : 



( f F 

^- I SOFT 
IRON DIAPHRAGM 

FI0.92 



B 



FIC.9I 



MAGNETISM AND ELECTROMAGNETISM 125 

set in vibration. The ear placed near the receiver receives the 
sound due to these vibrations. 

Telephones are installed in mines for conveying messages 
from one point to another. 

MOTION OF A CONDUCTOR, CONVEYING A CURRENT, IN A 
MAGNETIC FIELD 

Fig. 93 shows a strong elcctromagmet E, arranged so that its 
poles project over the edge of the bench. A soft-iron rod A is 
supported horizontally in a stand (not shown in the diagram) 
and a copper rod C, fitted with a hook, is suspended from one 
end. The rod C is adjusted so that it rests in and at right angles 
to the magnetic field between the pole-pieces N and S. One 
end of a piece of flex wire F is soldered to the lower end of the 
rod C and the other end of the flex is connected to the positive 
terminal of an accumulator B. The negative terminal of the 
accumulator is connected through a variable resistance R to a 
screw terminal D, attached to the rod A. With this arrangement 
the current passes up the rod C. 

Starting with the whole of the variable resistance in the cir- 
cuit, if the resistance is quickly decreased with a consequent 
increase in the current, the lower end of the rod C moves into 
the space between the coils of the electromagnet. When the 
terminals of the accumulator are interchanged so that the 
current flows down the rod C, and the current is again increased, 
the lower end of the rod moves in the opposite direction. 

If a plug key is placed in the circuit, by inserting the plug 
and starting the current, the rod C is again set in motion. 
Thus if a conductor is at right angles to a magnetic field and 
a current is started or increased, the conductor moves across 
the magnetic field so as to cut the lines of force. 

It will be seen from the diagram (Fig. 94) that the directions 
of the field, the current and the motion are mutually at right 
angles. We are thus led to Fleming's Left-hand Rule which can 
be stated as follows : // the thumb , the first finger and the second 
finger of the left hand are held mutually at right angles to one 
another and if the first and second fingers point in the directions 
of the field and the current respectively, then the thumb points in 
the direction of the motion. 

The Moving-coil Galvanometer 

A practical example of Fleming's Left-hand Rule is the 
action of a moving-coil galvanometer (Fig. 95a). M is a perma- 
nent magnet with shaped pole-pieces N and S. A cylinder of soft 



126 SCIENCE FOR COALMINING STUDENTS 




FIG.93 



THU 



N FIRST FINGER 




FIG.94 



MAGNETISM AND ELECTROMAGNETISM 127 

iron (shown in Fig. 95b) is mounted centrally between the 
pole-pieces and the combination of the soft-iron cylinder and 
the curved pole-pieces produces a radial field. 

PQ represents one side of a rectangular coil mounted on a 
spiral spring R. The opposite side is mounted on a similar spring 
(not shown in the diagram). The coil consists of several turns 
of thin insulated wire and is free to move in the space between 
the poles and the soft-iron cylinder. When the current enters at 




(a) 




the terminal X, it passes through the spring R, through the coil 
PO and out at the terminal Y. 

If the current goes down the right-hand side of the coil at 
Q and up the left-hand side at P, by the Left-hand Rule the 
coil, along with the pointer attached to it, rotates in a clock- 
wise direction. If the current enters at Y and leaves at X, 
the coil rotates in a cotmter-clockwise direction. 

The spring R, by winding or unwinding as the deflection 
increases, acts as a control and the coil takes up a position of 
equilibrium. 

It will be seen that the instrument both detects a current 
and also gives its direction. 



128 SCIENCE FOR COALMINING STUDENTS 

PRODUCTION OF A CURRENT BY THE MOTION OF A CONDUCTOR 
ACROSS A MAGNETIC FIELD 

In Fig. 96 A BCD is a coil, consisting of about 20 turns of 
ordinary connecting wire. A piece of double flex wire F connects 
the ends of the coil to the terminals of a galvanometer G. A 
strong horse-shoe magnet M is placed on the table and the 
coil ABCD is placed near it, with the sides AB and CD 
vertical. 

If the windings of the coil and the flex are examined the 
direction of the current in the coil which produces a galvano- 
meter deflection to the right or to the left can be deduced. 
If the coil is now moved into the space between the poles N 
and S of the magnet, it will be noticed that the deflection 
is in such a direction that a current flows from A to B. When 
the coil is moved away from the magnet it will be found that a 
current flows from B to A. 

The experiment shows that if a conductor which forms part 
of a complete circuit cuts across the lines of force of a magnetic 
field, (a) a current, known as an induced current, is produced in 
the conductor, (b) the current only lasts as long as the motion 
is taking place and (c) the direction of the current is given by 
the following rule (Fleming's Right-hand Rule) : If the first and 
second fingers and the thumb of the right hand are held mutually 
at right angles to one another and if the first finger and thumb 
point in the directions of the magnetic field and the motion of the 
conductor respectively , then the second finger points in the direction 
of the induced current (Fig. 97). 

THE DYNAMO 

The dynamo is based on Fleming's Right-hand Rule for 
induced currents. Fig. 98 shows a rectangular coil ABCD 
rotating in the space between the poles N and S of an 
electromagnet. Copper slip-rings E and F are mounted on a 
spindle the axis of which is coincident with the line PQ. The 
rings E and F are insulated from the spindle and are connected 
to the sides AB and CD of the coil respectively. As the spindle 
rotates so does the coil ABCD. 

Suppose the coil is horizontal (Fig. 98). As AB moves upwards 
and CD moves downwards, a current is induced in the directions 
A to B and C to D. The current leaves the coil by the carbon 
brush L which rests on the ring F, traverses the external circuit 
and re-enters by the carbon brush K which rests on the ring 



MAGNETISM AND ELECTROMAGNETISM 




IRST FINGER 




H 

R 



THuMl 

A ? 

8 



FINGER 



FIG. 97 




FIG.98 



130 SCIENCE FOR COALMINING STUDENTS 

E. When the coil is vertical the sides AB and CD move in a 
direction parallel to the field and, since no lines of force are 
cut, no current flows. But when CD replaces AB, the current 
enters the ring F by the brush L, flows from D to C in CD and 
from B to A in AB. Thus the current changes" its direction in 
the coil and in the external circuit and is known as an alternat- 
ing current. 

The Direct-current Dynamo 

As we have seen, the arrangement described above produces 
an alternating current both in the coil itself and in the external 
circuit. To produce a direct current, that is, a current flowing 
in the same direction in the external circuit all the time, a 
split-ring commutator is used. The commutator consists of 
two semi-rings X and Y which are made of copper. They are 
mounted on the spindle as before and arc also insulated from it. 
The sides AB and CD of the coil are connected to Y and X 
respectively. As AB moves upwards and CD downwards (Fig. 
gga) a current flows from A to B in AB and from C to D in CD. 
The current leaves the coil by the brush L which rests on X 
and, after passing round the external circuit, re-enters at the 
brush K which rests on Y. When the plane of the coil is vertical 
(Fig. ggb) no current flows and the brushes are in contact with 
the spaces between X and Y. When AB and CD interchange 
positions (Fig. 99c), the current in AB runs from B to A and 
the current in the external circuit flows in the same direction as 
before. But when the coil is again vertical no current flows and 
the brushes are again in contact with the spaces between X 
and Y. Thus it can be seen that the current changes direction 
in the coil every time the brushes are in contact with ttie spaces, 
but the current in the external circuit is maintained in the 
same direction for all positions of the coil. 

THE ELECTRIC MOTOR 

An A.C. dynamo may be converted into an A.C. motor by 
passing an electric current into it from another A.C. dynamo. 
Thus if the current enters by the brush K (Fig. 98), by Fleming's 
Left-hand Rule the side AB of the coil moves downwards and 
the side CD upwards. 

Similarly a D.C. dynamo may be converted into a D.C. 
motor by passing an electric current into the coil ABCD at 
the brush K (Fig. gga), and again the side AB moves down- 
wards and the side CD upwards. 



MAGNETISM AND ELECTROMAGNETISM 

The Armature 

In practice, both in motors and dynamos, the effect is multi- 
plied by having a number of coils mounted on a frame known 
as the armature, which is mounted on the rotating spindle. 





DYNAMOS AND MOTORS IN MINING 

In modern mines A.C. supply from the local authority is 
supplied to the colliery power house, from whence it is trans- 
mitted by cables to the underground roadways, where it is 



132 



SCIENCE FOR COALMINING STUDENTS 



transformed down (see p. 134) for various requirements. In 
addition an A.C. generator giving the same voltage may be in- 
stalled in the power house on the surface, and this supply is 
transmitted underground by cables, where it is employed when 
breakdown of the main supply occurs or at periods of light load. 

LENZ'S LAW 

Lenz's law for induced currents states that the direction of 
the induced current in a circuit is such as to oppose the motion 
which produces it. 

This law may be verified by means of the apparatus shown 
in Fig 100. The windings of the coil A and circuit are examined 
so as to ascertain 'whether the galvanometer deflection is to the 
right or left for a clockwise current in the coil when viewed 
from the right-hand side. If the north pole of the bar magnet 
is inserted in the coil and the deflection of the galvanometer, 
right or left, is noted, the direction of the current in the coil, 
clockwise or counter-clockwise, may be deduced. When the 
pole of the magnet is withdrawn, it will be found that the 
current flows in the opposite direction. A set of results is 
shown below. 



Motion of 
Pole 


Deflection of 
Galvanometer 


Direction of 
Current in Coil 


Polarity of face 
of Coil 


North pole approaches 
North pole recedes 
South pole approaches 
South pole recedes 


Right 
Left 
Left 
Right 


Counter-clockwise 
Clockwise 
Clockwise 
Counter-clockwise 


North 
South 
South 
North 



It will be seen from the above table that the polarity of the 
face of the coil is such as to oppose the motion of the magnet. 

MUTUAL INDUCTION 

We have already seen how an induced current is produced 
in a coil of wire by inserting a pole of a bar magnet inside it. 
We also saw that the induced current only lasts as long as the 
magnet is in motion, that is, during the time the number of 
lines of force through the coil is changing. 

If the bar magnet is kept stationary and the coil is moved 
from the position A to the position B (Fig. 101) there is an 
increase in the number of lines of force through it and this 
also produces an induced current. 

We are now in a position to understand how the starting or 



MAGNETISM AND ELECTROMAGNETISM 133 




FIG.IOO 




N 



FIG.IOI 




FIG. 102 



134 SCIENCE FOR COALMINING STUDENTS 

the breaking of a current in a circuit produces an induced 
current in another circuit placed near to it. 

A large coil P, consisting of several turns of ordinary con- 
necting wire is arranged in series with a plug key K and an 
accumulator (Fig. 102). The terminals of a smaller coil S, con- 
sisting of several turns of the same kind of wire, are connected to 
the terminals of a galvanometer by means of a piece of double 
flex wire F. The diameter of the smaller coil is such that it 
can be inserted in the larger coil. 

If the key K of the circuit is initially open and the key is 
then closed there is a deflection of the galvanometer needle 
which only lasts for an instant. When the current in the coil P 
is broken by withdrawing the plug, a momentary deflection of 
the galvanometer needle is again obtained but in the opposite 
direction. 

The coils P and S are known as the primary and secondary 
coils respectively. There are no lines of force through the primary 
coil before it is closed, but when the current flows, lines of force 
are linked through it. Thus the starting of the current produces 
an increase in the number of lines of force through the secondary 
coil. This gives rise to an induced E.M.F. (see p. 156) in the 
secondary coil, which produces an induced current through this 
coil. 

When the primary circuit is broken, there is a decrease in 
the number of lines of force through it and a corresponding 
decrease in the number of lines of force linked through the 
secondary coil. This gives rise to an induced E.M.F. in the 
secondary coil and an induced current flows in the opposite 
direction. 

THE TRANSFORMER 

The transformer is an application of mutual induction. Two 
coils of wire, the primary P and the secondary S, are wound on 
the same core of soft iron (Fig. 103) . When an alternating current 
passes through the primary coil, first in one direction and then 
in the other, the magnetic field in the primary coil suffers a 
series of reversals. This produces reversals of the magnetic 
flux through the secondary coil with a consequent induced 
alternating current. When the primary contains only a few 
turns and the secondary a large number of turns, a small poten- 
tial difference (see p. 141) applied to the ends of the primary 
coil produces a large potential difference between the ends of the 
secondary coil. Thisis known as a step-up transformer (Fig. iO3a) . 

Fig. I03b shows a step-down transformer, in which there is a 



MAGNETISM AND ELECTROMAGNETISM 135 













LOW VOLTAGE 






MM 

M 


HIGH VOLTAGE 


P 








S 



(a) 













HIGH VOLTAGE 


Mi 

tmm 


M 




^ 


MM 


LOW VOLTAGE 


P 






S 


(b) 

FIG.IO3 



SUPPLY 



PRING. 
CONTACT 




TO MACHINE 

large number of turns in the primary coil and a small number 
in the secondary coil. In this case a large potential difference 
applied to the terminals of the primary coil produces a small 
potential difference between the ends of the secondary coil. 



136 SCIENCE FOR COALMINING STUDENTS 
THE USE OF TRANSFORMERS IN MINES 

Transformers are used extensively in mining practice both 
on the surface and underground. They enable the supply 
voltage to be altered to suit the varying requirements of the 
electrical equipment used. For example, the supply voltage 
may be 2,200 volts A.C. and the following equipment may have 
to be supplied: 

(1) A pump motor at 220 volts A.C. 

(2) An electric coal-drilling machine at no volts A.C. 

(3) A coal-cutter at 440 volts A.C. 

(4) An underground lighting system at no volts A.C. 

(5) An electrically driven haulage engine at 440 volts A.C. 

In each of the above-mentioned cases, the supply voltage 
must be stepped down to meet the requirements of the motor 
involved and thus suitable transformers will have to be in- 
stalled. 

FURTHER PRACTICAL APPLICATIONS OF ELECTROMAGNETISM 

The No Volts Release 

In order to prevent accidents due to the sudden starting-up 
of a machine in the event of a failure in the electric supply, 
certain types of switch-gear incorporate a device known as the 
no volts release which operates if the power is cut off. This ren- 
ders the circuit between the switch-box and the machine dead 
even on recommencement of the supply and the circuit remains 
dead until the switch in the box is inserted again. 

The remote control type of electrically driven coal-cutter is 
protected in this manner and the student will readily appreciate 
the danger of sudden starting-up of the cutter chains and picks 
if such protection in the switch-gear is not provided. The 
magnetic property of a current-carrying solenoid is the prin- 
ciple on which the no volts release is based and the arrangement 
is shown in Fig 104. 

When the push is moved to the left, the contacts in the 
switch are closed and the power is supplied to the machine. 
The solenoid becomes a magnet and the soft-iron core is 
attracted. This keeps the contacts in the closed position and 
the springs are in considerable tension. 

On failure of the supply the magnetic effect of the solenoid is 
lost and the springs open the contacts, which can only be 
closed again by manual operation. 



MAGNETISM AND ELECTROMAGNETISM 



137 



The Overload Trip 

A common feature of machinery driven by electric power is 
the danger of overloading for excessive periods. In such circum- 
stances the heat produced in conductors carrying heavy load 
currents is much greater than normal. This may be detrimental 
to the insulation and may finally cause failure of the conductors. 

To prevent such occurrences, the machines may be protected 
by incorporating in the switch-gear an overload trip which, in 

SUPPLY MAINS 



J SOFT IRON / . 

I f i . ,<*-/ /. 

A | 1 



SOLENOID 



SPRING x 
CONTACT 



SWITCH 



SOFT IRON- 



-SOLENOID 
B 



^SPRING 



l 



PUSH 



TO MACHINE 



FIG. 105 



the event of the machine being overloaded to a dangerous 
extent, causes tripping of the switch, thus cutting off the 
supply to the machine and so preventing serious damage. 

The overload trip depends on the magnetic effect of a solenoid 
conveying a current. The solenoid B (Fig. 105) carries full 
load current and when overloaded attracts the soft-iron core 
against the action of the spring. This opens the switch O which 
causes the solenoid A, i.e. the no volts release, to be demag- 
netised. The circuit is thus opened and has to be remade by 
manual operation. 



138 SCIENCE FOR COALMINING STUDENTS 

Shot-firing Batteries 

The main types of shot-firing batteries are (i) batteries which 
rely for their operation on a contained dry cell and (2) batteries 
which utilise the effect of an induced E.M.F. when a coil is 
rotated in a magnetic field. 

A diagram and description of a shot-firing battery which is 
sometimes called an exploder is given below. 



SECTIONAL ELEVATION 




SIDE JLEVATiON 

k j i 




T 'V 



K 


SHORT CIRCUIT 
WITHIN BATTERY 

ARMATURE 


CIRCUIT 
DIAGRAM 


1 COIL | 


TO EXTERNAL CIRCUIT 



(C) 

FIC.IO6 



The battery is provided with a permanent magnet of the 
horse-shoe type (Fig. 106). Two brass plates B, enclosing the 
magnet M, carry the driving and armature shafts, while one of 
the plates and the battery casing C carry a short counter-shaft 
S (Fig. io6a). 

The driving shaft D carries a sector gear wheel E which 
engages with a pinion F on the counter-shaft. A spur wheel G 
carried by the counter-shaft engages the pinion H on the arma- 



MAGNETISM AND ELECTROMAGNETISM 139 

ture shaft A. This system of gears enables the armature to be 
rotated at a fairly high spewed when the driving shaft is turned 
by means of a removable handle. 

The armature carries a winding of copper wire which is 
provided with a silk covering and when it is rotated between 
the poles of the permanent magnet, an E.M.F. is induced 
in it. 

The armature winding is arranged with one end insulated 
from the armature shaft and casing. This end is connected to a 
terminal Tj which is insulated from the casing. The other end 
of the armature winding is connected to the armature shaft 
and the circuit is completed by means of the shafts and brass 
plates to the casing and the other terminal T 2 (Fig. io6b). The 
detonator is connected across the terminals T! and T 2 . 

The armature winding is short-circuited by means of a spring 
contact J which makes contact with a brass disc K, mounted 
on the driving shaft. This short circuit persists for the greater 
portion of the twist of the handle. Thus the E.M.F. induced in 
the armature coil is applied to both the short circuit and the 
external detonator circuit (Fig. io6c). 

In the case of the low-tension detonator, the induced E.M.F. 
in the armature coil produces a sufficient current to fire the 
detonator. In the case of the high-tension detonator, however, 
this induced current is insufficient to fire the detonator. The 
disc K is therefore designed with a slot in it and, near the end 
of the turn of the handle, the spring contact enters the slot, 
thus breaking the short circuit. At this stage the induced 
E.M.F. due to the sudden stopping of the current in the short 
circuit becomes very considerable and is superimposed on the 
E.M.F. already applied to the detonator circuit. The current 
produced is then sufficient to fire the detonator. 

A high-tension exploder has a far greater number of turns 
in the armature coil than a low-tension exploder and the induced 
current is correspondingly larger. 

EXERCISES 

1. How would you demonstrate experimentally the difference in 
the magnetic properties of soft iron and steel? 

Describe a mining appliance in which use is made of either (a) a 
temporary magnet, or (b) a permanent magnet. (Min. Sc. ; U.L.C.I.) 

2. What is meant by the term ''Magnetic Declination"? Describe 
the variations which may occur in the value of the magnetic declination 
and mention the importance, if any, of these variations in connection 
with mine surveying. (Min. Sc. ; U.L.C.I.) 



J40 SCIENCE FOR COALMINING STUDENTS 

3. Describe the construction and action of a telephone. What special 
precautions are necessary in the design and installation of telephones 
for use in mines? * (Min. Sc. ; U.L.C.I.) 

4. State the principle on which is based the use of the miner's dial 
for mine surveying. Why is it necessary to indicate on the plan the 
date of a survey which has been plotted from the magnetic meridian? 
Describe and explain the effects of a live cable on the needle of a dial 
set up near it, and state if the effect would remain the same if the dial 
were moved from place to place in the region of the cable. 

(Min. Sc.; U.L.C.I.) 

5. Give an account of electro-magnetic induction, and describe 
experiments to illustrate your answer. Give examples of mining electrical 
machines or apparatus in which electromagnetic induction is of import- 
ance. (Min. Sc.; U.L.C.I.) 



CHAPTER XIII 

OHM'S LAW 
POTENTIAL DIFFERENCE 

Fig. 107 shows a horizontal tube XY connected to a reservoir 
R. Vertical tubes project from the tube XY at intervals. The 
reservoir R contains water which is maintained at a constant 
level by means of a tap and water flows along the tube XY. 
The pressures at various points A, C, E, etc., in the horizontal 
tube are given by the heights of the water columns, viz. AB, 
CD, EF, etc. The heights of these columns decrease uniformly, 
which shows that there is a uniform fall of water pressure from 
Xto Y. 

In the same manner when an electric current flows through 
a uniform wire, there is a difference of electric pressure between 
the ends of the wire and also a uniform fall of electric pressure 
along the wire. This difference of electric pressure, which is 
known as potential difference (P.D.), is produced by connecting 
the ends of the wire to a dynamo, an accumulator, etc. Potential 
difference is measured in volts by an instrument known as a 
voltmeter. 

CURRENT STRENGTH 

Just as the strength of a water current at a given point in a 
tube is the quantity of water which passes the point per second, 
so the strength of an electric current is the quantity of electricity 
which passes a given point in a wire per second. 

Current strength is measured in amperes by an instrument 
called an ammeter. 

OHM'S LAW 

A definite relationship exists between the current in a wire 
and the potential difference between its ends. This relationship, 
which was discovered by Ohm, a German scientist, states that 
the potential difference between the ends of a conductor is directly 
proportional to the current flowing in the conductor. 

If AB is the conductor (Fig. 108), I the current strength 

141 



142 SCIENCE FOR COALMINING STUDENTS 

and V the potential difference between the ends, we 
have : 

__=a constant. 

The value of this constant is known as the electrical resistance 
of the conductor. 

Thus ~=R, 

where V=the potential difference between A and B, 

I==the current in AB, 
and R=the resistance of conductor AB. 



Units 

The units of potential difference, current and resistance are 
the volt, the ampere and the ohm respectively. 

If V=i volt and I=i ampere, then R=i ohm. 

Thus a potential difference of i volt maintains a current of 
i ampere in a resistance of i ohm. 

Example. An electric lamp has a resistance of 1,000 ohms and is 
placed across the mains, the voltage of which is 230 volts. Find the 
current in the filament of the lamp. 

V 

Using r=R, we have: 

=230 volts, R = i,ooo ohms 
2 f= 1,000 

1 = ^30 amp. 

I,OOO 

= 0.23 amp. 

Example. A wire of 30 ohms resistance has a current of 1-5 amperes 
flowing through it. Find the P.D. between the ends of the wire. 

V 

Using =R, we have: 

1 = 1-5 amp., R=3O ohms. 
V 



45 volts. 



OHM S LAW 



143 



H 



B 




Y C E C A 

FIC.IO7 



-V 

FIG.IO8 




FiG.ioq 



(a) 



(b) 



FIG.IIO 



144 SCIENCE FOR COALMINING STUDENTS 
RESISTANCES IN SERIES AND IN PARALLEL 

Series 

Fig. loga shows two resistances in series. In this case the 
current is the same in each resistance. If Rj and R 2 are the 
separate resistances and R is the combined resistance, we have: 

P.I). between__P.D. between ,P.D. between 
A and C "~~ A and B B and C. 



Example. Two coils of resistance 50 ohms and 75 ohms are arranged 
in series across the 230-volt mains. Find the current in the coils and 
the J*.D. between the ends of the 75 -ohm coil. 

Combined resistance -=75 +50 ohms. 
Y - 125 ohms. 

From --R, we have: 

230 
y--5 

T 2 3 

I = -2-, amp. 

125 

1*84 amp. 
P.D. between ends of 75-ohiu coil 1R 

=--1-84x75 volts. 
=-138 volts. 

Parallel 

Fig. iogb shows two resistances arranged in parallel. The 
current I splits up at A into two branch currents, Ij along the 
resistance R lf and I 2 along the resistance R 2 . These currents 
recombine at B. 

If V=P.D. between A and B, we have: 

V V T 
y- K! or ~ ~LI 

l! KI 

and V V 

ir 2 R 2 2 

Also V V 

-- ==R or ~ =1, where R is the combined resistance. 
1 K. 

But I=Ii+I 2 

V = V V 
R RI R 2 

I == i+ I 
R RI R 2 



OHM'S LAW 145 

If three or more resistances are arranged in parallel, the com- 
bined resistance R is given by : 

i i.i.i., 

R = R 1 + R 2 + R 3 + etC " 

Example. Two coils of resistance 10 ohms and 15 ohms are arranged 
(a) in series, (b) in parallel. Find the combined resistance in each case. 

(a) The combined resistance R is given by 

R=-Ri+R 2 
=-104-15 ohms 
=25 ohms. 

(b) Let R the combined resistance. 

_L = .L+I 

R 10 15 

^A+_ 2 
30 30 

_! 
"30 

R~ =6 ohms. 
5 

Example. Ten lamps, each of 1,000 ohms resistance, are connected 
across the 230- volt mams (a) in series and (b) in parallel. Find the 
current in each lamp in each case (Fig. noa and b). 

(a) P.D. across AB = i,oooI volts, where I is the current. 
P.D. across BC = 1,000! volts, etc. 

230 -~io x i,oool 

T 2 3 

1=^ - amp. 
10,000 

=0-023 amp. 

(b) In this case each lamp is connected directly across the mains. 

V 



Hence ~ 



230 

~|- = 1,000 

_ 230 



1,000 
=0.23 amp. 

The total current taken by the ten lamps is 10x0-23 amp., i.e. 2*3 
amp. 

Series and Parallel Circuits in Shot-firing Operations 

The use of delayed action detonators in shot-firing operations 
requires that a group of detonators should be connected to- 
gether in series (Fig. ma) or in parallel (Fig. nib). Each 

K) 



146 SCIENCE FOR COALMINING STUDENTS 

detonator contains a time fuse and the detonators are timed to 
go off at predetermined instants. 

In simultaneous shot-firing operations, the detonators are 
coupled in series as in Fig. ma and contain no fuses. The first 
detonator is coupled to one cable with one of the wires and the 
last detonator of the round is coupled to the other cable. 

Shunts 

When only a fraction of the current in a circuit is required to 
traverse an ammeter a piece of wire, known as a shunt, is placed 
in parallel with the ammeter (Fig. 112). The length of the shunt 
wire and consequently its resistance is so chosen that the 
required fraction of the current passes through the ammeter 
and the remainder through the shunt. The fall of potential 
from X to Y is the same whether taken through the ammeter or 
through the shunt. 

Example. An ammeter, reading o-i ampere, has a resistance of 0-05 
ohm. What shunt must be placed in parallel with the ammeter so that 
it may read o-io amperes? 

Again considering Fig. 112, let Y be the resistance of the shunt. The 
current of 10 amperes is split up at X, and the ammeter only takes 
i ampere. 

Hence P.O. between X and Y through ammeter i xo-o5 volt 
and 1M>. between X and Y through shunt =9 r volt 
9^ = 1x0-05 

0-05 _ . 

r = - -=0-0056 ohm. 
9 

AMMETERS AND VOLTMETERS 

The function of an ammeter is to measure the strength of the 
current in a circuit. The ammeter is placed in series with the 
various coils, etc., and its resistance must be very small. Thus 
the resistance of the circuit is scarcely affected and conse- 
quently the presence of the ammeter does not appreciably 
affect the current. In the last chapter the construction of the 
moving-coil galvanometer (Fig. gsa) was discussed. A moving- 
coil ammeter is constructed on similar lines, the only addition 
being a shunt wire AB (Fig. 113) of low resistance, placed in 
parallel with the moving coil. This shunt wire reduces the 
resistance of the instrument to the very small value which is 
essential in the case of an ammeter. 

As we have already seen, the function of a voltmeter is to 
measure the potential difference between the ends of a con- 
ductor. The voltmeter must therefore be placed in parallel 
with the conductor AB as in Fig. 114, and its resistance must be 



OHM S LAW 



147 



^ 


^Mi 

y 









M 




1 k 


1 


X 


r r 


Cb) 

FIG. Ill 




SHUNT 
FIG. 112 





FIG.II3 



very high so that the current in the conductor is not materially 
altered by its presence. The galvanometer shown in Fig. 95a 
(Ch. XII) may be made into a voltmeter by placing a very high 
resistance AB in series with the moving coil (Fig. 115). 



SCIENCE FOR COALMINING STUDENTS 

SPECIFIC RESISTANCE 

The resistance of a centimetre cube of a material is known 
as the specific resistance or resistivity of the material and is 
measured in ohms per centimetre cube. 

Fig. n6a represents a row of / cm. cubes. If s is the specific 






FIG. 116 



OHM S LAW 149 

resistance of the material (i.e. the resistance of each cm. cube), 
then the resistance of the row of cubes is equal to Is ohms. 
If we have A rows (Fig. n6b), each row consisting of / cm. 
cubes, the combined resistance of the A rows is given by R, 

where ^=T-+T-+ to A terms 

R Is Is 



ls 



Evidently R varies as I when A and 5 are constant 

and R varies as T when I and 5 are constant. 

A 

If we take the inch as the unit of length, the specific resistance 
is measured in ohms per inch cube. 

Example. What is the resistance of a mile of copper wire, if the di- 
ameter of the wire is o-i inch and the specific resistance 0-00000067 
ohm per in. cube? 

From R --, we have: 
A 

^ 0*00000067 X 1760 x 36 , 
K onms 

7 X (0-1)2 
4 
=5-40 ohms. 



EXERCISES 

1. The potential difference between the ends of a wire is 220 volts. 
If the resistance of the wire is 125 ohms, calculate the current in the 
wire. 

2. A current of 1-5 amperes passes through a wire of resistance 25 
ohms. Find the potential difference between the ends of the wire. 

3. A lamp requires a current of ampere. If the lamp is to be con- 
nected to a 220- volt supply, what must be its resistance? What would 
be the effect of putting this lamp on a loo-volt supply? 

(P.S.T.2.; U.L.C.I.) 

4. Current is taken from a 2 20- volt supply and passes first through 
a resistance RI of 50 ohms and then through a spiral coil R2 of resistance 
5 ohms. What is the reading on an ammeter placed in the circuit? 

(P.S.T.2.; U.L.C.I.) 

5. What resistance must be connected in series with a coil having a 
resistance of 60 ohms, if a current of 3 amperes is to flow through the 
circuit when supplied by mains at 210 volts? (P.S.T.2.; U.L.C.I.) 



150 SCIENCE FOR COALMINING STUDENTS 

6, Two wires of resistance 20 and 30 ohms are arranged (a) in series, 
(b) in parallel. Determine the resistance of the combination in each case. 

7. The specific resistance of copper is 0-00000067 ohm per inch cube. 
Calculate the resistance of a copper wire 100 yards long and 0-060 
inch diameter. 

8, 1,000 yards of 22 S.W.G. copper wire has a resistance of 39 ohms, 
the cross-sectional area of the wire being 0-000616 sq. in. Deduce from 
this the specific resistance or resistivity of copper. (U.L.C.l.) 

9. State Ohm's Law, and define the electrical terms you mention. 
An electrical measuring instrument, having a resistance of i ohm, 

shows its maximum deflection when a current of 2 amperes is flowing 
through it. Find (a) what resistance must be coupled in parallel with 
the instrument so that the maximum deflection will be obtained when 
the total current in the combined circuit is jo amperes, (b) what 
resistance must be coupled in series with the instrument so that the 
maximum deflection will be obtained when the total pressure across 
the combined circuit is 100 volts. (Min. Sc.; U.L.C.l.) 



CHAPTER XIV 

CELLS AND BATTERIES 
THE COMPLETE CIRCUIT 

ELECTROLYSIS 

The Electrolysis of Dilute Sulphuric Acid 

Fig. 117 shows two graduated tubes, provided with taps X 
and Y and connected by a branch tube which leads to the 
reservoir R. The lower ends of the tubes are closed by rubber 
stoppers through which pass copper wires which have thin 
sheets of platinum A and C soldered to them. The plate A at 
which the current enters is known as the anode and the plate C 
at which the current leaves the apparatus is known as the 
cathode. The apparatus is initially filled with the electrolyte, 
which consists of water, slightly acidulated with sulphuric acid. 
The sulphuric acid splits up into ions, viz. the hydrogen-ions 
and the sulphions. When the positive and negative terminals of 
an accumulator are connected to the anode and cathode 
respectively, the potential difference produces a current in the 
electrolyte and directs the hydrogen-ions to the cathode and 
the sulphions to the anode. 

Thus Sulphuric Acid=Hydrogen-ion+Sulphion. 

(appears at cathode) 

Sulphion+ Water== Sulphuric Acid+Oxygen 

(appears at anode) 

The volumes of hydrogen and oxygen liberated are in the 
ratio of two to one. 

CELLS 

The Simple Cell 

When two plates, one of copper and the other of zinc, each 
fitted with a screw terminal, are immersed in a vessel containing 
dilute sulphuric acid, and the terminals of the plates are con- 
nected to a galvanometer by means of ordinary connecting 
wire, taking care that the plates do not touch each other, a 
deflection is observed in the galvanometer, but after a few 
minutes the deflection decreases to zero (Fig. 118). 



152 



SCIENCE FOR COALMINING STUDENTS 



OXYGEN- 









-/ 

R 


f 


-HYDROGEN 


i^ 


I 




^ 


CC 

C 
^CATHODE 




\ \NODE 




FIG. 117 



FIG. 1(8 



/) 



M 



? 

^ 



FIG. 1 19 






T 5 






B 



FIG. 120 



r 



FIG. 121 



-VWWV ' 

(b) 



CELLS AND BATTERIES 153 

If the zinc plate is withdrawn and its surface is rubbed with 
mercury by means of a piece of cloth, when the plate is replaced 
in the acid to which a few crystals of potassium bichromate 
have been added the deflection of the galvanometer remains 
steady for a much longer period than before. 

The above arrangement constitutes a simple cell. A current 
flows from the copper plate to the zinc plate through the gal- 
vanometer and from the zinc to the copper plate through the 
cell. The terminal by which the current leaves the cell is called 
the positive terminal and the other is the negative terminal. 
The chemical t reaction is shown below: 

Zinc + Sulphuric Acid=Zinc Sulphate + Hydrogen 

The hydrogen produced collects on the copper plate and has 
the effect of reducing the current, which gradually decreases to 
zero. This effect is called polarisation and to prevent it a sub- 
stance such as potassium bichromate which is rich in oxygen 
is added. The hydrogen is oxidised to water in the manner 
shown below: 

Hydrogen + Oxygen Water. 

The potassium bichromate is known as the depolarising 
agent. 

Another defect of a simple cell is local action. Local action is 
the wearing away of the zinc plate due to impurities in the zinc, 
and it is prevented by amalgamating the plate, that is, by 
covering it with a coating of mercury. The zinc and the mercury 
form an amalgam and the defect of local action is practically 
eliminated. 

The Leclanche Cell 

The Leclanche cell (Fig. 119) consists of a porous pot A 
which contains a carbon rod B, surrounded by a mixture of gas 
carbon and manganese dioxide. The porous pot is contained in 
a glass vessel and the space between them contains a solution 
of ammonium chloride (sal-ammoniac). A zinc rod C is immersed 
in this solution and the zinc and the carbon rods are fitted with 
screw terminals. When these terminals are connected by a 
wire a current flows from the carbon (positive) to the zinc 
(negative) through the wire. The chemical reactions which take 
place are : 

Zinc + Ammonium Chloride =Zinc Chloride + Ammonia 

+Hydrogen. 



154 SCIENCE FOR COALMINING STUDENTS 

The manganese dioxide is the depolarising agent and supplies 
the oxygen required to oxidise the hydrogen. Thus we have : 

Hydrogen +Oxygen = Water. 

In spite of the fact that a depolarising agent is present, the 
current gradually decreases. This is because hydrogen is pro- 
duced at a greater rate than it can be oxidised. 

The Leclanch cell is therefore never used when a continuous 
current is required for a considerable time. For an intermittent 
current, however, such as that required for an electric bell, it 
is a most useful cell, because in the intervals between the ring- 
ings of the bell, the cell has time to recover. 

Leclanche cells are used in the mine for electric bell and 
signalling circuits, 

The Accumulator 

The accumulator, or storage cell as it is sometimes called, 
has a greater practical usage than a Leclanch6 cell, as it supplies 
a continuous current over a long period. In its simplest form it 
consists of two lead plates immersed in dilute sulphuric acid, 
contained in a vessel generally made of glass or celluloid. The 
positive plate is covered with lead dioxide, which has a choco- 
late colour, while the negative plate has a covering of spongy 
lead, which is slate-coloured. When the terminals attached to 
the plates are connected by a conductor, a current flows from 
the positive to the negative terminal through the conductor. 
In this case the cell is discharging and the chemical reactions 
taking place are : 

At positive plate * 

Lead Dioxide -f Sulphuric Acid + Hydrogen* 

Lead Sulphate + Water. 

At negative plate 

Lead + Sulphuric Acid -(-Oxygen* =Lead Sulphate + Water. 

The lead plates become covered with a coating of white lead 
sulphate and some of the sulphuric acid is used up. This causes 
the relative density of the acid to fall from 1-27 when fully 
charged to 1-2 when discharged. In Chapter II we saw how the 
relative density of the acid could be measured by a small 
hydrometer and how this test served to show when the cell is 
run down. The voltage of the cell also decreases from 2'i volts 
on full charge to 1-8 volts on discharge. 

To recharge the cell a current from a direct-current generator 



CELLS AND BATTERIES 155 

of suitable voltage is passed into the positive terminal of the cell 
and the following reactions take place : 

At positive plate 

Lead Sulphate + Wat er+ Oxygen* 

=Lead Dioxide + Sulphuric Acid. 

At negative plate 

Lead Sulphate +Hydrogen*= Lead + Sulphuric Acid. 

In the process of charging an accumulator, the lead sulphate 
on the positive plate is converted into lead dioxide and the 
lead sulphate on the negative plate is changed to spongy lead. 

The relative density of the acid increases owing to the 
formation of more sulphuric acid and the voltage rises to 2-1 
volts on full charge. 

In the manufacture of accumulators, lead-dissolving acid is 
added to the sulphuric acid and charging and discharging are 
repeated many times until the depth of the deposit on the 
plates is considerably increased. In this method, which is 
known as the Plant e process, the plates are said to be "formed/' 

An accumulator can only supply a limited quantity of elec- 
tricity, depending on its specification. This quantity is known 
as its capacity and if exceeded the cell becomes damaged. If an 
accumulator has a capacity of 50 ampere-hours, it is capable 
of supplying i ampere for 50 hours, 2 amperes for 25 hours, 
etc. If a greater quantity of electricity than specified is taken 
from the accumulator, excessive sulphating takes place, that 
is, too much lead sulphate is formed. There is also a safe upper 
limit to the strength of the current which can be taken from the 
accumulator. 

Dry Cells 

Dry cells are usually of the Leclanche type, the sal-ammoniac 
solution being replaced by a paste which is moistened with 
sal-ammoniac solution. The glass vessel is replaced by a zinc 
container which serves as the negative of the cell whereas the 
porous pot, containing the gas carbon, manganese dioxide and 
carbon rod, is replaced by a cardboard case. The cell is sealed 
at the top, a small vent hole being provided. 

* During discharge the current leaves at the positive plate, which becomes 
the cathode of an electrolytic cell, and hydrogen is liberated at this electrode. 
The current enters at the negative plate, which becomes the anode where 
oxygen is liberated. 

During charging the current enters at the positive plate, which becomes the 
anode at which oxygen is liberated. The current leaves at the negative plate, 
which becomes the cathode where hydrogen is liberated. 



156 SCIENCE FOR COALMINING STUDENTS 

In mining practice, dry cells are often used instead of the 
usual fluid cells. These dry cells possess the advantage of 
portability which enables them to be used in instruments such 
as the McLuckie methanometer. 



ELECTROMOTIVE FORCE 

If the tap T (Fig. 120) is open water flows from the vessel A 
to the vessel B. The water level in A falls, while that in B rises. 
In order to maintain the difference in water levels the same, 
the pump P draws water from the vessel B to the vessel A. 

A cell functions in a similar manner. We may imagine A and 
B to represent the positive and the negative plates of the cell 
respectively, and the difference in level when T is closed to 
represent the electromotive force of the cell. The tap T when 
open is analogous to the external resistance which is connected 
to the terminals of the cell, and the pump P is analogous 
to the chemical energy of the cell which causes the current 
to flow from the negative to the positive terminal in the 
cell. 

If R=the external resistance in ohms (Fig. I2ia), 

r=the internal resistance of the cell in ohms, 
I=the current in amperes, 

and E=the electromotive force of the cell in volts, 

then E.M.F.=P.D. to drive the current round the external 
circuit +P.D. to drive the current through the 
cell, 

i.e. E=IR+Ir 

T E 

' 



This relationship represents Ohm's Law for a complete circuit. 
Fig. I2ib shows a cell of electromotive force E in series with 
an external resistance and a plug key K. A voltmeter V is 
connected to the terminals of the cell. When the key K is open, 
no current flows through the external resistance and the volt- 
meter V, which takes an insignificant current, reads the electro- 
motive force (E.M.F.) of the cell. This is the total E.M.F. or the 
E.M.F. on open circuit. When the key K is closed, a current 
flows through the external resistance and the voltmeter 
registers the potential difference required to drive the current 
through this resistance or the E.M.F. on closed circuit, 



CELLS AND BATTERIES 157 

Example, A miner's electric lamp is connected across the terminals 
of a battery of E.M.F. 2-1 volts and internal resistance 0-2 ohm. If 
the resistance of the lamp filament is 5 ohms, calculate the current in 
the filament and the P.D. between the terminals of the battery. 

We have I ^- , 

R+r' 

where E E.M.F. of battery in volts, 

R -= external resistance in ohms, 
r internal resistance of battery in ohms. 
E 2-1 



r 5+0-2 

2-1 

0-404 amperes. 

P.I), between terminals of battery = 1R. 

=0-404x5 

= 2-02 VOltS. 



EXERCISES 

1. Distinguish between a primary and a secondary cell. Enumerate 
the common uses of each type at mines. Describe the construction and 
action of a primary cell commonly used in mines, and indicate how the 
cell is maintained in good condition. (Mm. Sc. ; U. L.C.I.) 

2. What is meant by the capacity of a battery? State the conditions 
which must be fulfilled by a battery suitable for use in a miner's electric 
hand lamp. Compare the advantages and disadvantages of lead-acid and 
nickel-iron cells for this purpose, and describe, with a sketch, the con- 
struction and action of one type of cell. (Min. Sc, ; U.L.C.I.) 

3. What conditions should be fulfilled by a battery suitable for use 
in a miner's electric hand lamp? Describe the construction of one type 
of cell, and indicate the chemical reactions which take place during 
charge and discharge. (Min. Sc.; U. L.C.I.) 

4. A miner's electric lamp is connected across the terminals of a 
battery of E.M.F. 2-0 volts and internal resistance 2-5 ohms. If the 
resistance of the lamp filament is 5 ohms, calculate the current in the 
filament and the P.D. between the terminals of the battery. 

5. A 1 2- volt battery whose resistance is 60 ohms is used to work a 
telegraph line 8 miles long, the resistance of the line wire being 10 ohms 
per mile. Calculate the current flowing in the circuit if the telegraph 
instrument, which is in series in the circuit, has a resistance of 100 
ohms. (P.S.T.2.; U.L.C.I.) 



CHAPTER XV 

HEATING EFFECTS OF CURRENT 

INTRODUCTION 

It is common knowledge that when an electric current passes 
through a wire heat is produced. In fact, the heat produced 
sometimes raises the temperature of the wire to such a value 
that it becomes incandescent, as in the case of an electric 
radiator or lamp. 

From the Principle of the Conservation of Energy, energy 
can neither be created nor destroyed, but it can be transformed 
from one form into another. Thus the appearance of heat in a 
wire due to the passage of an electric current suggests that 
energy is expended in maintaining the current and the conse- 
quent supply of heat associated with it. This energy comes 
from the dynamo or from the stored-up energy of the cell to 
which the wire is connected. 

The Joule 

The unit of electrical energy is the joule and a joule is the 
energy consumed when a potential difference of one volt 
maintains a current of one ampere in a conductor for one 
second. Thus: 

Energy consumed when a P.D. of i volt maifitains a 

current of I amp. for i second=i joule. 
Energy consumed when a P.D. of V volts maintains a 

current of i amp. for i second =V joules. 
Energy consumed when a P.D. of V volts maintains a 

current of I amp. for i second = VI joules, 
and Energy consumed when a P.D. of V volts maintains a 

current of I amp. for t seconds = Vlt joules. 

i.e. W=VIt, where W=the energy consumed in joules. 
But V=IR, where R=the resistance of the conductor. 
Hence W=PR* 

V 2 ' 
or W=-. 

158 



HEATING EFFECTS OF CURRENT 159 

But from the mechanical equivalent of heat, 

i calorie =4-2 joules. 
Hence if H is the heat produced (expressed in calories) we have 3 



4-2 

tr VM 

or H = 

4-2 

u V2t 
or H = -, 

4-2R 

where V, I, R and t are expressed in volts, amperes, ohms and 
seconds respectively. 

Example. How much heat is produced per hour in a cable 1,000 ft. 
long, if the resistance per foot is 0-05 ohm and a current of 5 amperes 
flows? 

Resistance of cable i, ooo X 0-05 ohms 
=50 ohms. 

Heat produced per hour ---- calories 

5 2 X 50 x 60 x 60 . 
^3 -J -- _ calorie* 

4^ 
1,071,428 calories. 

POWER 

We have already seen in Chapter V that the power of a 
machine is the rate at which the machine performs work. 

In Electricity, the rate at which a machine generates energy 
or the rate at which energy is consumed in a circuit is known as 
the power. 

From W=VI, we have: 

W 
Power =1 :=VI 

t 

i.e. P=VI, where P is the power. 

Power is measured in watts. Thus we have: 
Watts = Volts x Amperes. 

The Watt 

If one joule of energy is consumed per second in a circuit, 
the power of the circuit is a watt. 
The kilowatt ~i,ooo watts, 
and one Horse Power (H.P.)=746 watts. 



l6o SCIENCE FOR COALMINING STUDENTS 

Example. A 60- watt lamp is placed across the mains whose voltage 
is 230 volts. Find the current in the lamp filament. 

From P=VI, where V,T and P are the voltage, current and power 
respectively, we have: 

60230! 

T 60 

I amp. 

230 

0*26 amp. 
The Board of Trade Unit. 

Alargerunitof electricity than the jouleistheBoard of Trade 
Unit or the Kilowatt-hour, which is the energy consumed 
in a circuit in one hour when the power is one kilowatt. 

Thns i B.O.T. unit i.ooo ( ^-1x3,600 seconds. 

\ seconds/ " 

3,600,000 joules. 

Example. How long will one unit of electricity maintain a lamp of 
100 watts? 

100 watts = ioo joules per second, 
i unit of electricity =3, 600, ooo joules. 

,. 3,600,000 , 

rimc = - -- seconds 

100 

=36,000 seconds 

36,000 , 

hours 

60x60 

~io hours. 

Example. In a shaft inset there are 12 lamps arranged in parallel, 
each lamp taking 0-3 ampere at no volts. Calculate the cost of lighting 
for a period of 100 hours at 2d. per B.O.T. unit. 
Knergy consumed per lamp=VI/ joules 

= 1 10 X 0-3 X 100 X 60 x 60 joules. 

Energy consumed by 12 lamps no x 0-3 x 100x60 x 60x12 joules. 
Number of B.O.T. units con- _ 110x0-3 Xioo X6ox6o x 2 
sinned 3,600,000 

^39-6 

Cost of lighting =39-6x2 pence 
79-2 pence 
=6s. 7d. 

PRACTICAL APPLICATIONS 

The heat developed in a conductor due to an electric current 
is utilised in industry in a variety of ways, but mining practice 
is mainly concerned with the waste of energy in the cables 
used in electrical transmission. This energy loss, viz. I 2 R, where 
R is the resistance of the cable, I the current and i the time, has 
to be reduced as far as possible, especially when the electrical 
energy has to be conveyed considerable distances underground. 



HEATING EFFECTS OF CURRENT l6l 

In Chapter XII we discussed the use of transformers for 
stepping-down a voltage of say 2,200 volts for various require- 
ments. High-voltage supply is used in order that the required 
power (I 2 R) may be transmitted with a low current value. This 
reduces the I 2 R loss in the supply cables, since the value of the 
current is small, and it also enables conductors of smaller 
diameter to be introduced into the cables with a resulting 
decrease in the cost of manufacture. The overall effect is a con- 
siderable economic saving. 

Electric Lamps 

If the potential difference between the ends of a wire is 
gradually increased, the current increases in the same ratio 
(Ohm's Law). Moreover, the heat produced per second is pro- 
portional to the square of the current. Thus as the potential 
difference increases the heat produced increases at a greater 
rate. If the wire is composed of platinum or tungsten, which 
have high melting points, the temperature of the wire increases 
to such an extent that heat radiations are emitted, then dull 
red light, then bright red light and, if the temperature rise is 
sufficiently high, intense white light is emitted. Thus it can be 
seen that in order to obtain white light, the filament must be 
made of a metal of very high melting point, because the wire 
must not melt at a temperature lower than that required for 
such radiations. 

Also at high temperatures metals oxidise, i.e. they combine 
with the oxygen of the air. This produces disintegration of the 
filament, which was a serious defect in the earlier types of 
lamp. To prevent the oxidation of the material of the filament 
vacuum lamps were invented. But even in vacuum lamps the 
filament vaporises and ultimately disintegrates. Modern lamps, 
however, are first evacuated and then filled with some inert 
gas such as nitrogen or argon. This process practically eliminates 
disintegration. 

In Fig. 122 a gas-filled tungsten lamp is shown. XY is the 
glass rod, from which radial supports for the filament branch 
out. The leads P and Q pass from the contact plate C, through 
the glass rod to the ends of the filament AB. 

Fuses 

When a steady current passes through a wire, the heat 
developed raises the temperature of the wire to a steady value, 
when the heat developed per second is equal to the heat lost 



1 62 



SCIENCE FOR COALMINING STUDENTS 



from the surface of the wire per second. The greater the current, 
the higher is this steady temperature. With wires made of 
copper of moderately high melting point, the current carried 
by the wire may increase to such an extent that this steady 
temperature becomes very considerable. However, with wires 
made of lead or tin, whose melting points are low, the current 
may be such that the steady temperature is not reached before 
the wire melts. 

The maximum current which a wire will carry depends mainly 
on the diameter, the material of which it is composed and the 
condition of its surface. If we have two wires of the same 




FIG. 122 

material and the same surface conditions, the wire with the 
greater diameter will carry more current than the one of smaller 
diameter. Also if two wires have the same diameter, the same 
resistivity and the same surface conditions, the one with the 
higher melting point will carry more current than the other 
without being unduly heated. 

In an electric circuit where copper wires are in use, large 
currents may cause the steady temperature mentioned above 
to be very high and damage to the insulation or even a break- 
down may occur. To prevent excessive currents a safety 
device is employed. This is the principle of the fuse wire. Fuse 
wires are generally made of tin or an alloy of tin and lead. 
These wires are inserted in the circuit at some accessible place, 



HEATING EFFECTS OF CURRENT 163 

viz. the fuse-box, and melt when the current reaches some 
predetermined value. 

In many mining appliances fuses are utilised as a protection 
for the electrical gear. 

Efficiency of an Electric Motor 

Owing to the PR loss in the armature coils and other 
losses, the energy yielded at the pulley of an electric motor 
per second is less than the energy input per second. 

The efficiency of the motor is given by : 

Efficiency- Energy out P ut P er second at pulley 
Energy input per second 



EXERCISES 

1. A 6o-watt lamp is connected to a 230- volt supply. What current 
does it take and what is the resistance of the filament? 

2. A lamp consumes energy at the rate of 100 watts. How long can 
it be lit for one B.O.T. unit of electricity? 

3. A lamp consumes energy at the rate of 60 watts for 10 hours. 
What is the cost of the energy at 4d. per B.O.T. unit? 

4. In a colliery there are 40 lamps arranged in parallel, each lamp 
taking 0-3 amp. at 230 volts. Calculate the cost of lighting for a working 
week of 48 hours at 2d. per B.O.T. unit. 

5. An electric motor is receiving 4,500 watts of electrical power and 
the supply voltage is 230 volts. What is the current, and how many 
Board of Trade units of electrical energy will it consume if run for 
3 hours? If its efficiency be 85 per cent., what will be the horse-power 
yielded at the pulley? (i h.p. =746 watts.) 

6. A pump lifts 85 gallons of water per minute against a head of 35 
feet. The electric motor driving the pump takes a current of 10 amp. 
at 200 volts. 

Determine (a) the overall efficiency of the set, (b) the number of 
B.O.T. units taken by the motor in 2 hours, (c) the cost of running the 
motor for 2 hours at i Jd. per B.O.T. unit. (Min. Sc.; U.L.C.I.) 

7. State Joule's Law. How is excessive heating prevented in electric 
cables ? 

A cable transmits 60 kilowatts at 500 volts to a motor. If the loss in 
transmission between motor and generator is 6 per cent, of the power 
at the generator, calculate the quantity of heat developed in the cable 
per minute. (Min. Sc.; U.L.C.I.) 

8. Power is transmitted by means of a twin-core cable from a genera- 
tor to a D.C. motor taking 100 h.p. at 500 volts, and situated 1,500 yd. 
distant. If the cross-sectional area of the cable conductors is 0-15 
sq. in. calculate, as a percentage of the power at the generator, the 
power lost in transmission. (Specific resistance of the cable conductors = 
o66 x io~ 6 ohms per cu. in.) (Min. Sc. ; U.L.C.I.) 



CHAPTER XVI 

ILLUMINATION, REFLECTION AND 
REFRACTION OF LIGHT, LENSES 

PHOTOMETRY 

Sources of Illumination 

We have already discussed the lighting produced when an 
electric current passes through the filament of a lamp. Prior 
to this mode of lighting, the chief sources of illumination of 
an artificial character were the flames from the ordinary candle, 
the gas jet, the oil lamp, etc., while recently fluorescent lighting 
has come to the forefront. As far as the miner is concerned, 
however, the most important sources of illumination are the 
flame from the oil lamp and the incandescence of the filament 
in an electric glow lamp. 

Now different sources give out different quantities of light, 
as in the case of an electric lamp, which may give out 32 times 
as much light as a candle. The amount of light given out by a 
source is known as the intensity or the illuminating power of 
the source, and to express this illuminating power it is necessary 
to choose a unit. The unit agreed upon by all international 
scientific bodies is known as the standard candle power, which 
is one-tenth of the illuminating power of a Harcourt Pentane 
Lamp, adjusted to standard conditions. This lamp burns pen- 
tane and is shown in Fig. 123. We are therefore able to express 
the illuminating power of any source of light in standard 
candle power. 

RECTILINEAR PROPAGATION OF LIGHT 

Light travels in straight lines. This can be verified by using 
the simple apparatus shown in Fig. 124. The apparatus con- 
sists of a metallic box L in which an electric lamp is fitted and 
a small circular hole is cut in one side of the box. S is a white 
screen and a triangular sheet of metal ABC is held between 
the screen and the lamp box in a plane parallel to the plane 
of the screen. When the current in the lamp is switched on, a 
shadow Aj Bj Cj is produced on the screen and the shape 
of the shadow is exactly similar to that of the metallic sheet. 



ILLUMINATION, REFLECTION AND REFRACTION 165 




FIG.I23 




FIG.I24 




Intensity of Illumination 

The intensity of illumination* on a surface is the quantity 
of light received by unit area of the surface. 

* Intensity of Illumination on a surface is sometimes called the Illumination 
on the surface. 



i66 



SCIENCE FOR COALMINING STUDENTS 



THE INVERSE SQUARE LAW 

Fig. 125 shows a wire frame in the form of a pyramid. The 
point S represents a point source of light, and the distances of 
the squares IJKL, EFGH, MNPQ and ABCD from the point S 
are in the ratio 1:2 .'3:4. The squares ABCD, MNPQ and 
EFGH contain 16, 9 and 4 squares respectively, each of the 
same area as the square IJKL. The amount of light passing 
through the square EFGH is equal to the amount of light 
passing through IJKL. Thus the intensity of illumination on 

the square EFGH is , i.e. - 1 -, of the intensity of illumination 

on the square IJKL. Also the amount of light passing through 
the square ABCD is equal to the quantity of light passing 
through the square IJKL and thus the intensity of illumination 

on the square ABCD is -fV i.e. ~, of the intensity of illumina- 

4 
tion on IJKL. 



Distance 


i 


2 


3 


4 


Area .... 


i 


22 


3 2 


4 2 






j 


i 


I 


Intensity of Illumination 


i 


S 


T 2 


4 2 



The table given above shows that : 

T ' * 

I varies as , 
a 2 

where I is the intensity of illumination and d the distance from 
the source. 

Thus the intensity of illumination at a point due to a small 
source of light varies inversely as the square of the distance of 
the point from the source. 

RELATION BETWEEN THE ILLUMINATING POWER (CANDLE 
POWER) OF A SOURCE AND THE INTENSITY OF ILLUMINA- 
TION IT PRODUCES AT A POINT 

Definition 

Unit intensity of illumination is the amount of light received 
from a source of unit candle power by unit area placed at unit 
distance from the source and at right angles to the direction of 
the light. 



ILLUMINATION, REFLECTION AND REFRACTION 167 

Thus intensity of illumination due to i.C.P. at i foot 

=i ft.-candle. 
intensity of illumination due to P candle power at i foot 

=P ft.-candles, 
and intensity of illumination due to P candle power at d feet 

p 
= -~ft.-candles, 

P .... 

i.e. I 3^, where I the intensity of illumination. 

a 2 

Example. What is the intensity of illumination due to a miner's 
safety lamp, of candle power i -6, at a distance of 3 feet from the lamp ? 

p 

We have I = -r> 

fc 

where P=caiidle power, 

d distance in feet, 
and I = intensity of illumination in ft.-candles. 

I ^Lf ft.-candle 

3* 
0-177 ft.-candle. 

MODERN UNITS 

In recent years, a different approach to the theory of photo- 
metry has been made and new units have been defined. 

Solid Angle 

Unit solid angle is the angle subtended at the centre of a 
sphere by an area of the surface equal to the radius squared. 
Hence the total solid angle at a point (i.e. the centre of the 

sphere) is equal to ^~, i.e. 477. 

The Lumen 

The unit of light quantity in the modern theory is the lumen, 
which is the quantity of light given out by a source of one 
candle power per unit solid angle. Thus if a source has an in- 
tensity or illuminating power of P candle power, it gives out 
P lumens per unit solid angle or 4?rP lumens in all directions. 

The Intensity of Illumination or the Illumination at a Point 
The intensity of illumination at a point at a distance of d cm. 

from a source of P candle power (i.e. P lumens per unit solid 

angle) 

_47rP (lumens) 



. cm.) 
== lumens per sq. cm. 



l(>8 SCIENCE FOR COALMINING STUDENTS 

If the distances are measured in feet, we have: 

T j. -x MI 4.- 4*7 (lumens) 

Intensity of illumination =- 7 - r r 7 

J 477^2 (sq. ft.) 

- lumens per sq. ft. 

Thus it can be* seen that the intensity of illumination at a 
point can either be expressed in lumens per sq. cm. or in cm.- 
candles when the distance is measured in centimetres, and in 
lumens per sq. ft. or in ft. -candles when the distance is measured 
in feet. 

PHOTOMETERS 

Rumford's Photometer 

Suppose two lamps are placed, one at A and the other at B 
(Fig. I26a), and a cylindrical rod R is placed in front of a white 
screen CD. The lamp at A and the rod R produce a shadow XZ 
and the lamp at B produces a shadow of the rod at XY. When 
the position of the rod and the distances of the lamps are 
adjusted the two shadows can be made to touch at X, and 
when the line of demarcation between the shadows disappears 
the two intensities of illumination are the same. The portions 
XY and XZ are illuminated by the lamps A and B respectively. 
For equal intensities we have : 



where P^candle power of A 

and P 2 =candle power of B. 

If the lamp A has a known candle power, then th$ candle 

power (P 2 ) of B can be calculated. 

Rumford's shadow photometer is shown in Fig. ia6b. 

Example. A standard lamp of 32 candle power is placed at a distance 
of 3-5 feet from the screen of a photometer. A lamp of unknown candle 
power, placed at 1-5 ft. from the screen, produces the same intensity 
of illumination. Find the candle power of this lamp. 

Let # unknown candle power of lamp. 

Intensity of illumination on the screen due to 32 c.p. ~r ft.-candles 

\o j i 

Intensity of illumination on the screen due to x c.p. - ft.-candles 

(i'5) 2 
x _ 32 

(i'5) 2 (3'5) 2 



=5-88 c. 



ILLUMINATION, REFLECTION AND REFRACTION 169 




LAMP 
0) 



LAMP 
(2) 



B 




M 



FIG. 127 



FIG. 128 



I7O SCIENCE FOR COALMINING STUDENTS 

The Flicker Photometer 

In the flicker photometer (Fig. 127) the screen consists of a 
plaster of Paris cylinder A with a bevelled edge. The axis oJ 
the cylinder is parallel to the line joining the two lamps, the 
candle powers of which have to be compared. The light from 
one lamp falls on one side of the bevelled edge and the light 
from the other lamp falls on the other side. A microscope M 
and a total reflecting prism P (see p. 176) are used for viewing 
the edges and the cylinder is rotated by clockwork. During 
one half of a revolution one side of the bevelled edge is seen in 
the focal plane of the microscope, and during the other half the 
other side is seen. If the lamps do not give equal illuminations 
on the two sides, flickering will result. The distances of the 
lamps are adjusted so that flickering ceases and then the 

P P 

illuminations are equal. The usual formula -! = T% * s then used, 

i 2 2 

This photometer is suitable for comparing the candle powers 
of two lamps which emit light of different colours. 

Selenium Cell Photometers 

When a beam of light is incident on a strip of selenium, the 
resistance of the selenium changes, and if the selenium strip is 
in series with a battery and a suitable ammeter, a change in 
the ammeter reading is recorded. 

This principle is applied in the photometer shown in Fig. 128, 
A casing X is closed at one end and at the other end the strip 
of selenium S, fitted with terminals, is placed. An opening foi 
the lamp L is situated near the closed end of the casing. Since 
the selenium strip takes the place of the eye in estimating the 
quantity of light, it is imperative to allow only those radiations 
to which the normal eye is sensitive to be incident on the strip, 
Hence a light filter which transmits only the required radiations 
is placed in front of the selenium strip. The strip of selenium 
and the filter constitute the cell. The selenium cell S is placed in 
series with a battery B and a sensitive ammeter A. A currenl 
flows through the selenium before the lamp is switched on and 
the reading of the pointer on the ammeter scale is marked zero 
Lamps of known candle power are placed in position and the 
corresponding deflections of the pointer when the lamps are 
switched on are marked to read these known candle powers, 
In this way the ammeter scale is calibrated, and if a lamp oJ 
unknown candle power is placed in position, when the lamp is 
switched on, the calibrated scale gives its candle power directly 



ILLUMINATION, REFLECTION AND REFRACTION 

There are two important disadvantages of this method. In 
the first place, it is essential to keep the battery fully charged 
so that the pointer is always opposite the zero of the scale 
before the lamp is switched on. Secondly, calibration methods 
have not proved too successful, owing to "fatigue" which 
develops in the selenium. 

The photometer (Fig. 129) which is about to be described 
depends on (i) the selenium cell principle and (2) the flicker 
photometer principle. The photometer head is in the form of 
an enclosed box, containing two right-angled isosceles reflecting 
prisms (see p. 176) which are arranged, as shown, opposite 
apertures in the sides of the box. Light from the standard lamp 
X is incident on the prism P, which reflects the light upon the 
selenium cell LM,* and light from the lamp Y under test is 
reflected by the prism Q upon the space MN. The selenium cell, 
which is in series with a battery B and a sensitive ammeter A, 
is oscillated backwards and forwards, by clockwork, between 
the positions LM and MN. Thus the selenium cell receives light 
from one lamp and then from the other. If the illuminations due 
to the two lamps are different, there is a fluctuation in the 
current which is shown by a flickering of the ammeter pointer. 
But if the illuminations are the same the flickering ceases. 
Thus the distances of the lamps are adjusted to give no flickering 

P P 

of the pointer and then the formula --I = T% can be applied, 

l ^2 

where P^the candle power of the standard lamp 
and P 2 =the candle power of the lamp under test. 

MEAN HORIZONTAL CANDLE POWER (M.H.C.P.) 

The candle power of a lamp in various directions is shown in 
Fig. I3oa. The candle power in each direction is marked on the 
diagram and when the points are joined the horizontal distribu- 
tion curve of the lamp is obtained. 

In most cases the candle power is nearly always the same in 
all directions and it is more accurate to plot candle power 
vertically against angle horizontally (Fig. I3ob). The mean 
horizontal candle power (M.H.C.P.) is then obtained by finding 
the mean ordinate. 

MINER'S LAMPS 

The flame safety lamp which is used mainly in testing for 
methane has already been discussed in Chapter X. Lamps used 
for illumination are of two types, the oil lamp and the electric 

* The cell is fitted with a light filter. 



172 



SCIENCE FOR COALMINING STUDENTS 




180 



OO 




60 



1 2O ISO 24O 3OO 360 

ANGLE *- 

FIC.I3O 



lamp. These electric lamps, which derive their current from 
small batteries, have comparatively low candle powers, which 
become smaller after the lamps have been in use for some time. 



ILLUMINATION, REFLECTION AND REFRACTION 173 
LIGHTING IN MINES 

The use of electric lighting underground has received con- 
siderable attention and in recent years marked improvements 
have been made. The introduction of miners 1 electric hand 
lamps and cap lamps has provided much improved lighting 
conditions which in turn have helped to reduce the number of 
cases of nystagmus and have also aided the efforts to obtain 
safer working conditions. The provision of general lighting 
underground at shaft insets and other places has increased the 
standard of lighting and has resulted in increased safety and 
efficiency. The whitening of roadways (roofs and sides) at 
positions where general lighting was installed has improved the 
efficiency of lighting, since the rays of light are reflected to a 
greater extent from a white surface than from a black surface. 
The Coal Mines (Lighting) General Regulations, May iyth, 
1947, indicate the importance of methods and standards of 
lighting and also indicate the more recent efforts to improve 
further the standard of lighting underground : 

Provision of general lighting at the coal face and the use of 
fluorescent lighting are in the experimental stage. 

REFLECTION AT PLANE SURFACES 

To illustrate the reflection of light, an arrangement is 
required which gives a thin parallel beam of light. Such an 
arrangement is the lamp-box (Fig. 131), which consists of a 
metallic box A with a slit S cut in one side. A suitable cylin- 
drical lens is placed in a support just in front of the slit and a 
lamp with a frosted globe is placed in position in the box. By 
means of this arrangement a strong parallel beam of light 
issues from the slit. 

If a plane mirror XY is supported on a sheet of drawing 
paper by means of two wooden blocks (Fig. 132) and a beam 
of light from a lamp-box is incident on the mirror at the point 
B, the beam is reflected along BC. If the points S, B, C and the 
trace of the mirror are marked and the lines SB, BC and the 
normal BN are drawn, the angles SBN and CBN are found to 
be equal. A set of results is shown below. 



Z.SBN 


Z.CBN 


35 
48 
61 


35 
48 
61 



174 SCIENCE FOR COALMINING STUDENTS 

The beam SB is called the incident beam, BC the reflected 
beam and the angles SBN and CBN are the angles of incidence 
and reflection respectively. 

Laws of Reflection at Plane Surfaces 

(i) The incident ray, the reflected ray and the normal to 
the mirror at the point of incidence are in the same plane. 
(2) The angle of incidence is equal to the angle of reflection. 

Position of the Image due to a Small Object in a Plane Mirror 

Fig. 133 represents a plane mirror XY, supported vertically 
on a sheet of paper, with a pin inserted at O. If pins are inserted 
at A and B in line with the image of O, and at C and D, also 
in line with the image of O, the image of O is at the point of 
intersection of AB and DC, i.e. at 1. If the trace of the mirror 
XY is drawn and I and O are joined, it will be found that IN is 
equal to ON and that 10 is at right angles to XY. 

Thus we can deduce that (i) the line joining the image and 
the object is at right angles to the mirror and (2) the distance 
of the image behind the mirror is equal to the distance of the 
object in front. 

THE OPTICAL SQUARE 

A plan of the optical square is shown in Fig. 134. This instru- 
ment is used in surveying for setting out right angles and con- 
sists of an enclosed cylindrical housing inside of which are two 
plane mirrors A and M. The half-silvered mirror A is mounted 
rigidly at an angle of 60 with the line of sight S^, while the 
mirror M is mounted so that it is capable of rotation about a 
vertical axis and it is adjusted by a screw attachment to be at 
an angle of 45 with the mirror A. 

On viewing one object O 1 directly through the unsilvered 
portion of the mirror A, the other object O 2 is adjusted until 
the images of the two objects are coincident (see inset). Then 
the lines drawn from both objects O 1 and 2 to the instrument 
station are at right angles to each other. The rays, with their 
inclinations to the mirrors, are shown in the diagram. 

REFRACTION AT PLANE SURFACES 

If a lamp-box L is placed on a sheet of white paper and a 
rectangular slab of glass is placed in the path of the beam IO, 
it will be noticed that the incident beam changes its direction, 
i.e. suffers refraction as it enters the glass, and again changes 



ILLUMINATION, REFLECTION AND REFRACTION 175 




FIG. 131 




FIC.I33 



PLAN VIEW 
FIG. 134 



SIGHT HOLE 




O? >MIRROR 
MIRROR A" 

(SECTION) 



176 SCIENCE FOR COALMINING STUDENTS 

direction at the second surface (Fig. I35a). But if the incident 
beam is at right angles to the surface of the slab, the ray passes 
through the slab undeviated (Fig. I35b). 

In Fig. I35a, IO is the incident ray at the air/glass surface, 
NOM is the normal at O and the angles NOI and O t OM are the 
angles of incidence and refraction respectively. Also OOx is 
the incident ray at the glass/air surface, M^Ni is the normal 
at O t and OR is the refracted ray. 

The following deductions can be made : 

(1) When a ray of light passes from air to glass, the ray is 
bent towards the normal in the glass, except in the case when 
the incident ray is at right angles to the surface, and when a 
ray of light passes from glass to air, it is bent away from the 
normal in the air. 

(2) The ray OiR is parallel to the ray IO and a ray of light 
passing through a parallel slab of glass is undeviated (Fig. I35a). 

(3) When a ray of light strikes a plane surface of glass at 
right angles, it continues along the same line undeviated 
(Fig. i 35 b). 

Deviation of a Ray of Light through a Triangular Prism 

If a triangular slab of glass ABC is placed in the path of a 
beam of light from a lamp-box, as shown in Fig. 136, the beam 
changes direction at each surface. IE is the incident ray, EF 
the refracted ray, and FR the emergent ray. NEO and MFO 
are the normals at E and F respectively, while the angles NEI, 
MFR and TSR are the angles of incidence, emergence and 
deviation respectively. 

* 
Total Reflecting Prisms 

In Fig. 137 ABC represents a right-angled isosceles prism, 
the angles A, B and C being 90, 45 and 45 respectively, 
and AF is the perpendicular drawn from A upon BC. If IE is 
an incident ray at right angles to the surface AB and directed 
towards F, this ray continues undeviated along EF and is re- 
flected along FG where the angle EFA is equal to the angle 
AFG. 

Now /_BFE=i8o 90 45 =45. 

Hence ZEFA=45=: /.AFG. 

Also ^GFC=90-45=45 . 

Hence / FGC=i8o 45 -45 



ILLUMINATION, REFLECTION AND REFRACTION 177 

R 



N 




FIG. 135 



12 




B 



178 SCIENCE FOR COALMINING STUDENTS 

Hence the ray FG strikes the surface AC at right angles and 
continues along GD undeviated. 

The principle of the total reflecting prism is employed in 
many optical instruments used in mine surveying. 

LENSES 

A lens is made of transparent material and it is bounded by 
spherical surfaces. If Q and C 2 are the centres of curvature of 
the faces, the line C^ is known as the Principal Axis and it 
passes through the point P which is called the Optical Centre 
of the lens (Fig. I38a) 

Suppose a slab is cut out of the centre of the lens and that the 
slab is divided into sections (Fig. I38b). Each section, except 
the centre one, acts as a triangular prism and deviates rays of 
light. In the case of the central section, a ray of light coincident 
with the principal axis passes through the section undeviated. 
Also rays of light parallel to the principal axis of the lens are 
deviated by the lens through the point F, which is known as 
the Principal Focus. The distance between the principal focus 
and the optical centre of the lens is known as the Focal Length. 

The lens considered above is a converging or convex lens, 
since it converges parallel rays to a point. Fig. I38c shows a 
diverging or concave lens which diverges rays parallel to the 
principal axis. The diverging rays appear to proceed from the 
principal focus F. 

Object and Image for a Convex Lens 

Let P be the optical centre of the lens (Fig. 139), F the 
principal focus, and OPF the principal axis. Let AOJ3 be an 
object at right angles to the principal axis. The ray AD, which 
is parallel to the principal axis, is refracted through the point 
F and, since the central portion of the lens may be regarded as 
a thin parallel slab, the ray AP passes through the optical 
centre of the lens undeviated. Hence the image of A is on DF 
produced and is also on AP produced. Thus the image of the 
point A is at Aj. Similarly the image of B is at B^ Thus AjBx is 
the image of the object AB and is inverted. 

Using the construction already described, Fig. 140 shows the 
size and position of the image due to an object placed at various 
distances from the lens. As the distance of the object from the 
lens decreases, the corresponding distance of the image increases. 

In the diagram shown (p. 180) the images are real as the actual 
rays pass through them. They can be projected on a screen and 
are inverted. 



ILLUMINATION, REFLECTION AND REFRACTION 






FIC.I39 



180 SCIENCE FOR COALMINING STUDENTS 



F (I) (2) (3) 




(I) (2) (3) 



FIG.I4O 




FIG. 141 




B 



FIG. 142 



ILLUMINATION, REFLECTION AND REFRACTION l8l 

If the distance of the object from the lens is less than the 
focal length PF, we obtain the adjoining ray diagram (Fig. 141). 
It will be noticed that rays from A and B after leaving the lens 
are diverged and, when produced backwards, form an erect, 
enlarged image A^. If the eye is placed in the position shown, 
we obtain the ray diagram for a simple microscope. The image 
is virtual, since the actual rays do not pass through it. 

THE COMPOUND MICROSCOPE 

A compound microscope consists of two lenses, the objective 
O and the eyepiece E. The object which is to be viewed, viz. 
AB (Fig. 142), is placed near the objective, at a distance slightly 




(b) 

FIG. 143 

greater than its focal length OF. A real inverted image AxBj is 
produced. This image is formed just within the principal focus 
of the eyepiece E and a final virtual image A 2 B 2 is produced. 
The position of the image A 2 B 2 is adjusted by means of the 
rack and pinion of the instrument until it is clearly seen at the 
least distance of distinct vision, viz. 25 cm. from the normal eye. 
The compound microscope is used by geologists for examining 
rock and coal sections mounted on slides. 

THE TELESCOPE 

The telescope (Fig. I43a), which is used for viewing distant 
objects, consists of two lenses, the objective O and the eyepiece 
E, XY being the principal axis of the system. AB and CO 
represent two parallel rays from a point on the edge of a 



l82 SCIENCE FOR COALMINING STUDENTS 

distant object,removed from the principal axis, and a real image 
pq is formed by the objective O. The image just comes within the 
principal focus of the eyepiece E and a final virtual image PQ is 
produced at the least distance of distinct vision. The image is 
inverted. 

Telescopes are used in many surveying instruments such as 
the theodolite, sextant, etc. The telescope of the theodolite is 
shown in Fig. I43b. The eyepiece E is focussed on the cross 
wires X by adjusting the tube in which its lenses are mounted. 
The telescope is then turned towards the distant object and 
the distance between the objective and the eyepiece is adjusted 
by a rack and pinion until a clear image of the distant object 
is obtained on the cross wires. 

EXERCISES 

1. Explain the following terms: foot-candle, candle power, mean 
horizontal candle power. Describe how the mean horizontal candle 
power of a miner's lamp may be determined. (Min. Sc. ; U.L.C.I.) 

2. State the principles relating to the reflection and refraction of 
light. How is a uniform distribution of light obtained from an electric 
safety lamp? (Min. Sc.; U.L.C.I.) 

3. What is the difference between reflection and refraction of light? 
Make sketches to illustrate how light is refracted in its passage 

through (a) a glass plate, (b) a prism, and (c) a double-convex lens. 

(Min. Sc.; U.L.C.I.) 

4. Sketch any type of photometer you have seen, and explain how it 
is used to compare the candle powers of two lamps. 

(Min. Sc.; U.L.C.I.) 

5. What is meant by the term "principal focus" as applied to a 
convex lens? 

A convex lens is used to produce an image of a blue print! Draw a 
diagram showing how an image magnified three times may be produced 
on a screen. (Min. Sc.; U.L.C.I.) 

6. Describe some form of photometer, and explain how you would 
use it to find the candle power of a miner's electric lamp. 

What is the intensity of illumination due to a lamp of 1*5 candle 
power, at a distance of 3 ft. from the lamp? (Min. Sc. ; U.L.C.I.) 

7. Draw a diagram to show the refraction of a ray of light through 
(a) a slab of glass with parallel sides, (b) a triangular glass prism, and 
(c) a bi-convex lens. Also, draw a diagram which shows how the eye 
sees an object through a convex lens, as in the case of a simple magnify- 
ing glass. (Min. Sc. ; U.L.C.I.) 

8. What is meant by "mean horizontal candle power"? 

In a photometer a miner's electric lamp, placed at a distance of 20 in. 
from the screen, produces the same intensity of illumination as a 
standard lamp of 12-5 candle power, placed at a distance of 60 in. from 
the screen. What is the candle power of the miner's lamp? 

(Min. Sc.; U.L.C.I.) 



ILLUMINATION, REFLECTION AND REFRACTION 183 

9. State the law of reflection of light at a plane reflecting surface. 
A beam of light makes an angle of incidence of 30 on a plane mirror. 

If the incident beam remains fixed and the mirror is rotated through 
an angle of 17, find, by calculation or by drawing, the angle through 
which the reflected ray is rotated. (Min. Sc. ; U .L.C.I.) 

10. What is meant by (a) candle power, and (b) intensity of illumina- 
tion? 

A standard lamp of candle power 16 is placed at a distance of 6 ft. 
from a screen. What is the intensity of illumination on the screen? 
A flame safety lamp, placed at a distance of 2 ft. from the screen, pro- 
duces the same intensity of illumination. What is the candle power of 
the safety lamp? (Min. Sc. ; U.L.C.I.) 



CHAPTER XVII 

THE CHEMISTRY OF AIR 

INTRODUCTION 

Having studied the atmosphere with regard to its weight and 
pressure, we shall now consider it from the point of view of its 
chemical composition. What are the gases which constitute the 
atmosphere and in what proportions do these gases exist? When 
these questions have been answered we shall then proceed 
further and study the composition of mine air. How does the 
composition of mine air differ from that of ordinary air? What 
are the dangerous and obnoxious gases in mine air and what are 
the statutory limits for these gases, consistent with the safety of 
the miner? 

The Heating of Metals in Air 

If a piece of copper foil is placed in a porcelain crucible 
which rests on a porcelain triangle, standing on a tripod, and 
the crucible is heated strongly, after a time it will be noticed 
that the copper is coated with a black substance. If the process 
is repeated with magnesium and then with lead these substances 
become coated with a white and a grey powdery substance 
respectively. 

If the experiment with the magnesium is repeated by first 
weighing the crucible and magnesium complete with lid, and 
if .the crucible is heated with a small space left for thfe air to 
enter, after allowing to cool a gain in weight is noticed. The 
small space allows air to enter the crucible but prevents the 
products of combustion from escaping. 

Evidently the magnesium combines with some constituent of 
the air. Now copper, lead and magnesium are metals and when 
heated strongly in air they combine with oxygen, one of its 
constituents, producing black copper oxide from copper, grey 
lead oxide from lead and white magnesium oxide from mag- 
nesium. The metals are said to oxidise and the process is known 
as oxidation. 

The Percentage Composition of the Air by Volume 
Fig. 144 shows a uniform tube about 40 cm. long, 1-5 cm. in 
diameter and closed at one end. The tube contains a small 

184 



THE CHEMISTRY OF AIR 185 

quantity of water which occupies i cm. of its length. If the 
thumb is placed over the open end, the tube can be supported 
in a stand with this end underneath the surface of water in a 
trough. If the water levels in the trough and the tube are 
adjusted to be the same, the length of the air column in the 
tube may be measured at atmospheric pressure. 

The diagram also shows a rubber stopper, fitted with a small 
glass container in which are placed a few pieces of solid pyro- 
gallol together with one or two pellets of sodium hydroxide. 
This mixture is known as alkaline pyrogallol. 

If the stopper is inserted in the tube, with its end still under 
water, and the tube is withdrawn and shaken vigorously, the 
oxygen of the air is absorbed by the alkaline pyrogallol. 

When the end of the tube with the stopper is again placed 
under the water and the stopper is removed, water rises 
up the tube. If the water levels in the tube and trough are 
adjusted to be the same so as to reduce the volume of the 
remaining gas to atmospheric pressure, it will be found that the 
gas occupies four-fifths of the original volume of the air column. 
Thus one-fifth of the air by volume consists of oxygen while 
the remaining four-fifths consists almost entirely of a gas 
known as nitrogen. 

The Rusting of Iron 

Fig. 145 shows a small muslin bag, containing iron filings 
moistened with water and fastened to the end of a knitting 
needle. If the bag is placed in a gas jar which rests on a beehive 
shelf in a trough containing water, it will be found that, after 
allowing the apparatus to stand for a few weeks, the water has 
risen one-fifth of the length of the jar and the filings have rusted. 
Actually the iron combines with the oxygen of the air, but this 
process, which is known as rusting, is much slower than actual 
burning. 

When the jar is removed from the trough and a lighted 
splint is placed inside, the flame is immediately extinguished. 
This shows that the remaining gas, nitrogen, does not support 
burning. 

PHYSICAL AND CHEMICAL CHANGES 

Ice, water and steam are three states of the same substance 
and the state is determined by the temperature. The change 
from one state to another is known as a physical change and 
there is no alteration in the weight of the substance. 

The action of heat on a copper rod. so long as the temperature 



l86 SCIENCE FOR COALMINING STUDENTS 

does not exceed a certain value, simply causes an increase in 
length and this is a physical change. But if the temperature is 
high enough, the copper rod becomes coated with a black tar- 
nish known as copper oxide and in this case a different substance 
is produced. Such a change is known as a chemical change, and 
as we have seen in the experiment on the burning of magnesium, 
it is accompanied by a change in weight. 

Elements 

An element is a simple substance which cannot be decom- 
posed into other substances by ordinary chemical or physical 
processes. Elements are generally classified into metals and 
non-metals. Thus copper, iron, aluminium, zinc, magnesium, 
etc., are metallic elements, whereas oxygen, nitrogen, carbon, 
sulphur, etc., are non-metallic elements. 

Compounds 

In addition to elements there are other substances which are 
more or less complex. These substances are composed of two 
or more elements in chemical combination and are known as 
chemical compounds. A chemical compound is entirely different 
in properties from the elements of which it is composed. Taking 
copper oxide as an example, if this substance is divided into 
small particles, each particle still consists of copper oxide, and 
however far the sub-division is carried there is no alteration 
in the composition of the substance. 

LAW OF DEFINITE PROPORTIONS 

If the experiment on the burning of magnesium is repeated 
several times with different weights of magnesium, it will be 
found that a definite weight of magnesium combines* with a 
definite weight of oxygen to form magnesium oxide. Thus 12 
grams of magnesium combines with 8 grams of oxygen to form 
20 grams of magnesium oxide, and 24 grams of magnesium 
combines with 16 grams of oxygen to form 40 grams of mag- 
nesium oxide. The ratio of magnesium to oxygen is 24 to 16 or 
3 to 2 or 60 per cent, magnesium to 40 per cent, oxygen. Thus 
we have the Law of Definite Proportions, which states that the 
ratio by weight in which elements combine chemically to form 
a definite chemical compound is always the same. 

LAW OF MULTIPLE PROPORTIONS 

If we consider the oxidation of carbon, under certain con- 
ditions one type of oxide is formed and under other conditions 
another type of oxide is formed. In the one case 12 grams of 



THE CHEMISTRY OF AIR 187 

carbon combine with 16 grams of oxygen to form 28 grams of 
carbon monoxide, whereas in the other case 12 grams of carbon 
combine with 32 grams of oxygen to form 44 grams of carbon 
dioxide. The ratio by weight in which the oxygen combines 
with the same weight of carbon is 2 to i. This leads to the 
Law of Multiple Proportions, which states that if an element 
combines with another element to form more than one chemical 
compound the weights of the one which combine with a fixed 
weight of the other bear a simple relation to one another. 

MIXTURES 

Sulphur and iron filings form a mixture which can be separ- 
ated by mechanical means. Thus if a magnet be placed in a 
mixture of sulphur and iron filings, the filings are attracted by 
the magnet and the sulphur is left behind. The percentage 
composition of the mixture may vary according to the quanti- 
ties of sulphur and iron present. 

If, however, a mixture of sulphur and iron filings be heated 
strongly a definite compound is produced, in which the per- 
centage composition is always the same. If the sulphur or the 
iron is in excess one of them remains unchanged at the end of 
the process. 

OXYGEN 

Preparation 

Fig. 146 shows an ignition tube A, fitted with a cork and 
delivery tube B and containing a mixture of potassium chlorate 
and manganese dioxide in the proportion of 9 to i by weight. 
An inverted gas jar C,, filled with water, stands on a beehive 
shelf D, in a trough containing water. A piece of iron gauze is 
wrapped round the tube and when the tube is heated strongly 
by a bunsen burner, oxygen is driven off. If the oxygen is 
allowed to escape for a time so as to displace all the air from 
the apparatus, a few jars of the gas can be collected by the 
downward displacement of water. Then the delivery tube 
should be disconnected from the ignition tube so as to prevent 
water from being drawn into the apparatus as it cools. 

The manganese dioxide is unchanged at the end of the experi- 
ment. It is known as a catalyst and merely improves the rate 
at which oxygen is given off. 

Properties 

Oxygen is a colourless, odourless gas which is only slightly 
soluble in water. When a glowing splint is introduced into a jar 



l88 SCIENCE FOR COALMINING STUDENTS 




FIG. 144 



FIG.I45 




FIG. 146 




(a) (b) (c) 

FIG. 147 



THE CHEMISTRY OF AIR 189 

containing oxygen, the splint bursts into flame. A piece of 
magnesium burns in air with a white flame, but when intro- 
duced into a jar containing oxygen, the flame is far more 
intense. A piece of glowing charcoal introduced into a jar con- 
taining oxygen bursts into flame. Thus we see that oxygen 
supports burning to a greater extent than air, but it does 
not burn. Elements which burn in oxygen are said to oxidise 
and form oxides. 

Physiological Properties of Oxygen 

Oxygen is the essential constituent of air for the purpose of 
supporting life. Hence it is necessary in mining practice to 
observe certain standards. 

The Coal Mines Act, 1911, states that the oxygen content of 
the air in a working place, with certain exceptions, shall not be 
less than 19 per cent, by volume. Observations have been made 
relating to the deficiency of oxygen in the atmosphere and the 
corresponding increase of carbon dioxide which occurs in a 
limited atmosphere in which combustion or respiration has been 
taking place for some time. The results show that 16 per cent, 
of oxygen in the atmosphere produces excessive panting which 
results in severe distress. If only n per cent, of oxygen is present 
in the atmosphere the effect is to cause collapse and ultimately 
death. 

THE BUNSEN FLAME 

When a bunsen burner is lit with the holes at the bottom 
open, air is drawn through the holes and the flame is nearly 
invisible. When the holes are partly open, the flame becomes 
more luminous. 

A bunsen flame consists of two distinct portions, an inner 
cone X which consists of unburnt gas (hydrocarbons, hydrogen, 
etc.) and an outer envelope Y in which the gas is burning, i.e. 
combining with the oxygen of the air. The outer envelope, 
which is at a very high temperature, is known as the oxidising 
flame, and certain metals when placed in it are oxidised. 
The inner cone is relatively colder (Fig. I47a). 

The structure of the flame can be shown by holding a match 
horizontally over the flame at a point just underneath the top 
of the inner zone. The effect is shown in Fig. I47b. 

Metals can be oxidised by using a blowpipe such as the one 
shown in Fig. I47C. The metal is placed in a porcelain lid and 
the blowpipe directs the bunsen flame upon it, so that the tip 



IQO SCIENCE FOR COALMINING STUDENTS 

"of the outer zone is in contact with the metal. The blast of air 
increases the supply of oxygen available for the oxidation. 

NITROGEN 

When all the oxygen in a jar containing air is used up by the 
burning of a substance such as phosphorus or by absorption 
with alkaline pyrogallol, the remaining gas is mainly nitrogen. 

Nitrogen can be prepared by passing air over heated copper 
turnings (Fig. 148). When water is allowed to run out of the 
vessel A into the bottle B, air is driven through the hard glass 
tube C which contains copper turnings heated to a high tempera- 
ture. The copper, in forming black copper oxide, removes the 
oxygen from the air and the residual gas nitrogen is collected 
over water in the jar D. 

If a lighted taper is placed in a jar containing nitrogen, the 
flame is extinguished immediately, showing that nitrogen does 
not support combustion. Nitrogen does not support respiration 
or breathing, for human beings or animals in a confined space 
are suffocated when a considerable proportion of the oxygen 
is used up. Nitrogen is not poisonous. In air it simply acts as 
a diluent for the oxygen. 

CARBON DIOXIDE 

The apparatus shown in Fig. 149 consists mainly of an 
aspirator A and a Wolffs bottle B containing lime water. All 
the joints are airtight. Water is allowed to flow from the 
aspirator and in this manner air is drawn through the lime 
water, which becomes milky in appearance. If the bottle C 
which contains caustic soda solution is attached to the bottle 
B by means of a piece of rubber tubing and fresh lime water is 
placed in B, when water is again allowed to flow out of the 
aspirator, the lime water is unaffected. Evidently in the first 
case there is some gas in the air which turns lime water milky 
and in the second case this gas is removed by the caustic 
soda solution. This constituent of the air is called carbon 
dioxide. 

If air is drawn through the caustic soda solution as before 
it will be found that the lime water in B is unaffected. But on 
disconnecting the bottle B from the other apparatus, introduc- 
ing a lighted splint into the bottle and shaking, the lime water 
turns milky. 

Evidently the burning of the wood produces carbon dioxide. 
In fact the burning of all materials which contain carbon 
produces this gas. 



THE CHEMISTRY OF AIR 



191 




FIG. 148 




FIG. 149 



IQ2 SCIENCE FOR COALMINING STUDENTS 

Preparation of Carbon Dioxide 

Fig. 150 shows an apparatus used for generating carbon 
dioxide. The bottle A is fitted with a stopper through which 
pass the thistle funnel B and the delivery tube C. The bottle 
contains marble chips, a form of calcium carbonate. If dilute 
hydrochloric acid is poured down the funnel, the acid acts upon 
the marble, producing effervescence. Carbon dioxide gas is 
evolved and is collected by the upward displacement of air in 
the jar. The chemical reaction can be represented thus: 

Hydrogen Chloride +Calcium Carbonate 

(Hydrochloric acid) 

=Calcium Chloride +Carbon Dioxide + Water. 

Properties of Carbon Dioxide Gas 

Carbon dioxide is an odourless, colourless gas which as we 
have seen turns lime water milky. This property is a test for 
the gas. Carbon dioxide is much heavier than air, which can be 
shown by pouring the gas from one jar to another in the same 
way as the pouring of water. When the lower jar, to which lime 
water has been added, is shaken the lime water assumes a 
milky appearance. 

Another way of showing that carbon dioxide gas is heavier 
than air is to counterpoise a beaker on a balance and to pour 
carbon dioxide into it from a gas jar (Fig. 151). The carbon 
dioxide displaces the air in the beaker, and the balance shows 
an increase in weight. 

The greater density of carbon dioxide compared with air is 
very important from the point of view of the miner. Carbon 
dioxide (a constituent of blackdamp), being heavier than air, 
collects near the floor of the workings and may form an ex- 
tinctive atmosphere (see p. 229). 

In ordinary air carbon dioxide is produced by the burning of 
substances containing carbon, e.g. wood, coal, etc. The per- 
centage quantity of carbon dioxide in ordinary air remains 
practically fixed, viz. 0-06 per cent, by weight. This seems 
strange at first sight, as we should expect the air in industrial 
districts near mills, mines, etc., to contain a much larger per- 
centage of the gas. Nevertheless, on the whole, the percentage 
remains about the same. The reason for this will now be dis- 
cussed. 

When carbon dioxide is bubbled into water contained in a 
trough, some of the gas escapes but a large quantity dissolves 



THE CHEMISTRY OF AIR 



193 



B O 



FIG. 150 



- o_ 



FIG. 152 




CARBON 
DIOXIDE 



FIG. 151 

in the water. If a green plant is placed in the trough underneath 
a funnel (Fig. 152) and a test tube filled with water is supported 
over the funnel, after allowing the apparatus to stand in sun- 
light for a few weeks, a gas collects in the test tube, and 
when this gas is tested with a glowing splint it is found to be 
oxygen. 
13 



194 SCIENCE FOR COALMINING STUDENTS 

Respiration 

It has already been seen that the burning of wood, etc., 
produces carbon dioxide. This gas is also produced in the pro- 
cess of breathing. Animals and human beings breathe in air 
and some of the oxygen is converted into carbon dioxide in the 
lungs. Thus the air breathed out contains a larger percentage 
of carbon dioxide. Plants also breathe in air and convert some 
of the oxygen into carbon dioxide, but in sunlight green plants 
carry on a process known as photo-synthesis or light building, 
in which they convert carbon dioxide into oxygen. As far as 
plant life is concerned the total effect of the two processes 
results in plants absorbing carbon dioxide and converting it 
into oxygen. Thus a balance exists whereby the percentage of 
carbon dioxide in the atmosphere remains the same. 

The Air as a Mixture 

We have seen that air consists mainly of oxygen and nitrogen 
in the proportion of i to 4 by volume. Now air is soluble in 
water, but the oxygen dissolves more readily than the nitrogen. 
Thus air driven off from water by heating contains a larger 
ratio of oxygen to nitrogen. This could not be so if the gases 
were in chemical combination. We conclude therefore that air 
is a mixture. 

OCCURRENCE OF CARBON DIOXIDE IN MINES 

Small quantities of carbon dioxide are produced in mine 
workings by breathing, by the burning of lamps and also from 
the use of explosives. Larger quantities of carbon dioxide are 
due to the oxidation of coal and other carbonaceous substances 
and the action of acidic waters on limestone in the strata 
(see p. 192). Large quantities of carbon dioxide are also found 
after an explosion. This is commonly known as choke damp. 

Practical Uses of Carbon Dioxide 

Carbon dioxide is a constituent of carbogen which is used 
in modern reviving apparatus. Carbogen consists of a mixture 
of carbon dioxide and oxygen in definite proportions. These 
proportions are fixed by C.M.A. regulations appertaining to 
rescue appliances and are such that the percentage of carbon 
dioxide in the mixture shall not be less than 5 per cent, or 
greater than 7 per cent. The use of carbon dioxide in the mixture 
utilises the physiological effect of this gas, which is to increase 
the depth of breathing. This property together with the high 
percentage of oxygen present is of valuable use when applied 



THE CHEMISTRY OF AIR 

to a person rendered unconscious by noxious gases such as 
firedamp, blackdamp, etc. 

A cylinder containing carbon dioxide under pressure consti- 
tutes a modern type of fire extinguisher in use at collieries, 
These extinguishers may be used with particular advantage in 
combating fires in electrical appliances. 

WATER VAPOUR 

We have already seen (Ch. VIII) that water vapour is a very 
important gas as far as mining practice is concerned. Water 
vapour is produced in ordinary air by evaporation from sheets 
of water and, since currents of air from the outside atmosphere 
pass down the downcast shaft, water vapour is also present in 
mine air. Perspiration from the miner's body and evaporation 
of water from sump holes also increase the quantity of water 
vapour in mine air. 



EXERCISES 

T. How would you prepare oxygen in the laboratory ? Give an account 
of the properties of oxygen which are of particular interest from the 
point of view of the miner. (Min. Sc.; U. L.C.I.) 

2. How would you show that the atmosphere is a mixture of gases? 
How does exhaled air differ from normal air? What impurities are 
likely to be present in mine air? (Min. Sc. ; U.L.C.L) 

3. Describe the preparation of carbon dioxide, giving a sketch of the 
apparatus used. Give an account of the occurrence of this gas in coal 
mines and its effects on animal life. How may the amount of carbon 
dioxide in a sample of mine air be determined? (Mm. Sc.; U.L.C.I.) 

4. How would you prepare a sample of oxygen in the laboratory? 
What are the chief properties of oxygen and what is the minimum 

percentage required for safety in mines? (Min. Sc.; U.E.I.) 

5. A candle is lighted and placed in a wide-mouthed glass jar. The 
mouth of the jar is then covered with a piece of cardboard. What do 
you expect to observe, and why? (Min. Sc. ; U.L.C.I.) 

6. Explain carefully how you would prepare oxygen, and the experi- 
ments you would perform to show its properties. State and explain 
what happens in each experiment. (Min. Sc.; U.L.C.I.) 

7. Describe the preparation of oxygen, giving a sketch of the appara- 
tus used. Give a brief account of the properties of this gas, and describe 
its uses in rescue operations. (Mm. Sc.; U.L.C.I.) 



CHAPTER XVIII 

THE CHEMISTRY OF WATER, HYDROGEN, 
SULPHUR AND CARBON 

COAL DISTILLATION AND PROXIMATE 
ANALYSIS 

WATER AND ITS CONSTITUENTS 

Water is a tasteless, colourless and odourless liquid which 
under a pressure of 760 mm. of mercury boils at 100 C. and 
freezes at o C. 

Water in large quantities can easily be detected, but when in 
small quantities it may be difficult to distinguish it from other 
colourless liquids without some reliable test. White anhydrous 
copper sulphate is an infallible test for water, for the colour of 
this substance becomes blue in the presence of small traces of 
water. 

SOLUTIONS 

If one or two potassium permanganate crystals are shaken up 
with water in a flask, the water assumes a pinkish colour. If 
more water is added, the permanganate spreads out over the 
whole of the water and the colour becomes less pink. The 
potassium permanganate is said to dissolve in the wSier to 
form a solution, the permanganate being the solute and the 
water the solvent. When more water is added the solution is 
said to be diluted, and theoretically there is no limit to the 
amount of dilution a solution may undergo. 

When common salt is dissolved in water, a colourless solution 
is produced. If more salt is added more dissolves and the solu- 
tion is said to become stronger. When more and more salt is 
added, a stage is reached when no more salt will dissolve in the 
given quantity of water. There is excess of salt left at the 
bottom of the vessel and the solution is said to be saturated 
at the particular temperature. 

In the case of common salt (sodium chloride), heating the 
solution and shaking the vessel causes more salt to dissolve and 
the solution becomes saturated at a higher temperature. 

196 



WATER, HYDROGEN, SULPHUR AND CARBON 



Solubility 

The solubility of a solid in a solvent is measured by the 
weight of the solid in grams which can be dissolved in 100 
grams of the solvent at the given temperature. 

The following table gives the solubility of common salt and 
saltpetre in water at various temperatures. 



Substance 


oC. 


50 C. 


100 C. 


Common salt 


35'5 


37 


39-G 


Saltpetre . . 


13-3 


86 


246 



VJ240 



5160 
Of 120 



t 



2O 4O 6O 80 
TEMPERATURE -DEC.C. 
FIG. 153 



IOO 



The graph (Fig. 153) shows the solubility of saltpetre at different 
temperatures. 

In the case of some salts, however, increase of temperature 
does not cause more salt to dissolve in the water. In fact 
some salts are more soluble in cold than in hot water. 

When a solution of a non-volatile salt (i.e. one which does 
not vaporise) is boiled, water is driven off in the form of steam 
and if the boiling is continued until all the water disappears, 
the solid is left behind in the vessel. Thus in the case of a 
solution of sodium chloride, a white powder is left in the 
evaporating dish. Also a solution of blue copper sulphate when 
evaporated to dryness leaves a white- powder known as an- 
hydrous copper sulphate. 



198 SCIENCE FOR COALMINING STUDENTS 

Water of Crystallisation 

When a hot saturated solution of copper sulphate is rapidly 
cooled, crystals of a blue colour separate out. When these 
crystals are examined under a microscope, they are found to 
have a definite shape with flat regular faces. A small crystal of 
copper sulphate placed in saturated copper sulphate solution 
grows into a large crystal of the same shape. 

Crystals of copper sulphate, alum, etc., all contain water, 
yet appear quite dry to the touch. When copper sulphate 
crystals are heated in a test tube, the water is driven off as 
steam, which condenses on the colder portions of the tube. A 
white powder is left which, as we have seen, is anhydrous copper 
sulphate. If a little water is added the salt turns blue and 
crystals again form. The water is in chemical combination with 
the salt and is known as water of crystallisation. 

ANALYSIS OF WATER BY PASSING STEAM OVER RED-HOT IRON 

Fig. 154 shows a flask A, fitted with a rubber stopper through 
which pass the delivery tube B and the safety tube C. The 
flask is supported in an iron stand and rests on a piece of 
gauze fixed to a tripod. The delivery tube B is connected to 
an iron tube D which contains a quantity of iron nails and 
rests in a furnace. The tube E leads from the tube D to the 
inverted gas jar F. This jar is filled with water and rests on a 
beehive shelf in a trough G, containing water. 

When steam is generated it passes over the red-hot iron and 
the gas evolved together with the air in the apparatus is 
allowed to escape for a time. Then the gas is collected by the 
downward displacement of water. This gas is hydrogen and 
the reaction is shown below : 

Hydrogen Oxide + Iron = Iron Oxide + Hydrogen. 

(Water) 

If a lighted taper is applied to the mouth of a jar containing 
the gas, the gas burns with a blue flame. 

HYDROGEN 

Preparation 

If dilute sulphuric acid is poured down the thistle funnel A 
(Fig. 155) upon granulated zinc in the bottle B, effervescence 
takes place. The gas hydrogen is evolved and is collected in the 



WATER, HYDROGEN, SULPHUR AND CARBON 




FIG. 154 



At 




FIG. 155 



200 SCIENCE FOR COALMINING STUDENTS 

jar C by the downward displacement of water. The chemical 
reaction is shown below: 

Zinc + Hydrogen Sulphate =Zinc Sulphate + Hydrogen. 

(Sulphuric acid) 

Properties 

Hydrogen is a colourless, odourless and tasteless gas. If a 
lighted taper is introduced into a jar containing hydrogen, the 
flame is extinguished, but the gas burns with a blue flame. This 
shows that although hydrogen burns it will not support burn- 
ing. The burning of hydrogen is represented as below: 

Hydrogen -f Oxygen = Hydrogen Oxide 

(Water) 

Hydrogen can be burned at a jet in the same way as coal gas. 
Fig. 156 shows a jet of burning hydrogen impinging on a flask 
through which cold water is circulating. If a watch-glass is 
placed directly underneath the jet, a colourless liquid collects 
in the glass. When this liquid is examined with anhydrous 
copper sulphate it is found to be water. 

If a mixture of hydrogen and oxygen in the ratio of 2 to i by 
volume is ignited, an explosion takes place and the product of 
combustion is the minute volume of water formed. 

Hydrogen is much lighter than air; in fact hydrogen is the 
lightest gas known. If the tube A (Fig. 157) isfilledwith hydrogen 
and the tube B with air, after a time the air in B is displaced 
by the hydrogen from the tube A. If a lighted match is placed 
near the mouth of the tube B, there is a slight explosion, 
showing that hydrogen has ascended into the upper vessel. 

SULPHUR 

Sulphur occurs in nature combined chiefly with the metals 
zinc, mercury, iron and lead, forming zinc sulphide (zinc blende), 
mercury sulphide (cinnabar), iron disulphide (iron pyrites) and 
lead sulphide (galena) respectively. 

When iron disulphide (iron pyrites) is heated in a hard glass 
tube a gas is given off. This gas condenses on the colder portions 
of the tube as a yellow powder and in the hotter portions as a 
yellow or brown liquid. On further cooling, the contents of the 
tube assume a yellow solid state. 

The decomposition of the pyrites is represented thus : 

3FeS 2 =Fe 3 S 4 +S 2 (see_Ch. XIX). 



WATER, HYDROGEN, SULPIIUU AND CARBON 201 




FIG.I5S 




t 



FIG.I57 



202 SCIENCE FOR COALMINING STUDENTS 

Properties of Sulphur 

Sulphur is a yellow brittle substance which melts at 114-5 C., 
forming a pale yellow liquid. When heated to a temperature 
between 200 C. and 250 C. sulphur becomes darker in colour 
and extremely viscid. On further heating to its boiling point, 
viz. 445 C., sulphur changes into a brownish-yellow vapour. 
When sulphur is heated in oxygen to a sufficiently high tempera- 
ture, sulphur dioxide gas is produced. In combination with 
carbon, sulphur forms carbon disulphide, and in combination 
with hydrogen it forms hydrogen sulphide gas (sulphuretted 
hydrogen). 

FORMS OF SULPHUR 

Crystals of Sulphur 

When molten sulphur solidifies, the particles arrange them- 
selves in regular forms called crystals which have the appear- 
ance of yellow needles. In the case of the deposition of sulphur 
from a solution, the crystals are in the form of octahedrons 
with rhombic bases. 

Flowers of Sulphur 

Sulphur may also be obtained in the amorphous or un- 
crystallised form. When sulphur is heated to boiling point, it 
vaporises and becomes a brownish-yellow gas which on con- 
densing forms a yellow powder in the colder portions of the 
tube. This powder is known as flowers of sulphur. 

Plastic Sulphur 

Sulphur near its boiling point when poured into cold water 
forms a soft, yellow, translucent mass of plastic sulphur!' 

CARBON 

Carbon is a constituent of all plants and animals and may be 
prepared in the laboratory by heating sawdust, flour, sugar, 
coal, etc., to a high temperature, in iron spoons, in a limited 
supply of air. On a large scale, in the manufacture of coal gas, 
coal is heated in a limited supply of air, in iron or fireclay 
retorts. The gaseous products of the distillation pass through an 
opening in the retort and coke, a form of carbon, is left at the 
bottom. 

Animal charcoal, which is another form of carbon, is the 
residue produced when bones are heated strongly out of contact 
with air. When animal charcoal is heated strongly in air, the 
carbon burns away and a white ash is left. 



WATER, HYDROGEN, SULPHUR AND CARBON 203 

Charcoal is produced by the burning of stacks of wood, 
covered with turf so as to limit the supply of air. Part of the 
wood burns and the heat produced chars the remainder. 

Soot or lampblack is a form of carbon which is produced by 
the burning of substances rich in carbon when the air is in 
short supply. 

Graphite is a crystalline form of carbon which is produced 
by dissolving charcoal in molten cast iron. On cooling the carbon 
separates into a crystalline form and the iron can be removed 
by dissolving it in acid. 

Carbon is a very inert substance at ordinary temperatures, 
while at high temperatures it combines with oxygen, forming 
either carbon monoxide or carbon dioxide. Carbon is also a 
constituent of carbon disulphide and the various hydrocarbons 
such as ethylene, acetylene, methane, etc. Since carbon has the 
property of withdrawing oxygen from metallic oxides, it is a 
good reducing agent (see p. 210). 

COAL ITS ORIGIN AND FORMATION 

The decay of vegetation of bygone ages, and the compression 
of this vegetation by overlying strata of earth, has resulted 
in the formation of coal. The type of coal depends to a great 
extent on the nature of the vegetation and the amount of 
decay which has taken place. Since coal is formed from 
wood whose principal constituents are cellulose, resins, proteins 
and sap juices it is quite obvious that it contains practically 
all the substances produced by the decay of these constituents. 
A small amount of mineral matter, known as ash, is also a 
constituent of coal. 

The following table gives the approximate percentages by 
weight of the various elements present in bituminous coal and 
anthracite. 



Type of Coal 


Carbon 


Hydrogen 


Oxygen 


Nitrogen 


Bituminous coal 


83% 


5% 


10% 


i-% 


Anthracite .... 


93%. 


3% 


3% 


o-7% 



Destructive Distillation of Coal 

Coal gas, which is produced by the destructive distillation 
of coal, consists of combustible gases many of which are 
highly luminous. These gaseous substances include ethylene, 



204 



SCIENCE FOR COALMINING STUDENTS 



the defines, acetylene, hydrogen and carbon monoxide. Very 
important by-products are formed in the distillation of coal. 
These include coal tar, hydrogen sulphide, carbon disulphide, 
ammoniacal liquor, benzol and naphthalene. 

PROXIMATE ANALYSIS OF COAL 

For commercial purposes proximate analysis of coal samples 
is of considerable importance. The main essentials are (a) con- 
tained moisture, (b) volatile content and (c) ash content. 

The percentage of, moisture is an important consideration, 





FIG. 158 



FIG. 159 



since the consumer has to pay for it if the contract involves 
payment by weight of coal purchased. In the case of the volatile 
content this will depend on the consumer's object. The vola- 
tiles consist of gases and the vapours of liquids from which, 
after purification, coal tar, ammonia, toluene, benzene, naph- 
thalene and other by-products are obtained. Hence the above- 
mentioned by-products may be of considerable importance to 
consumers. The calorific value of some of the gases and the 
hydrocarbons is of course very important in steam-raising 
plants. 

Finally the ash content of the coal is of importance, since 
the higher the percentage of mineral matter in the coal then the 
higher is the overall cost to the consumer. In addition there is 
the expense of the removal of the ashes after combustion. 



WATER, HYDROGEN, SULPHUR AND CARBON 2O5 

Determination of the Moisture Content of a Sample of Coal 

The determination of the moisture content of a coal sample 
is carried out in the following manner. Two watch-glasses are 
weighed together. Then a small amount of the sample which 
has been ground to such a size that it will pass through a go- 
mesh sieve, is placed on one of the watch-glasses, the other 
being used as a cover. The watch-glass containing the sample is 
weighed, together with the cover glass. The cover glass is 
removed and the watch-glass and sample are placed in a water 
oven (Fig. 158) at a temperature of 100 C. The sample is heated 
in this manner for one hour, after which the watch-glass and 
sample are cooled in a dessicator (Fig. 159). The desiccator 
consists of a glass vessel with two compartments, one above 
the other and separated by gauze, the lower compartment con- 
taining strong sulphuric acid or calcium chloride, both of which 
absorb water. The watch-glass, sample and cover are reweighed, 
then reheated as before for 15 minutes and, after cooling, 
weighed again. This process is continued until there is no change 
in weight, and from the data collected the moisture content is 
calculated. 

The experiment is repeated with a further sample of coal 
and a mean value for the moisture content is found. A set of 
results is shown below. 

(i) Weight of two watch-glasses =17-51 gm. 

Weight of two watch-glasses +coal 22-51 gm. 

.'. Weight of coal 5 gm. 

Weight of two watch-glasses -{-coal 22-35 g m - 
(after i hour in oven) 

Weight of two watch-glasses +coal ^22-28 gm. 
(after further 15 minutes) 

Weight of two watch-glasses +coal =22-28 gm. 
(after further 15 minutes) 

.*. Weight of moisture in sample =22-51 gm. 

22-28 gm. 

=0-23 gm. 

Q./^O 

Hence percentage of moisture in sample = - - x 100 

=4-6 per cent. 



*UK uuALMi JN UNIT M u un is i ?> 



(2) Weight of two watch-glasses =17*03 gm. 

Weight of two watch-glasses +coal =22-03 gm. 
/. Weight of coal =5 gm. 

Weight of two watch-glasses+coal=2i-82 gm. 
(after i hour in oven) 

Weight of two watch-glasses 7)- coal =2179 gm. 
(after further 15 minutes) 

Weight of two watch-glasses +coal =21 79 gm. 
(after further 15 minutes) 

.*. Weight of moisture in sample ' =22-03 gm. 

-21-79 gm. 

=0-24 gm. 

Hence percentage moisture content = - Xioo 
in sample 5 

=4-8 per cent. 

/. Mean percentage of moisture in 

sample =4'7 P er cent. 



Estimation of the Volatile Matter in a Coal Sample 

A silica crucible with close-fitting lid is weighed empty and 
then exactly one gram of the coal sample, ground to such a 
fineness that it will pass through a go-mesh sieve, is weighed 
into the crucible. 

The crucible complete with lid is placed on a porcelain tri- 
angle, fitted on a tripod. The crucible is then heated for exactly 
7 minutes in a Meker gas flame which has a temperature of 
965 C. approximately, and the volatiles escape through the 
crevices between the lid and the crucible. After heating, the 
crucible is transferred immediately into a desiccator and 
allowed to cool. The crucible complete with lid and contents 
is then weighed and any loss of weight is due to loss of volatiles 
and moisture. Since the moisture content of the sample has 
already been determined, the volatile content is easily found. 
The experiment is repeated and a mean value is obtained as 
shown below. 

(i) Weight of crucible and lid = 11-734 gm. 

Weight of crucible, lid and coal =12-734 gm. 
Weight of crucible, lid and coke =12-350 gm. 



WATER, HYDROGEN, SULPHUR AND CARBON 207 

.*. Loss in weight due to volatiles and=i2734 gm. 

moisture 12-350 gm. 

=0-384 gm. 

But the weight of moisture in=^^ gm. 
i gm. of coal I0 

-=0-047 gm. 

.'. Weight of volatiles in i gm. of coal ^0-384 gm. 

-0-047 gm. 

-0-337 gm. 
Percentage of volatile matter in 

sample 33*7 per cent. 

(2) Weight of crucible and lid =11-732 gm. 

Weight of crucible, lid and coal =12-732 gm. 
Weight of crucible, lid and coke =12-348 gm. 

.*. Loss in weight due to volatiles =12-732 gm. 

-12-348 gm. 

=0-384 gm. 
But weight of moisture in i gm. 

of coal = 0-047 g m - 

.'. Weight of volatiles in i gm. of coal=o384 gm. 

0-047 gm. 
=0-337 g^ 
Percentage of volatile matter in 

coal sample =337 per cent. 

Mean percentage of volatile matter 

in coal sample. =337 per cent. 

Determination of Ash Content of a Coal Sample 

A porcelain crucible is weighed empty and then one gram of 
the powdered sample (go-mesh) is added. The crucible and 
contents are placed in a furnace and heated gradually for half 
an hour up to a temperature of 400 C. This heating is continued 
for a further half -hour until the temperature reaches 700 C. 
Then the temperature is maintained constant at 700 C. and 
the heating is continued for a further half-hour. 

The crucible and contents are then removed from the furnace 
and, after cooling in a desiccator, are again weighed. The cru- 
cible and contents are then placed in the furnace for 15 minutes 



208 SCIENCE FOR COALMINING STUDENTS 

and heated to 700 C. and, after cooling as before, are again 
weighed. If a change in weight is indicated, this heat treatment 
is applied until the weight remains constant. Periodic stirring 
of the sample by means of a platinum wire is carried out 
throughout the experiment. From the data obtained the ash 
content may be calculated and a mean value is obtained by 
carrying out the experiment in duplicate. A set of results is 
shown below. 

(1) Weight of crucible =7'7 I S m - 
Weight of crucible +coal =871 gm. 

Weight of crucible +ash =777 S m - 

(after 90 minutes heating) 

Weight of crucible +ash =777 S m - 

(after a further 15 minutes heating) 

.'. Weight of ash =0-06 gm. 

.". Percentage ash content of sample 6 per cent. 

(2) Weight of crucible =7'43 gm. 
Weight of crucible +coal =8-43 gm. 

Weight of crucible +ash =7*49 S m - 

(after 90 minutes heating) 

Weight of crucible +ash =7*49 g m - 

(after a further 15 minutes heating) 

.'. Weight of ash =0-06 gm. 

.". Percentage ash content of sample =6 per cent. 

/. Mean percentage ash content of sample =6 per pent. 

The three previous determinations form the basis of "Proxi- 
mate Analysis." Thus we may state the following properties 
of the coal sample : 

Moisture content = 47 per cent. 

Volatile matter =337 per cent. 

Ash content = 6'0 per cent. 

Total =44-4 per cent. 

The percentage of fixed carbon may be obtained by sub- 
tracting the above total from 100 per cent. 

i.e. Fixed carbon in coal sample =10044-4 

=55-6 per cent. 



WATER, HYDROGEN, SULPHUR AND CARBON 2OQ 

EXERCISES 

1. Describe the chief properties of carbon. What arc the chief ele- 
ments, besides carbon, normally present in coal? (Min. Sc.; U.L.C.I.) 

2. How is sulphur commonly obtained? Give an account of the action 
of heat on sulphur. 

3. Describe the methods of obtaining the crystalline forms of sulphur. 

4. Describe a method for the proximate analysis of coal. 

(Min. Sc.; U.L.C.I.) 

5. Describe one of the following: 

(a) The proximate analysis of coal. 

(b) The analysis of mine air, which may contain carbon dioxide and 
methane. (Min. Sc,; U.L.C.I.) (See Ch. XX.) 



CHAPTER XIX 

CHEMICAL THEORY, ACIDS, BASES AND 
ALKALIS, HARD AND SOFT WATERS 

REDUCTION 

A reducing agent is a substance which withdraws oxygen 
from another substance in the course of a chemical change. 
Important reducing agents are hydrogen, carbon monoxide and 
coal gas which contains a quantity of hydrogen. 

Fig. T 60 shows an apparatus in which A is a hard glass tube, 




FIC.I6O 

containing a quantity of black copper oxide and through 
which coal gas is passed. If the coal gas is lit at the jet B, so 
as to prevent its escape into the atmosphere, and the tube is 
strongly heated, the copper oxide is completely reduced to 
metallic copper. 

If the experiment is repeated with a known weight of copper 
oxide in a porcelain boat and coal gas is again passed through 
the heated tube, after allowing the tube to cool, the porcelain 
boat shows a loss in weight. If on repeating the experiment 
there is no further loss in weight, the reduction is complete. 

210 



CHEMICAL THEORY, ACIDS, BASES AND ALKALIS 211 

A set of results is shown below. 

Weight of dish+copper oxide=575 gm. 

Weight of dish =4-26 gm. 

.*. Weight of copper oxide =1-49 gm. 

Weight of dish+copper =5*45 gm. 

Weight of dish =4-26 gm. 

/. Weight of copper =1-19 gm. 

Hence 1-49 gm. of copper oxide are reduced to 1-19 gm. of 

copper, 
or 1-19 gm. of copper are in chemical combination with 

0-30 gm. of oxygen, 
i.e. i gm. of copper combines with 0-25 gm. of oxygen. 

CHEMICAL EQUIVALENT 
Experiments including the above, show that : 

4 gm. of copper combine with i gm. of oxygen. 
1-5 gm. of magnesium combine with i gm. of oxygen, 
and 0-125 g m - of hydrogen combines with i gm. of oxygen. 
Thus 4 gm. of copper are equivalent to 0-125 g m - of hydrogen, 
or 32 gm. of copper are equivalent to i gm. of hydrogen. 
Also 1-5 gm. of magnesium are equivalent to 0-125 g m - of 

hydrogen. 

or 12 gm. of magnesium are equivalent to i gm. of hydro- 
gen. 

In fact, we may conclude that the weights of the various 
elements which combine with one gram of hydrogen are equi- 
valent to one another. 

The chemical equivalent of an element is the number of grams 
of the element which will combine with or displace i gram of 
hydrogen, 8 grams of oxygen or 35-5 grams of chlorine. 

ATOMS AND MOLECULES 

An atom is the smallest conceivable part of an element 
which can enter into a chemical reaction. In some elements 
the atoms cannot exist alone but are arranged in pairs. Thus 
two atoms of oxygen always exist together as a molecule of 
oxygen and two atoms of hydrogen exist together to form a 
molecule of hydrogen. Such molecules are known as diatomic 
molecules. In the case of a compound such as water, two atoms 



212 



SCIENCE FOR COALMINING STUDENTS 



of hydrogen combine with one atom of oxygen to form a tri- 
atomic molecule. 

Whether the substance be element or compound subdivision 
cannot take place beyond the molecule if the chemical composi- 
tion of the substance remains unchanged. Thus we may define 
a molecule as the smallest conceivable part of a substance which 
is capable of separate existence. 

Atomic Weights 

The relative weights of atoms can be obtained by finding 
the proportions in which various elements combine. It has been 
found that one gram of magnesium combines with f gram of 
oxygen; but since an atom of magnesium combines with an 
atom of oxygen (see valency, p. 213), one gram of magnesium 
and f gram of oxygen contain the same number of atoms. 
Thus the weight of an atom of magnesium is f times the weight 
of an atom of oxygen. 

Taking hydrogen as a standard and representing its atomic 
weight by unity, the atomic weights of other elements are given 
in the following table. The symbols representing the various 
elements arc also given. 



Element 


Symbol 


Atomic 
Weight 


Element 


Symbol 


Atomic 
Weight 


Hydrogen 
Oxygen 
Nitrogen 
Carbon 
Sulphur 
Calcium 
Sodium 
Potassium 


H 
O 

N 
C 
S 
Ca 

Na 
K 


i 
16 
14 

12 

32 
40 

23 

39 


Tin . 
Zinc . 
Lead . 
Mercury 
Iron 
Copper 
Magnesium 
Chlorine . 


Sn 
Zn 
Pb 
Hg 
Fe 
Cu 
Mg 
Cl 


118 

65 
207 

200 

56 
*63 
24 
35'5 



VALENCY 

It might be imagined that chemical equivalent and atomic 
weight are represented by the same number. This is only the 
case when one atom of an element combines with or displaces one 
atom of hydrogen. In the case of chlorine, one atom of chlorine 
combines with one atom of hydrogen and the combining power 
of chlorine is unity. Thus the formula for hydrochloric acid 
(hydrogen chloride) is HC1. But in the case of water one atom 
of oxygen combines with two atoms of hydrogen, the combining 
power of oxygen is two and the chemical formula for water is 
H 2 0. Again, one atom of carbon combines with four atoms of 



CHEMICAL THEORY, ACIDS, BASES AND ALKALIS 213 

hydrogen to form the gas methane. The combining power of 
carbon is four and the formula for methane is CH 4 . Also one 
atom of carbon combines with two atoms of oxygen to form 
the gas carbon dioxide (CC^) and since the combining power of 
oxygen is two, that of carbon is four. 

We may therefore define the combining power or valency of 
an element as the number of atoms of hydrogen it combines 
with or displaces from an acid. 

Generally speaking : 

Valency x Equivalent = Atomic Weight 

e.g. Valency of Oxygen x Equivalent 2x8 

=16 
= Atomic Weight. 



MOL. 




CHEMICAL EQUATIONS 

Chemical reactions can be represented by equations as far as 
the elements and molecules which enter into the reactions are 
concerned. Considering the equation which represents the burn- 
ing of hydrogen, since there are two atoms in a molecule of 
oxygen and a molecule is the smallest conceivable part of a 
substance which is capable of separate existence, instead of 
writing the chemical equation as : 

H 2 + O = H 2 O 
we write : 

2H 2 + 2 - 2H 2 

2 molecules i molecule 2 molecules 

This reaction is shown diagrammatically in Fig. 161. 



214 SCIENCE FOR COALMINING STUDENTS 

The oxidation of carbon is represented as follows : 
C + O 2 = C0 2 

i atom i molecule i molecule 

Also the oxidation of magnesium is shown thus . 
2Mg + O 2 = 2MgO 

2 atoms i molecule 2 molecules 

It should be noted that there is the same number of atoms 
of each element on each side of the equation. 

An atom of zinc combines with a molecule of sulphuric acid 
to form a molecule offline sulphate and a molecule of hydrogen, 
viz. : 

Zn | H 2 S0 4 == ZnSO 4 + H 2 

i atom i molecule i molecule i molecule 

AVOGADRO'S LAW 

Avogadro's Law states that equal volumes of all gases at the 
same temperature and pressure contain the same number of 
molecules. 

Suppose there are n molecules in one litre of oxygen and n 
molecules in a litre of hydrogen. The burning of hydrogen is 
represented thus : 

2H 2 + O 2 = 2H 2 O 
2 molecules i molecule 2 molecules 

i.e. 2 molecules of hydrogen combine with i molecule of oxygen 
to form 2 molecules of steam. Hence 2n molecules of hydrogen 
combine with n molecules of oxygen to form zn molecules of 
steam. Therefore 2 litres of hydrogen combine with i litre of 
oxygen to form 2 litres of steam. 

Again, suppose there are n molecules in i litre of oxygen. 
The oxidation of carbon to carbon dioxide is shown below: 

C + 2 - C0 2 

i atom i molecule i molecule 

i.e. i atom of carbon combines with i molecule of oxygen to 
form i molecule of carbon dioxide. Hence n atoms of carbon 
combine with n molecules of oxygen to form n molecules of 
carbon dioxide. Therefore a definite quantity of carbon is 
oxidised by i litre of oxygen to form i litre of carbon dioxide. 
Again, in the oxidation of carbon monoxide into carbon 
dioxide, we have: 

2CO + O 2 = 2CO 2 

2 molecules i molecule 2 molecules 

i.e. 2 molecules of carbon monoxide combine with i molecule of 



CHEMICAL THEORY, ACIDS, BASES AND ALKALIS 215 

oxygen to form 2 molecules of carbon dioxide. Hence 2n mole- 
cules of carbon monoxide combine with n molecules of oxygen 
to form zn molecules of carbon dioxide. Therefore 2 litres of 
carbon monoxide combine with i litre of oxygen to form 2 litres 
of carbon dioxide. 

The above considerations lead to the Law of Gay-Lussac, 
which states that the volumes in which gases combine bear a 
simple relation to one another and to the volume of the resulting 
product if that be gaseous. 

CALCULATIONS 

T. Find the weight of oxygen to oxidise 50 grams of magnesium. 

2Mg + O 2 =- aMgO 
(2x24) (16x2) 
Hence 48 gm. of magnesium combine with 32 gm. of oxygen. 

50 gm. of magnesium combine with 32 x J- gm. of oxygen 

"=33*3 gm- f oxygen. 

2. Find the weight of oxygen which is required to oxidise 50 gm. 
of carbon to carbon dioxide. 

C + O 2 ^ CO 2 

12 (l6X2) 

12 gm. of carbon require 32 gm. of oxygen. 
50 gm. of carbon require 32 xff gm. of oxygen 

= 133-3 gm. of oxygen. 

3. Find the weight of oxygen which is required to oxidise 100 gm. 
of carbon monoxide to carbon dioxide. 

2CO + O 2 = 2CO 2 
2(12 + 16) (2x16) 

56 gm. of carbon monoxide require 32 gm. of oxygen. 
TOO gm. of carbon monoxide require 32 X 1 /^ gm. of oxygen 

57-1 gm. of oxygen. 

4. What volume of oxygen is required to oxidise 30 cub. ft. of carbon 
monoxide to carbon dioxide? 

2CO + O 2 - 2CO 2 
2 molecules i molecule 
2 volumes i volume 

i.e. 2 volumes of carbon monoxide require i volume of oxygen. 
30 cub. ft. of carbon monoxide require J X 30 cub. ft. of oxygen 

15 cub. ft. of oxygen. 

5. What volume of oxygen is required to oxidise 10 cub. ft. of 
methane? (See p. 225.) 

Methane + Oxygen Carbon dioxide -f Water (Steam) 

CH 4 + 20 2 = CO 2 + 2H 2 O 

i molecule 2 molecules i molecule 2 molecules 

i volume 2 volumes i volume 2 volumes 

Hence i volume of CH 4 requires 2 volumes of oxygen. 
10 cub. ft. of CH 4 require 20 cub. ft. of oxygen. 



2l6 SCIENCE FOR COALMINING STUDENTS 

ACID-FORMING OXIDES 

If a piece of glowing carbon is placed in a gas-jar containing 
oxygen, the carbon burns more brightly and carbon dioxide gas 
is produced. When a quantity of water is added and the jar is 
shaken with a lid over the top, the carbon dioxide dissolves in 
the water. If a little blue litmus solution is added the litmus 
is turned red. Now the turning of blue litmus red indicates the 
presence of an acid and evidently when carbon dioxide is dis- 
solved in water an acid is formed. This acid is known as carbonic 
acid and the chemical reactions which take place in its formation 
are shown below: 

C + O 2 = CO 2 

H 2 + C0 2 = H 2 C0 3 

Carbonic acid 

If the above process is repeated by burning sulphur in a jar of 
oxygen, sulphur dioxide gas is produced. When the products of 
combustion are shaken up with water and a little blue litmus 
solution is added the blue litmus is turned red. An acid is thus 
produced in this case also, and the acid is known as sulphurous 
acid. The chemical reactions are shown below : 

S + O 2 = SO 2 

Sulphur dioxide 

H 2 + SO 2 = H 2 SO 3 

Sulphurous acid 

Under certain conditions when sulphur is burned in oxygen 
sulphur trioxide is produced, and when dissolved in water an 
acid known as sulphuric acid is produced. The chemical reactions 
are shown below: 

2$> + 30 2 = 2SO 3 ' 

Sulphur trioxide 

H 2 O + SO 3 = H 2 SO 4 

Sulphuric acid 

BASES AND ALKALIS 

When a little sodium is placed in an evaporating dish and 
the sodium is ignited, an oxide known as sodium oxide is 
formed. When a little water is added and the contents of the 
dish are stirred with a glass rod, sodium hydroxide solution is 
produced. When a little red litmus solution is added, the litmus 
turns blue. The turning of red litmus blue indicates the presence 
of an alkali and evidently sodium hydroxide solution, or 



CHEMICAL THEORY, ACIDS, BASES AND ALKALIS 217 

caustic soda solution as it is commonly called, is an alkali. The 
chemical reactions are shown below: 

4Na + O 2 = 2Na 2 O 

Sodium Sodium oxide 

Na 2 + H 2 - 2NaOH 

Sodium hydroxide 

Similarly calcium oxide (lime) and potassium oxide when dis- 
solved in water produce calcium hydroxide (lime water) and 
potassium hydroxide (caustic potash) respectively. Both calcium 
and potassium hydroxides turn red litmus solution blue and are 
therefore alkalis. 

Metallic oxides are generally known as bases and bases which 
dissolve in water form alkalis. 

SALT FORMATION 

If an evaporating dish contains a quantity of dilute hydro- 
chloric acid, coloured with a little red litmus solution, and caustic 
soda solution is run into the dish, drop by drop, eventually 
the red litmus begins to turn blue. The acid is just neutralised 
by the alkali (caustic soda solution), and if the solution is 
evaporated to dryness a salt known as sodium chloride (com- 
mon salt) is left. The chemical reaction is shown below: 

NaOH + HC1 = NaCl + H 2 O 

Sodium Hydrochloric Sodium Water 

hydroxide acid chloride 

The salt remaining is neutral and has no effect on either red or 
blue litmus. 

Another salt known as sodium sulphate can be prepared in a 
similar manner. If an evaporating dish contains a quantity of 
caustic soda solution together with a little blue litmus and 
dilute sulphuric acid is run into the dish, drop by drop, the 
litmus eventually turns red. Evidently the acid neutralises 
the alkali and the extra drop of acid turns the blue litmus red. 
The equation of the reaction is shown below : 



2NaOH + H 2 SO4 = Na 2 S0 4 + 2H 2 O 

Caustic Sulphuric Sodium Water 

soda acid sulphate 

If the solution is evaporated to dryness a solid, namely sodium 
sulphate, is left. 

If a little dilute sulphuric acid is poured upon a quantity of 
black copper oxide in an evaporating dish, a chemical reaction 
takes place and a blue solution is produced. If the solution is 



2l8 SCIENCE FOR COALMINING STUDENTS 

heated strongly and all the water is driven off, a white powder 
is left. This powder is the salt known as anhydrous copper 
sulphate. 
The previous experiments show generally that : 

Non-metallic oxide + Water = Acid. 
Metal +Oxygen =Base. 
Also : Base + Acid =Salt + Water. 
Alkali + Acid =Salt +Water. 

Hydrochloric, sulphuric, carbonic and nitric acids when 
neutralised by an alkali or a base give salts known as chlorides, 
sulphates, carbonates, and nitrates respectively. 

Precipitation 

As we have seen, some salts are soluble in water and others 
are insoluble. In a chemical reaction in which an insoluble 
salt is produced, this salt is deposited or precipitated. Thus both 
calcium chloride and sodium carbonate are soluble in water but 
when the solutions of the two salts are mixed the following 
reaction takes place : 

CaCl 2 +Na 2 CO 3 =2NaCl+CaCO 3 

(precipitated) 

Sodium chloride and calcium carbonate are produced. The 
sodium chloride remains in solution, whereas the calcium 
carbonate, being insoluble in water, is precipitated. 

Solutions of calcium sulphate and sodium carbonate mixed 
together give a precipitate of calcium carbonate according to 
the following reaction: 

CaSO 4 +Na 2 CO3==Na 2 SO 4 +CaCO3 

(precipitated) 

The precipitate in each case can be washed and dried and this 
is a method of preparing a sample of the substance. 

IMPURITIES IN WATER 

In connection with steam-raising plants at collieries a 
sufficient supply of water must be available. This water may 
be obtained from a nearby stream or river or a natural under- 
ground supply, and the presence of impurities may be such 
that the water requires treatment before use. 

If the water carries mineral matter in suspension it may be 
clarified by passing the water through filters before use or by 
allowing it to stand in settling tanks or ponds. 



CHEMICAL THEORY, ACIDS, BASES AND ALKALIS 

The treatment of impurities which have a soluble nature 
is more complex. Samples of the water must be taken and 
analysed and the impurities, if any, noted. Then the required 
treatment may be applied. 

Soluble impurities may have deleterious effects on the plant, 
the two most important being a corrosive effect and formation 
of scale, and impurities may have either or both of these proper- 
ties. The most common soluble impurities which require treat- 
ment are certain salts of calcium, iron and magnesium. 

Soluble Impurities forming Scale 

The following salts are the most common : calcium bicarbon- 
ate (CaH 2 (CO 3 ) 2 ), calcium sulphate (CaSO 4 ), ferrous sulphate 
(FeSO 4 ), iron oxide (hydrated), magnesium carbonate (MgCO 3 ) 
and magnesium bicarbonate (MgH 2 (CO 3 ) 2 ). 

Soluble Impurities having a Corrosive Effect 

Impurities which produce a marked corrosive effect are 
ferrous sulphate and magnesium chloride (MgCl 2 ). 

TREATMENT OF BOILER FEED WATERS 

Soluble mineral impurities form "hard" waters and such 
"hardness" may have two distinct forms, (a) temporary and 
(b) permanent hardness. Temporary hardness may be removed 
by boiling the water, whilst permanent hardness requires 
chemical treatment. Distilled water is often used in high- 
pressure steam plants and is the most suitable type of feed 
water which can be used. The water is provided by using a 
low-pressure boiler in the plant. 

Temporary Hardness 

Temporary hardness is caused mainly by the bicarbonates of 
calcium and magnesium. In the case of calcium bicarbonate, if 
the water is boiled calcium carbonate is precipitated since it is 
insoluble in water, and in this manner the hardness is removed. 
The sediment formed may be removed from the boiler by 
"blowing-down." The chemical reaction is shown below: 

CaH 2 (CO 3 ) 2 = CaCO 3 + CO 2 + H 2 O 

Calcium Calcium 

bicarbonate carbonate 
(precipitated) 



220 SCIENCE FOR COALMINING STUDENTS 

Calcium bicarbonate may also be removed by the addition of 
lime (calcium hydroxide) according to the following reaction: 

CaH 2 (CO 3 ) 2 + Ca(OH) 2 = 2CaCO 3 + 2H 2 O 

Calcium Calcium Calcium Water 

bicarbonate Hyciroxide carbonate 

(precipitated) 

In the case of magnesium bicarbonate in feed water the boil- 
ing reduces it to magnesium carbonate, which, however, is 
soluble. Magnesium carbonate is also a scale-former, so lime 
must still be added either initially or after boiling : 

MgH 2 (C0 3 ) 2 - MgC0 3 + H 2 + C0 2 

Magnesium Magnesium 
bicarbonate carbonate 

MgC0 3 + Ca(OH) 2 = CaC0 3 + Mg(OH) 2 

Magnesium Calcium Calcium Magnesium 
carbonate hydroxide carbonate hydroxide 
(precipitated) (precipitated) 

Permanent Hardness 

Permanent hardness is commonly caused by the sulphates, 
chlorides and nitrates of calcium and magnesium. Treatment of 
the water with lime and/or commercial soda as shown in the 
following equations is a method which may be applied to remove 
the hardness. 

Combined treatment to render magnesium sulphate harmless: 

MgS0 4 + Ca(OH) 2 = CaS0 4 + Mg(OH) 2 

Magnesium Calcium Calcium Magnesium 

sulphate hydroxide sulphate hydroxide 

(precipitated) 

CaSO 4 + Na 2 CO 3 = CaCO 3 + Na 2 S0 4 

Calcium Sodium Calcium Sodium 
sulphate carbonate carbonate sulphate 
(precipitated) 

Combined treatment to render magnesium chloride harmless : 
MgCl 2 + Ca(OH) 2 = Mg(OH) 2 <+ CaCl 2 

Magnesium Calcium Magnesium Calcium 

chloride hydroxide hydroxide chloride 

(precipitated) 

CaCl 2 + Na 2 CO 3 = CaCO 3 + 2NaCl 

Calcium Sodium Calcium Sodium 
dilorido carbonate carbonate chloride 
(precipitated) 



CHEMICAL THEORY, ACIDS, BASES AND ALKALIS 221 

Treatment to render magnesium nitrate harmless: 

Mg(N0 3 ) 2 + Ca(OH) 2 = Mg(OH) 2 + Ca(NO 3 ) 2 

Magnesium Calcium Magnesium Calcium 

nitrate hydroxide hydroxide nitrate 

(precipitated) 

Ca(NO 3 ) 2 + Na 2 CO 3 = CaCO 3 + 2NaNO 3 

Calcium Sodium Calcium Sodium 

nitrate carbonate carbonate nitrate 

(precipitated) 

In the above equations it will be seen that a sodium salt is left 
in solution in each case. These salts are harmless but may be 
removed by blowing-down the boilers. This treatment reduces 
the salinity of the water. 

Calcium impurities are treated by the addition of commercial 
soda only: 

CaCl 2 + Na 2 CO 3 - CaCO 3 + 2NaCl 

Calcium Sodium Calcium Sodium 

chloride carbonate carbonate chloride 
(precipitated) 

Ca(NO 3 ) 2 + Na 2 CO 3 = CaCO 3 + 2NaNO 3 
Calcium Sodium Calcium Sodium 

nitrate carbonate carbonate nitrate 

(precipitated) 

CaSO 4 + Na 2 CO 3 = CaCO 3 + Na 2 SO 4 

Calcium Sodium Calcium Sodium 
sulphate carbonate carbonate sulphate 
(precipitated) 

The sodium salts left in solution are treated as stated before 
by "bio wing-down/' 

THE WATER-SOFTENING PLANT 

A water-softening plant of the Kennicot type is shown in 
Fig. i62a. The reagent is added to the water by means of the 
lift-pipe method (Fig. i62b). The hard water from the supply 
enters the plant and is divided at an adjustable weir. Only a 
small portion is diverted to the regulator tank whilst the main 
part of the supply water passes on to a water wheel which is 
used to operate the stirring mechanism in the reagent tank. 
From the water wheel the water enters the reaction tank and 
mixes with the reagent. Then the water is passed on to the 
filter beds and thence to the boiler plant. 

The reagent tank (Fig. i62b) holds a definite amount of 
reagent, i.e. lime or soda or a mixture of the two which is dis- 
solved in water. When the regulator tank is full of water, the 



222 



SCIENCE FOR COALMINING STUDENTS 



reagent tank is empty. At this stage the main supply of water 
which has entered the plant has been treated with the correct 
amount of reagent. But when the regulator tank is nearly empty 

I HARD WATER 
SUPPLY 



















&< 

= ; 


REGULATOR 
TANK 


REAGENT! 

TANK 1 


1 




ipi 



'ADJUSTABLE 
WEIR 



WATERWHEEL 
AND STIRRING 
MECHANISM 



REACTION 
TANK 



FEED WATER 
TO BOILER PLANT 



(a) 





LIGHT CHAIN 
PULLEYS 



PERFORATED 
PIPE 




WATER 
FROM WEIR 



FLOAT 






REAGENT TO 
REACTION 
TANK 



FLEXIBLE 
;O,N, 



REGULATOR 
TANK 



FIG. 162 



the reagent tank is nearly full of reagent. The supply of reagent 
to the reaction tank is controlled by the float in the regulator 
tank, which in turn is controlled by the rate of flow of water 
passing into the regulator tank from the adjustable weir. 



CHEMICAL THEORY, ACIDS, BASES AND ALKALIS 223 

EXERCISES 

1. Give an account of the impurities occurring most frequently in 
mine waters. Explain, briefly, what you understand by the terms 
" hard" and "soft" in relation to water. How may one kind of hardness 
be removed from water? (Min. Sc.; U.L.C.I,) 

2. 100 Ib. of limestone containing 95 per cent, of calcium carbonate 
are heated in a lime kiln and converted to quicklime. Calculate the 
weight (in Ib.) of carbon dioxide evolved. (Atomic weights: C 12, 
Ca=40, = 16.) (Min. Sc. ; U.L.C.I.) 

3. Outline the methods by means of which you could test the suita- 
bility of a natural water for use in a boiler. What treatment would be 
necessary in the case of hard water? (Min. Sc.; U.L.C.T.) 

4. One gram of coal when completely burned in oxygen gave 2*64 
grams of carbon dioxide. Calculate the percentage of carbon in the coal. 
(Atomic weights: C^=i2, O = i6.) (Min. Sc.; U.L.C.i.) 

5. Give an account of the impurities present in mine waters. How 
can the temporary and permanent hardness be removed from water? 

(Min. Sc.; U.L.C.I.) 

6. Ten grams of carbon are burned in an unlimited supply of air. 
What is the name of the compound obtained, and what is its weight in 
grams? (Atomic weights: C~i2, O = i6.) (Min. Sc.; U.L.C.I.) 

7. 100 Ib. of coal containing 90 per cent, of carbon are burned in 
excess of air. Calculate the weight (in pounds) of carbon dioxide pro- 
duced. (Atomic weights: C-=i2, O^i6.) (Mm. Sc.; U.L.C.I.) 

8. Define the terms "element" and "compound." Name six of each 
met with in coalmining, and describe briefly their properties from the 
miner's point of view. ' (Min. Sc.; U.L.C.I.) 

9. Give one example to illustrate the use of each of the following 
terms, choosing, as far as possible, examples which may be met with in 
mining: element, compound, acid, salt, chemical reaction, oxidation. 

(Min. Sc.; U.L.C.I.) 



CHAPTER XX 

MINE GASES AND EXPLOSIONS 

MINE GASES 

The gases oxygen, nitrogen and carbon dioxide which consti- 
tute ordinary air have already been discussed in Chapter XVII. 
Mine air consists of these gases, not quite in the same proportions 
as in ordinary air, but mixed with other gases produced by 
various causes underground. These additional gases are 
mainly methane, carbon monoxide and hydrogen sulphide, 
while carbon dioxide usually exists in greater proportions than 
in ordinary air. Pure methane rarely occurs in mines, but when 
existing in percentages by volume of 60 to 100 with varying 
percentages of oxygen, nitrogen and carbon dioxide, it is 
known as firedamp. Carbon monoxide and hydrogen sulphide 
arc known as whitedamp and stinkdamp respectively and the 
term blackdamp refers to mixtures of carbon dioxide with 
nitrogen, etc. 

Methane 

Methane is prepared in the laboratory by heating to a high 
temperature a mixture of one part of sodium acetate and two 
parts of caustic soda (sodium hydroxide) in a hard glass tube, 
provided with a cork which is fitted with a delivery tubp (Fig. 
163). A quantity of quicklime assists in the reaction and is un- 
altered at the end. The methane is collected over water and the 
chemical reaction is shown below: 

CH 3 COONa + NaOH = Na 2 CO 3 + CH 4 

Sodium Sodium Sodium Methane 

acetate hydroxide carbonate 

Properties of Methane 

Methane is a colourless, odourless, tasteless and non-poison- 
ous gas. If lime water is added to methane, the lime water is 
unaffected. When a light is applied to the mouth of a jar con- 
taining methane, the gas burns with a non-luminous flame. 
Then on adding lime water to the jar and shaking, the lime 
water assumes a milky appearance, showing the presence of 

224 



MINE GASES AND EXPLOSIONS 



225 



carbon dioxide. The burning of the methane is shown by the 
following reaction: 

CH 4 + 2O 2 = CO 2 + 2H 2 O 

Methane Oxygen Carbon Steam 
dioxide (water) 

Methane is a lighter gas than air in so far as it can be poured 
upwards from one jar to another. If a light is applied to the 
upper jar, the contained gas burns, showing that the methane 
has displaced the air. Drops of water are formed on the sides 
of the jar in accordance with the equation shown above. The 




FIG. 163 

fact that methane is lighter than air is very important from the 
point of view of mining. 

When methane and air are mixed in certain proportions and 
a flame is applied, the mixture explodes. In the above equation 
which represents the burning of methane we have : 

Methane + Oxygen = Carbon Dioxide + Steam, 
i molecule 2 molecules i molecule 2 molecules 

i volume 2 volumes i volume 2 volumes 

When the steam condenses to water, this is equivalent to 
three volumes of the mixture producing one volume of carbon 
dioxide. The outrush during a methane explosion due to the 
increase in the volume of the gases at the high temperature 
and then the inrush (after-blast) due to the partial vacuum 
formed by the condensation of the steam can now be under- 
stood. 

15 



226 SCIENCE FOR COALMINING STUDENTS 

Ignition Temperature 

The ignition temperature of a substance is the temperature 
at which the substance begins to burn. 

EXPLOSIONS 

When ordinary coal gas is burned at a jet the supply of gas 
is controlled, i.e. the rate at which the gas is allowed to come 
into contact with the air is slow and regular and the flame 
cannot spread. If a large quantity of coal gas is allowed to 
escape into a room, under certain conditions, a light applied to 
the mixture of gas and air results in an explosion. This is due 
to the burning of the whole quantity of coal gas at once. The 
flame, once started, spreads instantaneously. 

Similarly if hydrogen is burned at a jet, the burning is 
orderly. But if hydrogen and oxygen are mixed in certain pro- 
portions, and the mixture is ignited, the flame spreads and 
cannot be controlled. If the hydrogen and oxygen are mixed in 
the proportion of 2 to i by volume, the explosion has its 
maximum force. 

Methane, like coal gas and hydrogen, can be burned at a jet 
in a controlled manner without an explosion taking place. But 
if the methane and air are mixed in certain proportions by 
volume, and a small quantity of the methane is ignited, the 
flame spreads suddenly and the result is an explosion. Evidently 
the flame once started cannot be controlled. 

During an explosion the gaseous products are at a very high 
temperature. This tends to cause a great increase in volume 
and, since there is opposition from the surrounding air, the 
pressure increases suddenly. The effect is more marked^in the 
case of explosive liquids owing to the greater increase in 
the volume of the gaseous products of combustion compared 
with the volume of the liquid. 

If steam is one of the products of combustion, when the 
steam condenses, a partial vacuum is formed which, as we have 
seen, is the cause of the after-blast in methane explosions. 

Explosive and Non-Explosive Mixtures of Methane and Air 

Tests have been made on the explosibility of mixtures con- 
sisting of various percentages of methane in air and the follow- 
ing results have been obtained. If the quantity of methane in 
the air is less than 5-6 per cent, by volume, the applied flame 
simply causes the methane to burn near the flame, but with 
increasing percentages of methane up to 5*6 per cent, the 



MINE GASES AND EXPLOSIONS 227 

volume of the burning space increases. For 5-6 per cent, of 
methane in the air, a flame just passes silently through the 
mixture even when the source of ignition is removed. With 
increasing percentages of methane above 5*6, a weak explosion 
occurs, which gradually increases in intensity until the per- 
centage becomes 9-4. In this case the explosion is most violent. 
As the percentage of methane increases from 9*4 to 15, the 
violence of the explosion diminishes, until at 15 per cent, the 
flame simply passes silently through the mixture. Then for 
higher percentages, the methane again burns near the applied 
flame, until at 25 per cent, the methane ceases to burn. 

From the above it can be seen that for percentages of methane 
in air from 5-6 to 15 the mixtures are inflammable, 5-6 being 
the lower limit of inflammability and 15 per cent, the upper 
limit. (N.B. The above percentages are only approximate.) 

Thus we may define an inflammable mixture as one through 
which a flame travels independently away from the source of 
ignition. 

In the above tests the spark from an electric arc is a more 
convenient and safer method of ignition. 

FIREDAMP 

Methane is a naturally occurring gas which may be found in 
variable quantities during the working of certain seams. This 
gas is often known as firedamp. Firedamp, however, is not pure 
methane, but consists of a mixture of various gases, the greatest 
percentage being methane, viz. 60 to 100 per cent, methane 
with varying amounts of oxygen, nitrogen and carbon dioxide. 

Deeper seams of coal are usually associated with a higher 
firedamp emission than more shallow seams. This feature is due 
to the fact that gases may escape more easily from shallow seams 
to the surface by means of faults, joints and other passages. 
Firedamp is generally found in cavities and caunches as well 
as in rise headings or roadways. The production of firedamp 
during the working of a particular seam may be reduced by 
improving the method of roof support on the face. This prevents 
crushing of the coal and minimises the emission of the gas. 

Detection of Firedamp Estimation of Methane Percentage 

by Caps 

The structure of a bunsen flame burning in air has already 
been discussed in Chapter XVII. What is the structure of a 
flame which burns in a mixture of methane and air? 

The flame safety lamp is supplied by oil and when the lamp 



228 



SCIENCE FOR COALMINING STUDENTS 



is introduced into a mixture of methane and air containing less 
than 5*6 per cent of methane, a pale blue "cap" is formed at 
the top of the flame. If the original flame is lowered so as to 
possess no luminosity, this blue cap can easily be seen. The 
cap is caused by the ignition of the methane particles near the 
flame, the ignition temperature of methane being 700 C. 
approximately. The heat produced by the burning methane 
particles is transmitted by convection currents to the adjacent 
methane particles and these ignite and so on. But since the 
initial source of heat is limited and much of the heat has to be 
used in raising the temperature of the particles of air, the cap 




1234 
-^PERCENTAGE OF METHANE 
FIG. 164 



extends only a limited distance, for beyond this distance the 
particles of methane do not reach their ignition temperature. 
If a greater concentration of methane particles is present, i.e. 
if the methane particles are nearer together, less air particles 
are present for absorbing the heat. Thus more heat is available 
for raising the temperature of the methane particles to the 
ignition point and the cap spreads farther. It would -seem, 
therefore, that the height of the cap is in some way connected 
with the concentration of the methane particles, and if we use 
a flame of standard size and heating value, the height of the cap 
is an approximate measure of the percentage of methane in 
mine air. 

The graph (Fig. 164) shows the relation between the height of 
the cap and the percentage of methane present. 



MINE GASES AND EXPLOSIONS 22Q 

Flame safety lamps are generally constructed on the "pro- 
tector" pattern, which possesses an arrangement for raising or 
lowering the flame. The lamp has two gauzes as a safety pre- 
caution in the event of the inner gauze being defective. If the 
lamp is taken into an area where the percentage of methane 
is above the lower limit of inflammability the height of the 
flame increases too much and the lamp is withdrawn from the 
danger zone. 

For normal testing purposes the flame is adjusted until it is 
almost non-luminous. Then the cap will form according to the 
percentage of methane present. 

The estimation of the percentage of methane in mine air by 
means of caps is very important from the point of view of safety 
in mines. In the first place it is important to remember that the 
Coal Mines Act gives certain statutory limits with regard to 
the percentage of inflammable gas, i.e. methane, present. Men 
are withdrawn from the face when the percentage of inflam- 
mable gas in the general body of the air is 2-5 and electrical 
machinery is cut off when mine air contains 1-25 per cent, of 
inflammable gas. 

BLACKDAMP 

Carbon dioxide has already been discussed in Chapter XVII. 
Blackdamp is a mixture of carbon dioxide, nitrogen and oxygen, 
the carbon dioxide being present in percentages ranging from 
5 to 20 per cent. The nitrogen is merely the residual nitrogen 
of the air, the oxygen of which has been partially or wholly 
converted into carbon dioxide. 

Blackdamp is produced by the liberation of naturally formed 
carbon dioxide from the strata into the mine atmosphere and, 
as we have seen, when the oxygen of the air is replaced by 
carbon dioxide due to oxidation. In mines where the use of 
naked lights is permitted a direct estimation of the percentage 
of carbon dioxide present can be made by means of the Haldane 
tube. 

Extinctive and Irrespirable Atmospheres 

When phosphorus burns in air in a confined space the whole 
of the oxygen in the space is used up, provided there is sufficient 
phosphorus present. In the case of a candle burning in air in 
a small air-tight gas jar, the flame is extinguished long before 
all the oxygen is used up and an extinctive atmosphere is 
produced. Thus we may define an extinctive atmosphere as one 
which does not support combustion. 



230 



SCIENCE FOR COALMINING STUDENTS 



On the other hand, when the oxygen in an atmosphere is 
deficient in quantity, breathing is very difficult and when the 
percentage of oxygen is low enough the atmosphere will not 
support life. An atmosphere which will not support life is 
known as an irrespirable atmosphere. Both these types of 
atmosphere may be found in underground workings after an 
explosion or a fire and may also be produced by an accumula- 
tion of gases such as firedamp or blackdamp. 

In all cases the atmosphere formed is deficient in oxygen 
due either to replacement by oxidation or by the presence of 
other gases. If the quantity of oxygen is not more than 17-9 
per cent, by volume, a flame is readily extinguished and under 
these conditions no atmosphere should be entered. An atmo- 
sphere containing less than 11 per cent, of oxygen by volume 
will not support life. The effect of various percentages of 
carbon dioxide in mine air on an oil flame and on human beings 
is shown in the following table. 



Normal breathing 


N (79 per cent.) 


O (21 per cent.) 




Deeper breathing 


N (79 per cent.) 


O (20-4 per cent.) 


No effect on 
flame 


Panting 


N (79 per cent.) 


O (20 per cent.) |i% 


Dull flame 


Increased panting 


N (79 per cent ) 


O (19 per cent.) CO 2 


Dull smoky 
flame 


Excessive panting 


N (79 per cent.) 


O(i8 per cent.) CO 2 
3% 


Flame ex- 
tinguished 


Severe distress 


N (79 per cent ) 


O (16 per CO 2 
cent.) 5% 


* 


Death 


N (79 per cent.) 


O (n per CO 2 
cent ) (10%) 





Estimation of the Percentage of Carbon Dioxide by Volume 
in a Mine Air Sample 

The percentage of carbon dioxide in a sample of mine air is 
determined by means of the Haldane Apparatus (Fig. 165). 
The sample is passed into an absorption pipette, containing 
caustic potash solution, which absorbs the carbon dioxide 
present. If the volume of the sample is measured before and 
after introduction to the pipette the reduction in volume due 
to absorbed carbon dioxide may be measured and the percentage 
of carbon dioxide in the sample calculated. 

The diagram indicates the arrangement of the apparatus. 



MINE GASES AND EXPLOSIONS 



23T 




CAUSTIC 
POTASH 
SOLUTION 




2 


] 


i 




'4 

~c 


1 


1 


\\ 


1 

1 

1 
(l 


1 

l) 


~ 




D 






B 



WATER ' 



FROM 

AIR 

PUMP 



-MERCURY 




FIG.I65 

The movement of the mercury reservoir A in conjunction with 
the two-way tap T x is used to draw in or expel the mine air 
sample to or from the graduated burette B. The absorption 
pipette E contains a solution of caustic potash, the level of which 
is regulated by the reservoir F, while the burette C which is 



232 SCIENCE FOR COALMINING STUDENTS 

exactly similar to B is used solely for the purpose of compen- 
sating for changes of temperature and pressure. 

The taps T 2 , T 3 and T 4 are opened to the atmosphere and the 
reservoir F is adjusted in such a manner that the caustic potash 
solution is brought up to the mark M 2 . The taps T lf T 3 and T 4 
are then placed in communication with the pipette E and the 
reservoir A is adjusted to bring the level ot the caustic potash 
solution to the mark M x . T x , T 3 and T 4 are then closed. The two- 
way tap T! is opened to the atmosphere and the air is expelled 
from the graduated gas burette B, by raising the mercury 
reservoir A. The bladder containing the mine air sample is then 
connected to the tap T! and a sample of the air is drawn into 
the burette B by lowering the reservoir A, the volume being 
read by means of the graduations. 

The taps TI, T 3 and T 4 are then adjusted to put the burette B 
in communication with the pipette E. The reservoir A is raised 
and the air sample is forced out of the burette B into the pipette 
E, where it comes into contact with the caustic potash solution. 
After the air sample has been in contact with the caustic 
potash solution for several minutes the reservoir A is lowered 
and the sample is drawn back into the burette B. When the 
caustic potash solution reaches the marks Mj and M 2 , the 
volume of the sample is read by means of the graduated 
burette B. 

The sample is again passed over to the pipette E, and the 
previous operations are repeated. If there is no change in 
volume, it may be taken that all the carbon dioxide was 
absorbed in .the previous operation. The reduction in volume 
is determined and this is expressed as a percentage of the initial 
volume of the sample. 

CARBON MONOXIDE 

Reduction of Carbon Dioxide to Carbon Monoxide by 
Red-hot Carbon 

Carbon dioxide is generated in the bottle A (Fig. 166) by the 
action of dilute hydrochloric acid on marble chips and it is 
passed over red-hot carbon in the iron tube B. The carbon 
dioxide gas is reduced to carbon monoxide according to the 
following reaction : 

C + CO 2 = 2CO 

Carbon Carbon Carbon 
dioxide monoxide 

The carbon monoxide along with any carbon dioxide which 



MINE GAvSES AND EXPLOSIONS 2J3 




FIG.Ibb 




FIG. 167 



234 SCIENCE FOR COALMINING STUDENTS 

has not been reduced is bubbled through caustic soda solution 
in the bottle C. The carbon dioxide gas is absorbed while the 
carbon monoxide proceeds through the delivery tube D and is 
collected over water in the vessel E which stands- on a beehive 
shelf, in the trough F. 

Preparation of Carbon Monoxide Gas 

Carbon monoxide can be prepared in the laboratory by the 
action of strong sulphuric acid on oxalic acid. The flask A con- 
tains oxalic acid and strong sulphuric acid is poured down the 
thistle funnel into the flask (Fig. 167). A mixture of carbon 
dioxide and carbon monoxide is produced and the sulphuric 
acid simply absorbs the water from the oxalic acid. The two 
gases produced are passed through the bottle B which contains 
caustic soda solution and the carbon dioxide is absorbed. The 
carbon monoxide, together with any trace of carbon dioxide, 
is passed through the bottle C, which contains solid caustic 
soda in small pieces, and any carbon dioxide is absorbed. 
The pure carbon monoxide is then collected over water in the 
jar D. The chemical reaction is represented as follows: 

C 2 H 2 4 + [H 2 S0 4 ] - [H 2 S0 4 ] + H 2 +CO +CO 2 

Oxalic Carbon 

acid monoxide 

Properties of Carbon Monoxide 

Carbon monoxide is an odourless, colourless and tasteless 
gas which is only slightly soluble in water. When a light is 
applied to the mouth of a gas- jar containing carbon monoxide, 
the gas burns with a bluish flame. If lime water be added tt) the 
jar, after shaking, the lime water assumes a milky appearance. 
Evidently the burning of carbon monoxide produces carbon 
dioxide, viz. : 

2CO + O 2 = 2CO 2 

Carbon Oxygen Carbon 
monoxide dioxide 

Occurrence of Carbon Monoxide in Mines 

We have already seen how carbon dioxide is reduced by red- 
hot carbon to carbon monoxide. A similar process takes place 
in underground fires. Any carbon dioxide produced by the 
combustion of the carbon in the coal passes over red-hot coal 
and is reduced to carbon monoxide. In effect, when carbon 
burns in a limited supply of oxygen, carbon monoxide is 
produced. 



MINE GASES AND EXPLOSIONS 235 

Carbon monoxide may be detected in the mine by its effect 
on warm-blooded animals such as mice, birds, etc. The per- 
centage of carbon monoxide in mine air may be estimated by 
the analysis of mine air samples. 

Physiological Effects of Carbon Monoxide 

The effect of carbon monoxide on human beings can be very 
serious. In the absence of carbon monoxide, the haemoglobin of 
the blood absorbs oxygen. But if carbon monoxide is present, 
the affinity of the haemoglobin for the carbon monoxide is 
greater than its affinity for oxygen and a compound known as 
carboxy-hsemoglobin is formed. Under these conditions the 
blood ultimately loses its affinity for oxygen and the miner 
suffers from carbon monoxide poisoning. 

Experiments show that the extent of carbon monoxide 
poisoning depends on the amount of gas present, the length of 
time the miner is exposed to the gas and the physical condition 
of the miner. Much work has been done by the Safety in Mines 
Research Board to protect the miner from carbon monoxide 
poisoning. 

Small warm-blooded animals are affected by carbon mon- 
oxide poisoning in a much smaller time than human beings, and 
to test for the presence of carbon monoxide, a mouse or a linnet 
is exposed to the gas in a cage which allows free movement. 

HYDROGEN SULPHIDE 

Occurrence 

Hydrogen sulphide, or sulplniretted hydrogen as it is com- 
monly called, is liberated during the decay of animal and veget- 
able matter containing sulphur. The formation of the gas is- due 
to bacterial action. 

Preparation 

The apparatus used for preparing hydrogen sulphide is shown 
in Fig. 168. Iron sulphide in small pieces is placed in the bottle 
A and dilute sulphuric acid (i part of acid and 7 parts of water) 
is poured down the funnel. The gas evolved is washed in the 
bottle B which contains water and it is then collected in the 
jar C. The chemical reaction is given below: 

FeS + H 2 S0 4 - FeSO 4 + H 2 S 

Ferrous Hydrogen 

sulphide sulphide 



236 SCIENCE FOR COALMINING STUDENTS 

Hydrogen sulphide in the pure state is produced by the 
action of hydrochloric acid on antimony sulphide, thus : 



Sb 2 S 3 

Antimopy 
sulphide 



6HC1 = 2SbCl 3 



3H 2 S 

Hydrogen 
sulphide 



Properties 

Hydrogen sulphide is a colourless gas with an offensive smell 
like that of bad eggs. It is poisonous in its pure state but even 
when diluted it gives dizziness and headache. Some animals 
are more affected by the gas than others. 

O 



V*Vft\ 



B 



M 



FIG. 168 



Hydrogen sulphide is soluble in cold water, so that in its 
preparation the water in B (Fig. 168) must be warm. A solution 
of hydrogen sulphide in water has a slight acid reaction with 
blue litmus and quickly decomposes in air. The hydrogen of the 
hydrogen sulphide combines with the oxygen of the air and the 
liquid becomes contaminated with a precipitate of sulphur. 
Hydrogen sulphide is inflammable and burns with a bluish 
flame according to the following reactions: 



2H 2 S + 30 2 = 2H 2 

Hydrogen Excess 
sulphide oxygen 

2H 2 S + O 2 = 2S + 
Limited Sulphur 
oxygen 



+ 2S0 2 

Sulphur 
dioxide 

2H 2 O 



MINE GASES AND EXPLOSIONS 237 

Detection of Hydrogen Sulphide 

A filter paper moistened with lead acetate is turned black by 
the action of hydrogen sulphide upon it. The chemical reaction 
is shown below: 



Pb(CH 3 COO) 2 H 

Lead 
acetate 


hH 2 S 


= PbS -f 

Lead 
sulphide 
(black) 


- 2CH 3 COOH 

Acetic acid 



COAL DUST EXPLOSIONS 

Explosions of mixtures of methane and air have already been 
discussed. These are not the only types of explosion which take 
place in mines. Explosions of coal dust may occur even though 
methane is absent, and an explosion will take place if the 
following conditions exist : (i) A flame is brought into the dust 
suspension at a sufficiently high temperature, (2) oxygen or air 
is present in sufficient quantity, (3) the coal duSt is in fine sus- 
pension, (4) the density of suspension is such that the particles 
of coal dust are near enough together to allow the heat produced 
in the combustion of the particles at the source of ignition to 
ignite the adjacent particles and so on. 

In dealing with mixtures of methane and air we have already 
seen that an explosion is an uncontrolled flame which spreads 
with great rapidity through the mixture. In the same manner 
the fine particles of coal dust, under the above conditions, 
burst into flame on reaching their ignition temperature and 
the flame spreads rapidly through the whole of the dust. The 
heat developed causes the air in the explosion area to expand 
suddenly against the resisting pressure of the surrounding air 
and in this way tremendous pressures are set up. 

Coal dust explosions may be caused by local explosions of 
firedamp, during shot-firing operations or by an open flame 
such as an electric arc. It is therefore essential to guard against 
these possible sources of ignition by observing C.M.A. regula- 
tions concerning ventilation, use of explosives and use of 
electricity. 

The spreading of incombustible dust in mine workings is a 
method widely adopted to render coal dust harmless. Thus if a 
few particles of coal dust become ignited some of the heat pro- 
duced is absorbed by the stone dust which if present in sufficient 
quantity leaves insufficient heat to ignite the adjacent coal dust 
particles. The C.M.A., 1911, and general regulations provided 
certain specifications regarding the spreading of incombustible 
dust and the use of water for consolidating the coal dust. 



238 SCIENCE FOR COALMINING STUDENTS 

Water is often used to prevent the formation of clouds of coal 
dust. This is done by using sprays at loading points, at conveyor 
transfer points, at intervals along haulage roads, on drilling 
machines and on coal-cutters. In such a manner the production 
and subsequent spreading of coal dust may be considerably 
reduced, thus lessening the risk of explosions. 

SPONTANEOUS COMBUSTION 

During the slow oxidation of a substance all the heat 
developed may be carried away by conduction, connection, etc., 
and the temperature of the substance remains constant. But 
in the case of substances which oxidise rapidly the heat is 
produced at such a rate that it cannot be carried away quickly 
enough and the temperature of the substance is raised. This 
process continues, with increased oxidation, until the ignition 
temperature is reached, when the substance begins to burn and 
the amount of heat is then sufficient to maintain the tempera- 
ture of the substance above its ignition point. If, in the oxidation 
of a substance, the heat developed is insufficient to raise the 
temperature to its ignition point, the substance cannot ignite 
and spontaneous combustion cannot take place. 

There are various factors which assist the spontaneous com- 
bustion of a substance. As there is no applied source of ignition, 
the substance must oxidise very readily. Also the heat developed 
in the oxidation of the substance must not be readily conducted 
away, and hence the substance must be surrounded by poor 
thermal conductors. A low ignition temperature is another 
factor which aids the initial burning of the substance and also 
a plentiful supply of oxygen is required for the combustion to 
continue. But air currents must not be strong enough to carry 
away the heat developed in the combustion. Lastly, the area 
exposed to oxidation must be as large as possible and this 
condition is satisfied if the material is in a finely divided state. 

The Spontaneous Combustion of Coal 

It is found that some coals combine with oxygen from the air 
more readily than others. This, combination gives rise to an 
increase in temperature, especially if the air-flow is slow, and if 
oxidation is allowed to continue it will increase in tempo with 
rise in temperature, until the ignition temperature of the coal is 
reached. 

In mines liable to spontaneous combustion the method by 
which the coal is worked should be chosen carefully, and 



MINE GASES AND EXPLOSIONS 239 

during working strict supervision should be enforced. A suffi- 
cient supply of fire-fighting material, e.g. stone dust, sand, water 
and fire-extinguishers, should be kept readily available. 

Spontaneous combustion may be aided by the oxidation of 
materials such as timber, brattice and pyrites. In the long wall 
system of mining care should be taken to prevent such materials 
or coal as far as possible from being left in the packs or wastes. 



EXERCISES 

T. Give an account of the occurrence, preparation and properties 
of methane. Describe how the presence of firedamp in coal mines may 
be detected by means of gas caps. (Min. Sc.; U. L.C.I.) 

2. Describe the chief properties of sulphur. What substances found 
in coal mines contain sulphur? How may stinkdamp be detected in 
mines? (Min. Sc.; U.L.C.I.) 

3. What are the chief causes of colliery explosions? What factors 
affect the cxplosibility of coal dust? (Min. Sc. ; U.L.C.I.) 

4. Describe the preparation and properties of carbon monoxide 
How do you account for the production of carbon monoxide in coal 
mines ? Describe an experiment to explain the occurrence of whitedamp 
at underground fires. (Min. Sc.; U.L.C.T.) 

5. Describe the preparation and properties of hydrogen sulphide. 
What are the sources of "stinkdamp" in coal mines, and what tests 
are used for its detection? (Min. Sc.; U.L.C.I.) 

6. Describe the preparation and properties of methane. How do you 
account for the presence of " firedamp " in coal mines, and how would 
you carry out tests for its detection in the workings of a mine? 

(Min. Sc.; U.L.C.I.) 

7. What is meant by the term " spontaneous combustion "? Describe 
how a "gob fire" might start, and some simple precautions which 
would reduce the probability of such occurrences. (Min. Sc. ; U.L.C.I.) 

8. How should you test for firedamp in the pit? Make a sketch of 
the " cap " you would expect to find when 3 per cent, of firedamp was 
present in the mine air. What is the smallest percentage of firedamp 
which, when mixed with air, will form an explosive atmosphere ? 

(Min. Sc.; U.L.C.I.) 

y. Explain how a coal dust explosion differs from a firedamp explo- 
sion. What gases would you expect to find in the atmosphere (a) after 
a firedamp explosion, and (b) after a coal dust explosion. 

(Min. Sc.; U.L.C.I.) 

10. Mention three substances which burn in air and three which do 
not burn. How are substances which do not burn used in mines (a) 
to prevent explosions, and (b) to put out fires? Explain their action in 
each of the cases you mention. (Min. Sc.; U.L.C.I.) 

j i . What percentages of firedamp in a mine do you consider to be 
dangerous and what percentage is explosive? Where in the mine is fire- 
damp most likely to be easily detected? (Min. Sc.; U.L.C.I.) 



240 SCIENCE FOR COALMINING STUDENTS 

12. Describe in detail, sketching the apparatus required, how you 
would prepare methane. How do you account for the presence of 
methane in coal mines? How would you estimate the percentage of 
fired amp at the ripping lip in a drawing road of a coal mine by means 
of a flame safety lamp? (Min, Sc.; U.L.C.I.) 

13. How would you determine the percentage of methane and carbon 
dioxide in a given sample of mine air? (Min. Sc.; U.L.C.I.) 

14. What are the chief gaseous products of the combustion of coal 
in air? Describe briefly their properties. (Min. Sc. ; U.L.C.I.) 

15. State the main properties of firedamp. What are the chief causes 
of explosions in mines and what precautions are taken to prevent them? 

(Min. Sc.; U.E.I.) 

1 6. Describe fully what you understand by the chemical equation: 
CO 2 +C~2CO. What experiment would you perform to illustrate the 
reaction shown by the equation? Explain the importance of this 
reaction, (Min. Sc. ; U.L.C.I.) 

17. Describe how each of the oxides of carbon may arise in mine 
atmospheres. State how you would detect their presence, and compare 
their physiological effects. (Min. Sc.; U.L.C.I.) 

1 8. Give a general account of the physiological effects of the presence 
of (a) carbon monoxide, (b) firedamp, and (c) moisture in mine air. 

(Min. Sc.; U.L.C.I.) 

19. Describe the properties of methane and carbon monoxide and 
explain how they are detected. (Min. Sc. ; U.L.C.I.) 

20. How would you test for firedamp in the pit? State (a) the lowest 
percentage of firedamp in air, and (b) the highest percentage of firedamp 
in air that will form an explosive mixture. What is the most explosive 
mixture of firedamp and air? (Min. Sc.; U.L.C.I.) 

21. Mention two frequent causes of colliery explosions and state 
what precautions are taken to avoid explosions due to these causes. 

(Min. Sc.; U.L.C.I.) 



CHAPTER XXI 

EXPLOSIVES 

EXPLOSIVES 

Explosives are used extensively in colliery work for the pur- 
pose of breaking down coal at the face as well as in caunch 
ripping, sinking and tunnel work. The use of explosives is 
regulated by the Coal Mines Act, 1911, and General Regulations. 
Due to these regulations many collieries may only use "Per- 
mitted explosives." Such explosives are permitted for use 
underground by the Ministry of Fuel and Power, after passing 
certain tests. 

High and Low Explosives 

Explosives are commonly referred to as "High" or "Low" 
explosives, this classification referring to the strength of the 
explosive. High explosives have a high rate of detonation due to 
the chemical combination of the fuel and the oxygen within 
the explosive material. This combination renders the oxygen 
more readily available and thus increases the rate of detonatioji. 
A typical example is nitro-glycerine. 

Low explosives have a lower rate of detonation due to the 
fact that the fuel and the oxygen are provided by separate 
materials which are in a finely divided state and are mixed 
together to form the explosive material. In this case the oxygen 
is not as readily available during combustion and thus the rate 
of detonation is comparatively slow. Black powder or gun- 
powder, which consists of a mixture of charcoal, sulphur and 
potassium nitrate (saltpetre), is a typical low explosive. How- 
ever, gunpowder is not included in the list of permitted ex- 
plosives because of the danger of incomplete combustion which 
results in the formation of a hot residue, capable of igniting 
inflammable gas. 

Sensitivity of an Explosive 

The term sensitivity, as applied to an explosive, refers to 
its tendency to explode due to vibration, ignition, etc. Some 
explosives are more safely handled than others, the former 
being less sensitive than the latter. 

16 2/tI 



242 SCIENCE FOR COALMINING STUDENTS 

Permitted Explosives 

Permitted explosives have varying compositions which de- 
pend largely on the type of work for which they are designed. 
High and low explosives are used separately or combined in 
order to obtain the desired strength and sensitivity. Other 
materials with non-explosive properties, such as wood meal, 
sodium chloride or clay, arc usually added for the purpose of 
altering the strength and sensitivity of the explosive. 

High and low explosives such as nitro-glycerine and am- 
monium nitrate respectively are used in permitted explosives. 
Nitro-glycerine is very sensitive and may be mixed with 
ammonium nitrate which has a low sensitivity in order to 
produce an explosive of medium sensitivity. Materials such as 
mtro-cotton and tri-nitro-toluene may be added to nitro- 
glycerine to reduce its sensitivity and also to enable it to be 
handled more easily during manufacture. 

Materials such as borax, hydrated salts, etc., are added to 
the explosives to act as cooling agents, whilst other materials 
are added to facilitate the process of cartridging. Sheathed 
explosives have a covering of sodium bicarbonate which acts as 
a cooling agent and also provides a protective cloud of carbon 
dioxide around the products of combustion. 

Finally potassium nitrate or sodium nitrate is added with 
the object of supplying oxygen during combustion. 

Permitted explosives may be divided into three classes: 
(a) ammonium nitrate explosives, (b) nitro-glycerine explosives 
and (c) explosives containing ammonium nitrate and nitro- 
glycerine. * 

Since ammonium nitrate explosives are not very sensitive, 
they usually contain some tri-nitro-toluene. Ammonium 
nitrate explosives are safer to handle and do not freeze as in 
the case of nitro-glycerine explosives. The explosives must be 
waterproofed as they are affected by moisture and should be 
carefully stored. The action of these explosives is not concen- 
trated and they form a useful general-purpose explosive. 

Nitro-glycerine explosives are very powerful and have a con- 
centrated action, which makes them suitable for blasting 
tunnels in hard rock. These explosives have a good resistance 
to moisture but are susceptible to freezing. Careful storage 
should be arranged to prevent such occurrences. 

Explosives containing ammonium nitrate and nitro-glycerine 
are used extensively in mining operations. The type of work 
for which they are designed is dependent on the percentage of 



EXPLOSIVES 



243 



nitro-glycerine present in the explosive. The higher the per- 
centage of nitro-glycerine the more suitable the explosive is 
for tunnel and ripping work. With smaller percentages of nitro- 
glycerine, the explosive may be used satisfactorily as a general- 
purpose explosive. Explosives suitable for coal usually con- 
tain only a small percentage of nitro-glycerine. 

The choice of an explosive for mining work depends mainly 
on conditions, i.e. safety regulations which decide between the 
use of permitted and other explosives, the nature of the work 

DETONATOR 
EADS 



NEOPRENE 
PLUG 

SOLDER 

LAMINATED 
CARDBOARD 

FUSEHEAD 

PRIMING COMPOSITION 

(LEADAZIDE AND 
LEADSTYPHNATE) 

DETONATING 

COMPOSITION 

(TETRYL) 




COPPER 

TUBE 



FIG. 169 
(SECTIONAL VIEW) 

to be done by the explosive and the natural conditions such 
as the presence of water. 

DETONATORS 

In underground work the use of electric detonators for 
igniting the explosive charges is common practice. These 
detonators are generally of two types, namely low-tension and 
high-tension. 

The construction of high- and low-tension detonators is 
almost identical apart from the fusehead. In the low-tension 
detonator (Fig. 169) a bridge wire is used to ignite the fusehead 

16* 



244 SCIENCE FOR COALMINING STUDENTS 

material, whilst in the high-tension detonator the introduction 
of carbon or other material in the fusehead mixture enables 
the mixture to pass a current and thus ignite itself. 

Delayed-action detonators, which are becoming increasingly 
popular for certain classes of work, have already been discussed 
in Chapter XIII. 

Production of Fumes when using Explosives 

The use of explosives underground often gives rise to the 
production of fumes which have an irritating effect on the 
eyes and respiratory organs. The fumes consist largely of fine 
dust together with liquid in a fine suspension and also a variety 
of gases. 

The liquid matter may be present in the form of nitrous or 
nitric acid in fine suspension, whereas the commonest gases 
formed are carbon dioxide, nitrogen, carbon monoxide, sul- 
phuretted hydrogen, ammonia, nitrogen peroxide, nitrous oxide 
and nitrogen dioxide. Apart from the first two mentioned gases 
the remainder have poisonous properties, but they are present 
in such small percentages that they are not directly dangerous. 
Their effect, however, may be to cause a certain amount of 
discomfort due to irritation. 



INDEX 



Accumulator, 114, 120, 122, 154 

capacity of, 155 

charging of, 155 

discharging of, 154 
acetylene, 203, 204 
acid, 216, 218 
agate knife-edge, 4 
air, 

composition of, 184, 185 

mine, 184, 224 
airway, return, 99 
alkali, 216, 217, 218 
amalgam, 153 

ammeter, 116, 141, 146, 170, 171 
ammonia, 204, 244 
ammoniacal liquor, 204 
ammonium chloride, 153 
ampere, 116, 142 
ampere-hour, 155 
anemometer, 35 
animal charcoal, 202 
anode, 151, 155 
anthracite, 203 
Archimedes' principle, 10 
argon, 161 
armature, 131, 139 

coil, 131, 139 

shaft, 138, 139 
atmosphere, 12 

extinctive, 192, 229 

irrespirable, 230 

pressure of, 14 
atom, 211 
atomic weight, 212 
Avogadro's law, 214 

Barograph, 15 
barometer, 

aneroid, 15 

Fortin, 2, 14 

height of, 14 

mercurial, 14, 91 

water, 30 
base, 217, 218 
battery, 123, 170, 171 
benzine, 204 
benzol, 204 

blackdamp, 39, 192, 224, 229 
blowpipe, 189 
Board of Trade unit, 160 
boiling point, 62, 86, 87, 93 
bomb calorimeter, 82 



borax, 242 

Bourdon gauge, 19, 20 

Boyle's law, 28, 29 

practical applications of, 30 

verification of, 28, 29 
brattice cloth, 39, 40 
British Thermal Unit, 75 
bunsen flame, 189 

Calcium 

bicarbonate, 219, 220 

carbonate, 192, 218, 220, 221 

chloride, 7, 25, 192, 218 

hydroxide, 217, 220 

nitrate, 221 

oxide, 217 

sulphate, 218, 219, 220, 221 
calorie, 75 

pound, 75 

calorific value, 81, 82 
calorimeter, 80, 82 
candle power, 164, 166, 167 
cap, 228 
capacity, 2 
carbogen, 194 
carbon, 202, 203 

fixed, 208 

gas, 153, 155 

carbon dioxide, 39, 190, 192, 194, 
227, 229, 242, 244 

occurrence of, 194 

preparation of, 192 

properties of, 192 

statutory limit of, 41 
carbon disulphide, 202, 204 
carbon monoxide, 203, 210, 224, 232, 
244 

occurrence of, 234 

physiological effects of, 235 

preparation of, 234 

properties of, 234 
carbonic acid, 216, 218 
carboxy-haemoglobin, 235 
catalyst, 187 
cathode, 151, 155 
caustic potash, 217, 231, 232 
caustic soda, 190, 217 
cells, 151, 153 

dry, 155 
cellulose, 203 
centigrade scale, 62 
centigrade heat unit, 75 

245 



246 INDEX 

centimetre-candle, 168 
centrifugal force, 33 
charcoal, 203, 241 
Charles' law, 68 
chemical chango, j86 
chemical equations, 213 
chemical equivalent, 211 
choke-clamp, 194 
circuit, electric, 116, 120, 156 
coal, 203 

ash content of, 207, 208 

bituminous, 82, 203 

by-products ol, 204 

moisture content of, 205 

volatile content of, 206 
coal-cutting machines, 20, 136 
coal-gas, 202, 210 

Coal Mines Act, 191 1, 189,229,237,241 
general regulations, 173, 194, 241 
coal-tar, 204 
combustion, 184, 189, 190, 229, 241 

spontaneous, 238 
common balance, 4 
commutator, split-ring, 130 
compass needle, 118, 120 
compound, 186 
compressed air, 20 
compressed-air receiver, 19, 20 
compression pump, 95 
condenbation, 87, 96, 99 
conduction of heat, 100 
conductors, 

electrical, 114 

thermal, 106, 108, 114 
convection, 106 

currents, 106, no, 112 

forced, 33, 112 
copper oxide, 184, 210, 217 
copper sulphate, 197, 198 

anhydrous, 196, 197, 198, 218 
counter-shaft, 138 
couple, 54 
crowbar, 52 
current, electric, 116, 141, 142 

Definite proportions, law of, 186 
density, 7, 24, 25 
depolarising agent, 153, 154 
dessicator, 205, 206, 207 
detonator, 

delayed-action, 145, 146, 244 

fusehead of, 243 

high-tension, 139, 243 

low-tension, 139, 243 
deviation, angle of, 176 
dew point, 96 
diffusion, 37 

Graham's law of, 38 

practical applications of, 39 
dumpy level, 5 



dynamo, 128 

alternating-current, 130 
carbon brushes of, 128 
commutator, 130 
direct-cunent, 130 
slip-rings of, 128 

Ebullition, go 

electric bell, 123 

electric coal-drilling machine, 136 

electrically driven haulage engine, 

136 

electrolysis, 151 
electrolyte, 151 
electromagnet, 122, 123, 125 
electromotive force, 156 
element, 186 
energy, 101, 158 

conservation of, 102, 158 

kinetic, 101 

potential, 101 
engine, 

boiler, 52, 95 

connecting-rod of, 47 

crank, 47 

furnace, 101 

petrol, 101 
equilibrant, 45 
equilibrium, 45 
ether, 91 
ethylene, 203 
evaporation, 90, 99, 195 
expansion, 61 

coefficient of linear, 64, 65 

fitting, 67 

of gases, 61, 68 

of liquids, 61 

of solids, 61, 64 
exploder, 138 

high-tension, 139 

low- tension, 139 
explosions, 226 

after-blast, 225, 226 

coal dust, 99, 237 

methane, 225, 226 
explosives, 241 

ammonium nitrate, 242 

general-purpose, 242, 243 

high, 241, 242 

low, 241, 242 

nitro-glycerme, 241, 242 

permitted, 241, 242 

sensitivity of, 241 

sheathed, 242 

Fahrenheit scale, 62 
fan, centrifugal, 33, 112 
ferrous sulphate, 219 
fire extinguisher, 195 
firedamp, 39, 224, 227 



INDEX 



247 



floating solids, 23 
fluid pressure, 25 

transmissibility of, 25 
flywheel, 47 

shaft, 67 

foot candle, 167, 168 
force, 42 

representation of a, 42 
forces, 

parallelogram of, 44 

resultant of, 42, 44 

triangle of, 45 
freezing point, 62, 87 
friction, 54 

coefficient of, 55 
frictional force, 55 

limiting, 55 
fuel, 81, 82, 101 
fuse, 162, 163 
fuse-box, 163 

Galvanometer, 125, 128, 146, 151 

gauze, 1 08, 109, no 

Gay Lussac's law, 215 

gob fires, 99 

graphite, 203 

gunpowder, 241 

Ilaldane's apparatus, 230, 231, 232 

heat stroke, 99 

heat units, 75 

horse-power, 59, 159 

hot- water system, no 

hydraulic jack, 25 

hydrocarbons, 189 

hydrochloric acid, 192, 217, 218, 236 

hydrogen, 151, 153, 154, 189, 198, 

200, 210 

properties of, 200 
hydrogen-ion, 151 
hydrogen sulphide, 202, 204, 224, 237 

preparation of, 235 

properties of, 236 
hydrometer, 23 
hygrometer, 

wet- and dry-bulb, 96 

whirling, 97 
hypsometer, 62 

Ignition temperature, 108, 226, 228, 

237. 238 

illuminating power, 164 
illumination, intensity of, 165, 166, 

167, 168 
image, 

optical, 174, 178 

virtual, 181, 182 
impeller wheel, 33 
incidence, angle of, 174, 176 



incombustible dust, 237 
induced current, 128, 132, 134 
induced electromotive force, 134, 138 
induction, mutual, 132, 134 
inflammability, 

lower limit of, 227 

upper limit of, 227 
inflammable mixtures, 227 
insulator, 

electrical, 114 

thermal, 108 
inverse square law, 166 
iron 

filings, 120 

rusting of, 185 

Joule, 158 
Joy loader, 25 

Kilogram, 4 
kilometre, I 
kilowatt, 159 

Latent heat, 86 

of fusion, 86, 87 

of vaporisation, 86, 87, 90 
lamp, 

electric, 161, 171, 172 

flame safety, 108, 229 

Harcourt pentane, 164 

miner's oil, 171 
lampblack, 203 
lead, 154, 155 

acetate, 237 

dioxide, 154, 155 

oxide, 184 

sulphate, 154, 155 
Leclanche cell, 153 
Left-hand Rule, 125, 127 
length, measurement of, i 
lens, 178 

optical centre of, 178 

principal axis of, 178 

principal focus of, 178 
Lenz's law, 132 
lever, 50 

bent, 50 

haulage wheel brake, 52 

safety valve, 52 
Liebig's condenser, 93 
lighting system, underground, 136 
lime water, 190, 192 
lines of force, 120, 134 
liquid air, 67 
litmus, 216, 217 
local action, 153 
loop joints, 68 
lumen, 167, 168 



248 INDEX 

Magnesium 

bicarbonate, 219, 220 

carbonate, 219, 220 

chloride, 219, 220 

hydroxide, 220, 221 

oxide, 184 

nitrate, 221 

sulphate, 220 
magnet, 

bar, 1 1 8, 120 

horse-shoe, 120 
magnetic 

declination, 118 

field, 120 

materials, 123 

meridian, 118 

pole, 1 1 8, 120 
magnetism, 118 

permanent, 122 

temporary, 120, 122 
manganese dioxide, 153, 155 
manometer, 17, 19 
mass, i, 4 

mean horizontal candle power, 171 
measuring cylinder, 2 
mechanical equivalent, 102, 103 
melting point, 86, 87 
meniscus, 2 

meridian, geographical, 118 
methane, 39, no, 203, 224, 225, 226, 
227, 228 

preparation of, 224 

properties of, 224, 225 

statutory limits of, 41, 229 
methanometer, McLuckie, 156 
method of mixtures, 79, 88 
metric system, i, 4 
micrometer screw, 67 
microscope, 170, 181 
miner's dial, 5, 118 
Ministry of Fuel and Power, 241 
mirror, plane, 173, 174 
mixtures, 187 
molecule, 39, 211, 212 
moments, 47 

principle of, 49 
motor, 

efficiency of, 163 

electric, 130 

pump, 136 
multiple proportions, law of, 187 

Naphthalene, 204 

nitric acid, 218, 244 

nitro-cotton, 242 

nitrogen, 185, 190, 194, 227, 229, 244 

dioxide, 244 

peroxide, 244 
nitrous acid, 244 
nitrous oxide, 244 



no volts release, 136 
non-explosive mixtures, 226 
non-magnetic materials, 123 
nystagmus, 173 

Ohm, 142 
Ohm's law, 141 
defines, 204 
optical square, 174 
overload trip, 137 
oxidation, 184, 189, 229, 238, 239 
oxide, acid-forming, 216 
oxygen, 82, 151, 153, 154, 155, 187, 
iHo, 193, 194, 211, 216, 227, 229, 
230, 238, 241 

physiological properties of, 189 

preparation of, 187 

Perspiration, 98, 99, 195 
photometer, 

flicker, 170 

Kumford's, 168 

selenium cell, 170, 171 
photometry, 164 
photo-synthesis, 194 
physical change, 185, 186 
pipes, 

lagging of, no 

hot-water, 68 
Plante process, 155 
plug key, 116 
polarisation, 153 
potassium, 

bichromate, 153 

chlorate, 187 

hydroxide, 217 

nitrate, 241, 242 
potential difference, 134, 135, 141, 

J 5 

pound, standard, 4 
power, 59, 159 
precipitation, 218 
pressure, n 

absolute, 19, 28 

due to air column, 17 

due to liquid column, 12 
primary coil, 134 
proteins, 203 
pump, 

compression, 95 

centrifugal oil, 25 

force, 32 

lift, 30 

reciprocating, 32 
pyrogallol, alkaline, 185 

Radiant heat, 106 
radiation, 106 
reaction tank, 221 
reagent tank, 221 



INDEX 



249 



reducing agent, 210 
reduction, 210 
reflection, 173 

angle of, 174 
refraction, 174 

angle of, 176 
refrigeration, 91 
regulator tank, 221 v 
relative density, 8, 23, 154 
relative humidity, 96, 99 
resins, 203 
resistance, 116 

electrical, 142 

external, 156 

fixed, 116 

internal, 156 

specific, 148 

variable, 116, 120, 122 
resistances 

in parallel, 144, 145 

in series, 144 
respiration, 189, 190, 194 
respiratory organs, 244 
reviving apparatus, 194 
Right-hand Rule, 128 

vSafety lamp, 

asbestos ring, 108, 109, no 

bonnet, 108 

control ring, 109, no 

Flame, 108 

flame-proof joints, no 

frame, 108, 109 

frame ring, 108, 109 

fuel tank, 108, 109 

magnetic lock, 108 
Safety in Mines Research Board, 235 
sal-ammoniac, 153, 155 
salt, 197, 218 

formation, 217 
saltpetre, 197 
secondary coil, 134 
sensible heat, 86 
sextant, 182 
shaft, 

disused, 21 

downcast, 17, 112 

upcast, 17, 112 
shot-firing 

battery, 138 

operations, 145, 146 
shrink fit, 67 
shunt, 146 
siphon, 32 

discharge tap, 33 

non-return valve of, 33 

priming tap, 33 
sodium 

acetate, 224 

bicarbonate, 242 



sodium 

carbonate, 218, 220, 221 

chloride, 196, 217, 218, 220, 221 

hydroxide, 216 

nitrate, 221, 242 

sulphate, 217, 220, 221 
solenoid, 136, 137 
solid angle, 167 
solubility, 197 
solute, 196 
solution, 196 
solvent, 196 
specific gravity, 8, 25 
specific heat, 76, 77 
spirit level, 5 
steam pipes, 68 
stinkdamp, 224 
stone dust, 237 
sulphides, 200 
sulphion, 151 
sulphur, 200, 241 

dioxide, 202, 216 

flowers of, 202 

plastic, 202 

trioxide, 216 

sulphuretted hydrogen, 244 
sulphuric acid, 24, 151, 154, 200, 216 
sulphurous acid, 216 
sweating of strata, 99 
switch-gear, 136, 137 

Telephone, 123 

carbon diaphragm of, 123 

microphone, 123 

receiver, 125 
telescope, 181, 182 
temperature, 61 

absolute, 69, 70, 71 

absolute zero of, 69, 72 

scales, 62 

terminal, screw, 114 
theodolite, 2, 5, 182 
therm, 75 

thermal capacity, 78 
thermometer, 62 

fixed points of, 62 
thrust, n 
toluene, 204 
torque, 47, 59 

total reflecting prism, 170, 171, 176 
transformer, 134, 161 

step-down, 134 

step-up, 134 
tri-nitro-toluene, 242 

Up thrust, 10 

Vacuum, 14, 19, 30 
valency, 212, 213 



250 

vapour, 

saturated, 92 

unsaturated, 92 
vapour pressure, 91 

saturation, gi, 93 
ventilation, 30, 39, 40, 112 

pressure, 18 
vernier, 2, 14 

calipers, 2 
volatile liquid, 91 
volt, 141, 142 
volt metre, 141, 146, 156 
volume, 2 

Water, 196 

analysis of, 198 

of crystall ibation, 198 

oven, 205 

permanent hardness of, 220 



INDEX 



Water, 

softening plant, 221 

temporary hardness of, 219 

vapour, 90, 195 
water equivalent, 78, 82 
water gauge, 17, r 8 
watt, 159 
weight, 4 
white damp, 224 
wood meal, 242 
work, 57 

done by torque, 58, 59 

unit of, 57 



Zinc 

chloride, 153 
rod, 153 
sulphate, 153, 200