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ESSAYS,

POLITICAL, ECONOMICAL,

AND

PHILOSOPHICAL.

VOL. II.

/

./

^^^, 9-3-4- i^^^.

ESSAYS,

POLITICAL, ECONOMICAL,

A N D

PHILOSOPHICAL.

By benjamin Count of RUMFORD,

KNir.HT OK THE ORDERS or THE WHITS KAGLE,AND ST. STANISLAUS}

Chamberlain, Privy Counjellor of State, and Lieutenant-General in the Service

ofhii Mofi Serene Highnejs ^^« Elector? a la tine, Reigning Duke

«/',Bavari A ; Colonel of his Regiment of Artillery, and Commander in

Chief of the General Staff of his Army ; F. R. S. Acad. R.

Hiber. Berol. Elec. Boicoe. Palat. et Amer. Soc.

VOLUME THE SECOND.

LONDON:

PRINTED FOR T. CADELL JUN. AND W. DAVIES IN THE STRAND.

1798. >>

lA

ADA!KISl^l«0,«

CONTENTS

OF THE

EIGHTH ESSAY.

CHAP. I.

./xN" Account of the Injlruments that were pre- pared for making the propofed Experiments. — A Ther?nometer is conJiruBed whofe Bulb is fur- rounded by a Torricellian Vacuum. — Heat is found to pafs in a Torricellian Vacuum with greater Difficulty than in Air, — Relative conduc- ing Powers of a Torricellian Vacumn and of Air with regard to Heat determined by Experiment.—' Relative conducing Powers of dry Air ai$d of moifi Air. — Relative conducing Powers of Air of different Degrees of Denftty. — Relative conducing Powers o/" Mercury J Water j Air; and a Torricellian Vacuum. - Page 391

- CHAP. 11.

The relative Warmth of various Subfiances ufed

in making artificial Cloathing, determined by Ex-

periment.'—'Relative Warmth of Coverings of the

fame Thicknefs, and formed of the fame Subfiance,

but of different Denfities, — Relative Warmth of

VOL. II, ' d Coverings

CONTENTS.

Coverings formed of equal ^antities of the fame $uhflance^ difpofed in different Ways. -^^Experi- ments made with a View to determining how far the Power which certain Bodies poffefs of confining Heat depends on their chemical Properties, — Ex' periments with Charcoal— w/V^ Lampblack— -zcZ/y^ Wdodafhes — Striking Experiments with Semen Lycopodii. — j411 thefe Experiments indicate that the Air which occupies the Interfiices of Subjiances ufed informing Coverings for confining Heat, a6ls a very important Part in that Operation. — Thofe Sub- jiances appear to prevent the Air from conducing the Heat. — An Inquiry concerning the Manner in which this is effe^ed. — This Inquiry leads to a decijive Experiment from the Refult of which it appears that Air is a perfed Non-condudor of Heat. — This Difcovery affords the Means of ex- plaining a Variety of interejiing Phenomena in the CEconomy of Nature^ - - Page 428

ESSAY IX.

An Inquiry concerning the Source of the Heat which is EXCITED by Friction. Page 467

.ESSAY VIIL

OF THE

PROPAGATION of HEAT

IN

VARIOUS SUBSTANCES:

BEING

An Account of a Number of New Experiments made with a View to the Inveftigation of the Causes of the Warmth of Natural and Artificial Clothing.

Firft publifhed in the Philofophical Tranfadiions.

VOL. II. E E

C 389 3

INTRODUCTION,

THIS ElTay contains nothing that wHl be new to philofophical readers j for it is little more than the fubftance of two Papers which have already appeared in the Philofophical Tranfaftions of the Royal Society of London ; one in the year 1786 i and the other (for which the Author had the honour to receive from the Society the Copleian Annual Medal) in the year 1792.

As reference has frequently been made to thefe Papers in fevefal of the preceding ElTay s ; and as many of the Experiments of which an account is given in them are not only interefting in themfelves, but are neceflary to be known in all their details in order to judge of feveral important conclufions that have been founded on their refults, the Author has thought that it would not be improper to republifh them under the prefent form. He was alfo defirous of adding the fubftance of thofe Papers to his Sixth and Seventh EiTays, in order that all that he has written on the Science of Heat might be brought together in one volume.

The EiTays which are deftined to compofe the next volume (many of which are already in great forwardnefs) are all on pradical fubjeds of a po? pular nature, and of general utility j and on that

E E 3 account

390 INTRODUCTION.

account it was judged befl to keep them feparate from thofe contained in this volume, which par^ take more of the nature of abflrufe philofophical invefligations.

Various unforefeen events have contributed to retard the pubhcation^ of the promifed EiTays on Kitchen Fire-places — on Cottage Fire-places— and on Clothing ; but the Author has well-founded hopes of being able to bring them forward in the courfe of a few months.

t 391 1

ESSAY VIII.

Of the Propagation of Heat in various

Subftances.

CHAP. I.

An Account of the Injlruments that were prepared for making the propofed Exper'unents, — A Ther- mometer is conjiructed whofe Bulb is furrounded by a Torricellian Vacuum. — Heat is found to pafs in a Torricellian Vacuum with greater Diffi- culty than in Air. — Relative conducing Powers of a Torricellian Vacuum and of Air with regard to Heat determined by Experiment. — Relative conduc- ing Powers of dry Air and ofmoiji Air. — Relative conducting Powers of Air of different Degrees of Denfity. — Relative conducing Powers of Mer- cury; Water J Air; ^w^f « Torricellian Vacuum.

[Read before the Royal Society, March 9, 1785.]

E^

XAMiNiNG the conducing power of air, and of various other fluid and folid bodies, with re- «^ gard to Heat, I was led to examine the conducing

power of the Torricellian vacuum. From the (Irik- '

ing analogy between the eledric fluid and Heat refpeding their conduftors and non-condudors, I

E E 3 (having

392 Of the Propagation of Heat

(having found that bodies, in general, which are conductors of the eleftric fluid, are likewife good condudlors of Heat, and, on the contrary, that eledlric bodies, or fuch as are bad conductors of the eleCtric fluid, are likewife bad conductors ©f Heat,) I was led to imagine that the Torricellian vacuum, which is kno\vn to afford fo ready a pafl!age to the eleCtric fluid, would alfo have afforded a ready paffage to Heat.

The common experiments of heating and cooU ing bodies under the receiver of an air-pump I con- ceived to be inadequate to determining this queftionj not only on account of the impoffibility of making a perfect void of air by means of the pump ; but alfo on account of the moift vapour, which exhaling from the wet leather and the oil ufed in the ma- chine, expands under the receiver, and fills it with a watery fluid, which, though extremely rare, is yet capable of conducting a great deal of Heat : I had recourfe therefore to other contrivances.

I took a thermometer, unfilled, the diameter of whofe bulb (which was globular) was juft half an inch, Paris meafure, and fixed it in the centre of a hollow glafs ball of the diameter oi i^ Paris inch^ in fuch a manner, that the fliort neck or opening of the ball being foldered faft to the tube of the thermometer j\ lines above its bulb, the bulb of the thermometer remained fixed in the centre of the ball, and confequently was cut off from all communication with the external air. In the bot- tom of the glafs ball was fixed a fmall hollow tube or point J which projecting outwards was foldered

7 to

in various Sub/iances» 393

to the end of a common barometer tube about 32 inches in length, and by means of this opening the fpace between the internal furface of the glafs ball and the bulb of the thermometer was filled with hot mercury, which had been previoufly freed of air and moifture by boiling. The ball, and alfo the barometrical tube attached to it, being filled with mercury, the tube was carefully inverted, and its open end placed in a bowl in which there was a quantity of mercury. The inflrument now be- came a barometer, and the mercury defcending from the ball (which was now uppermofl) left the fpace furrounding the bulb of the thermometer free of air. The mercury having totally quitted the glafs ball, and having funk in the tube to the height of 28 inches, (being the height of the mercury in the common barometer at that time) with a lamp and a blow-pipe I melted the tube together, or fealed it hermetically, about three-quarters of an inch below the ball, and cutting it at this place with a fine file, I feparated the ball from the long barometrical tube. The thermometer being after- wards filled with mercury in the common way, I now polfeifed a thermometer whofe bulb was con- fined in the centre of a Torricellian vacuu?n, and which ferved at the fame time as the body to be heated, and as the inflrument for meafuring the Heat communicated.

Experiments N° i.

With this inflrument (fee Fig, i.) I made the following Experiment. Having plunged it into a

E E 4 veffel

394 Q/* f^^^ Propagation of Heat.

veflel filled with water, warm to the i8th degree of Reaumur's fcale, and fuffered it to remain there till it had acquired the temperature of the Water, that is to fay, till the mercury in the in- clofed thermometer ftood at 1 8°, I took it out of this veffel and plunged it fuddenly into a velTel of boiling water, and holding it in the water (which was kept conflantly boiling) by the end of the tube, in fuch a manner that the glafs ball, in the centre of which was the bulb of the thermometer, was jufl fubmerged, I obferved the number of degrees to which the mercury in the thermometer had arifen at difl'erent periods of time, counted from the moment of its immerfion. Thus, after it had remained in the boiling water i min. 30 fee. I found the mercury had rifen from iB"* to 27°. After 4 minutes had elapfed, it had rifen to 44°tc ; ^"^^ at the end of 5 minutes it had rifen to 48%V«

Experiment^ N° 2.

Taking it now out of the boiling water I fuifered it to cool gradually in the air, and after it had ac- quired the temperature of the atm.ofphere, which was that of 15° R. fthe weather being perfeftly fine) I broke off a little piece from the point of the fmall tube which remained at the bottom of the glafs ball, where it had been hermetically fealed, and of courfe the atmofpheric air rufhed imm.e- diately into the ball. The ball furrounding the bulb of the thermometer being now filled with air, (injRiead of being emptied of air, as it was in the before-mentioned Experiment,) I refealed the end'

of

in various Subjia7ices. 395

of the fmall tube at the bottom of the glafs ball hermetically, and by that means cut off all com- munication between the air confined in the ball and the external air ; and with the inftrument fo pre- pared I repeated the Experiment before-mentioned ; that is to fay, I put it into water warmed to 18% and when it had acquired the temperature of the water, I plunged it into boiling water, and obferved the times of the afcent of the mercury in the thermometer. They were as follows :

Time	Heat
elapfed.	acquired.
H^at at the moment of Ijeing plunged into "i the boiling water, - - - - j	18= 11,
M. S.	0
After having remained in the boiling w^ter 0 45	27
1 0	34t^
2 10	44r^fi
2 40	4?/^
4 0	56r\-
5 0	6ot^

From the rufult of thefe Experiments it appears evidently, that the Torricellian vacuum, which affords fo ready a palTage to the eledric fluid, fo far from being a good condudlor of Heat, is a much worfe conductor of it than common air, which of itfelf is reckoned among the worfl: : for in the lafl Experiment, when the bulb of the thermometer was furrounded with air, and the inflrument was plunged into boiling water, the mercury rofe from 1 8° to 27*^ in 45 feconds ; but in the former Ex- periment, when it was furrounded by a Torricellian vacuum, it required to remain in the boiling v/atei i

minute

$g6 Of the Propagation of Heat

minute 30 feconds zz 90 feconds, to acquire that degree of heat. In the vacuum it required 5 minutes to rife to 48 V-o ; but in air it rofe to that height in 2 minutes 40 feconds ; and the propor- tion of the times in the other obfervations is nearly the fame, as will appear by the following Table.

The bulb of the thermometer placed in the
	centre of the	glafs ball, .	ind
r furrounded by a Torricellian vacuum.	furrounded by air,,
(Exp	N'^i.)	(Exp.	N'' 2.)
Time	Heat	Time	Heat
elapfed.	acquired.	elapfed.	acquired.
Upon being plunged into 7 boiling water - - j	18°		18''
M. S.	0	M. S.	0
After remaining in it I 30	27	0 45	27
		I 0	30t^5
4 0	44to-	2 10	44tq
5 0	4Br^	2 40	48t\-
		4 0	56t\
		5 0	60/*

Thefe Experiments were made at Manheim, upon the firfl day of July 1785, in the prefence of Profeflbr Hemmer, of the Eleftoral Academy of Sciences of Manheim, and Charles Artaria, me- teorological inftrument maker to the academy, by v/hom I was afiifted in m.aking them.

Finding the conftruftion of the inftrument made ufe of in thefe Experiments attended with much trouble and riik, on account of the difficulty of foldering the glafs ball to the tube of the thermo- meter without at the fame time either clofmg up, ■or otherwife injuring, the bore of the tube, I

had

in 'Various Subjlances* ^gy

had recourfe to another contrivance much more commodious, and much eafier in the execu- tion.

At the end of a glafs tu.be or cylinder about eleven inches in length, and near three quarters of an inch in diameter internally, I caufed a hollow globe to be blown i^- inch in diameter, with an opening in the bottom of it correfponding with the bore of the tube, and equal to it in diameter, leaving to the opening a neck or Ihort tube, about an inch in length. Having a thermometer prepared, whofe bulb was juft half an inch in diameter, and whofe freezing point fell at about 2-1 inches above its bulb, I graduated its tube according to Reaumur's fcale, beginning at o**, and marking that point, and alfo every tenth degree above it to 80°, with threads of fine filk bound round it, which being moiflened with lac varnifh adhered firmly to the tube. This thermometer I introduced into the glafs cylinder and globe jufl defcribed, by the opening in the bottom of the globe, having firft choaked the cylinder at about 2 inches from its junction with the globe by heating it, and crowding its fides inwards towards its axis, leaving only an opening fufficient to admit the tube of the thermometer. The thermometer being introduced into the cylinder in fuch a man- ner that the centre of its bulb coincided with the centre of the globe, I marked a place in the cylinder, about three-quarters of an inch above the 80th degree or boiling point upon the tube of the inclofed thermometer, and taking out the thermo- meter, I choaked the cylinder again in this place.

Introo

39 S Of the Propagation of Heat

Introducing now the thermometer for the lafl time, I clofed the opening at the bottom of the globe at the lamp, taking care before I brought it to the fire, to turn the cylinder upfide down, and to let the bulb of the thermometer fall into the cylinder till it refled upon the lower choak in the cylinder. By this means the bulb of the thermo- meter was removed more than 3 inches from the flame of the lamp. The opening at the bottom of the globe being now clofed, and the bulb of the thermometer being fuffered to return into the globe, the end of the cylinder was cut off to within about half an inch of the upper choak. This being done, it is plain, that the tube of the thermometer projected beyond the end of the cylinder. Taking hold of the end of the tube, I placed the bulb of the thermometer as nearly as poffible in the centre of the globe, and obferving and marking a point in the tube immediately above the upper choak of the cylinder, I turned the cylinder upfide down, and fuffering the bulb of the thermometer to enter the cylinder, and reft upon the firft or lower choak, (by which means the end of the tube of the thermo- meter came further out of the cylinder) the end of the tube was cut off at the mark juft mentioned, (care having firft been taken to melt the internal cavity or bore of the tube together at that place) and a fmall folid ball of glafs, a little larger than the internal diameter or opening of the choak, was foldered to the end of the tube, forming a. little button or knob, which refting upon the upper choak of the cylinder ferved to fufpend the thermo- meter

in various Subjiances> ^95

meter in fuch a manner that the centre of its bulb coincided with the centre of the globe in which it was fliut up. The end of the cylinder above the upper choak being now heated and drawn out to a point, or rather being formed into the figure of the fruftum of a hollow cone, the end of it was foldered to the end of a barometrical tube, by the help of which the cavity of the cylinder and globe containing the thermometer was completely voided of air with mercury j when, the end of the cylinder being hermetically fealed, the barometrical tube was detached from it with a file, and the thermo- meter was left Completely fhut up in a Torricellian vacuum, the centre of the bulb of the thermome- ter being confined in the centre of the glafs globe, without touching it in any part, by means of the two choaks in the cylinder, and the button upon the end of the tube. (See Fig. 2.)

Of thefe inftruments I provided myfelf with two, as nearly as poffible of the fame dimenfions ; the one, which I fhall call N^ i. being voided of air, in the manner above defcribed ; the other, N° 2. being filled with air, and hermetically fealed.

With thefe two inftruments (fee Fig. 2.) I made the following Experiments upon the nth of July laft at Manheim, between the hours of ten and twelve, the weather being very fine and clear, the mercury in the barometer ftanding at 27 inches 1 1 lines, Reaumur's thermometer at 15°, and the quill hygrometer of the academy of Manheim ^t 47^

Experiments^

400 Of the Propagation of Heai

Experiments, N° 3, 4, 5, and 6.

Putting both the Inflruments into a mixture of pounded ice and water, I let them remain there till the mercury in the inclofed thermometers refted at the point 0°, that is to fay, till they had acquired exadly the temperature of the cold mixture ; and then taking them out of it I plunged them fud-» denly into a large veflel of boiling water, and ob^^ ferved the time required for the mercury to rife in the thermometers from ten degrees to ten degrees, from o** to 80", taking care to keep the water conllantly boiling during the whole of this time, and taking care alfo to keep the inflruments im« merfed to the fame depth, that is to fay, juft fo deep that the point o** of the inclofed thermometer was even with the furface of the water.

Thefe Experiments I repeated twice with the ut-? mofl care ; and the following Table gives the refult of them.

k

in various Sub/lances.

401

Thermometer N	°i. 1	Thermometer N ^ 2 .
Its bulb half an inch in dia-	Its bulb half an inch in dia-
meter, {hut up in the	centre of	meter, {hut up in the centre of
a hollow glafs globe,	l| inch in	a hollow glafs globe, 1 1 inch in
diameter, void of air,	and her-	6.\z.mttcr, Jilled lulth air, and her-
metically fealed.		metically fealed.
Taken out of freezing water.	Taken out of freezing ivaler.
and plunged into boiling	ivater.	and plunged Into boiling water.
Time elaplcd.	Heat acquired.	Time elapfed.	Heat acquired.
Exp. No 3. Exp. No 4.	Exp. No 5. Exp. No 6.
M. S M. S.	0^ 0	0° M. S. M. S. 0
0 51 0 51	10	0 30 0 30 10
0 59 0 59	20	0 35 0 37 20
II 12	30	0 41 0 41 30
I 18 I 22	40	0 49 0 53 40
I 24 I 23	50	II 0 59 50
20 I 51	60	I 24 I 20 60
3 30 36	70	2 45 2 25 70
11 41 10 27	80 total time	9 10 9 38	80
22 44 21 i —	16 S5 173 =	total time
of heating from 0° tc	) 80^.	of heating from 0" to 80°.
Total time from 0'	to 70° :	Total time from 0° to 70° :
M	. S.	M. S.
In Exp. N"3.=:ii	3	In Exp. N''5. = 7 45
In Exp. N°4. — K	5 34	In Exp. N"6.=: Medium —	■7 25
Medium=ic	48?	-7 35

It appears from thefe Experiments that the con- ducing power of air to that of the Torricellian va- cuum, under the circumftances defcribed, is as y^l- to 10^ inverfely, or as 1000 to 702 nearly ; for the quantities of Heat communicated being equal,- the intenfity of the communication is as the times inverfely.

In thefe Experiments the Heat paifed through the fiirrounding medium into the bulb of the thermo- meter :

402

Of the Propagation of Heat

meter : in order to reverfe the Experiment, and make the Heat pafs out of the thermometer, I put the inftruments into boiling water, and let them remain therein till they had acquired the tempera? ture of the water ; that is to fay, till the mercury in the inclofed thermometers flood at 80° ; and then, taking them out of the boiling water, I plunged them fuddenly into a mixture of water and pounded ice, and moving them about conti- nually in this mixture, I obferved the times em- ployed in cooling as follows :

Thermometer N° i.

Surrounded by a Torricellian

'vacuum.

Taken out of boiling water, and

plunged into freezing -water.

Time ehipfed.

Exp. No 7. Exp. NO 8.

Thermometer N° 2.

Surrounded by air.

Tahen out of h oiling ivater, and plunged into freezing tuater.

Heat loft.

M. S. I 2

0 58

1 17

1 46

2 5

3 14 5 42

M. S.

0 54

1 2 I 18

1 37

2 16

3 10 5 59

80''

70 60

50

40

30 20

10

Time elapfed.

Exp. N° 9. Exp. No 10.

Heat loft.

Not obferved. Not oblerved. o

VI. s.	M. s.
0 33	0 33
0 39	0 34
0 44	0 44
0 SS	0 ss
I 17	I 18
I 57	I 57
3 44	. 3 40

40 10 Not obferved.

80°

o

70 60

50 40

30

20 10

O

Total time of cooling from 80° to 10".

M. S,

In Exp. N' 7. — 16 4 In Exp. N°8.=:i6 16

Medium m 6 10

Total time- of cooling from 80' to 10°.

M. S, In Exp. N° 9.^=9 49 In Exp. N° 10. — 9 41

Mediumir9 45

By

in njarious Subjlances.

403

By thefe Experiments it appears, that the con* dufting power of air is to that of the Torricellian vacuum as 9-J^ to i6|-| inverfely, or as 1000 to 603.

- To determine whether the fame law would hold good when the heated thermometers, inflead of be- ing plunged into freezing water, were fuffered to cool in the open air, I made the following Experi- ments. The thermometers N" i and W 2 being again heated in boiling water, as in the laft Experi- ments, I took them out of the water, and fufpend- ed them in the middle of a large room, where the air (which appeared to be perfeftly at reft, the win- dows and doors being all fhut) was warm to the 1 6th degree of Reaumur's thermometer, and the times of cooling were obferved as follows :

(Exp. N°XT.)	(Exp. N^i2.)
Thermometer N" i .	Thermometer N"^ 2.
Surrounded by a Torricellian	Surrounded by air.
vacuum.
Heated to 80", end fufpended in	Heated to 80°, and fufpended in
the open air <warm to l6\	the open air ivarm to 16''.
Time elapfed. Heat loft.	Time elapfed. Heat loft.
8o«	80°
M. S. 0	M. S. 0
Not obferved. 70	Not obferved. 70
I 24 60	0 51 60
I 44 50	I 5 50
2 28 40	I 34 40
4 16 30	2 41 30
10 12 =r total time em-	6 ti ~ total time em-
ployed in cooling from70°to30°.	ployed in cooling from 70^10 30°.

Here the difference in the conducing powers of

air and of the Torricellian vacuum appears to be

VOL. II. F F nearly

404 Of the Propagation of Heat

nearly the fame as in the foregoing Experiments, being as 6^~ to lo^-^ inverfely, or as looo to 605. I could not obferve the time of cooling from 80' to 70% being at that time bufied in fufpending the Inftruments.

As it might poffibly be objeded to the conclu- fions drawn from thefe Experiments that, notwith- flanding all the care that was taken in the con- fl:ru=6i;ing of the two inftruments made ufe of that they fhould be perfectly alike, yet they might in reality be fo far different either in fhape or fize, as to occafion a very fenfible error in the refult of the Experiments ; to remove thefe doubts I made the following Experiments :

In the morning towards eleven o'clock, the wea- ther being remarkably fine, the mercury in the ba- rometer ftanding at 27 inches 1 1 lines, Reaumur's thermometer at 1 5", and the hygrometer at 47", I repeated the Experiment N° 3, (of heating the ther- mometer N" I in boiling water, &c.) and imme- diately afterwards opened the cylinder containing the thermometer at its upper end, where it had been fealed, and letting the air into it, I re-fealed it hermetically, and repeated the Experiment again with the fame inftrument, the thermometer being now furrounded with air, like the thermometer N° 2. '

The refult of thefe Experiments, which may be feen in the following Table, fhews evidently, that the error arifing from the diiference of the (hapes or dimenfions of the two inftruments in queftion was inconfiderable, if not totally imperceptible. 15

in various Sub/iances,

405

(Exp. N° 13.)

Thermometer N° I.

Its bulb half an inch in dia- meter fhut up in the centre of a glafs globe i * inch in dia- meter, voided of air^ and her- metically fealed.

Taken out of freezing ivater', and plunged into boiling water.

Time clapfed.

M. S.

o 55

55

7

15

29

2 21

13 44

Heat acquired.

o 10

20

30 40

50 60

70

80

24 48 n total time heating from o" to 80". Total time from o** to 70** II 4 .

of

(Exp. N^ 14.) The fame Thermometer (N° I.)

The glafs globe, containing the bulb of the thermometer, being now filed with air, and hermetically fealed.

Taken out of freezing water, and plunged into boiling ivater.

Time elapfed.

M. S.

O 32

o

o

o

I

I

2

32

43

50 I

24

3B 10 25

Heat acquired. 0°

o

10 20

30

40

50 60 70 80

18 5 =r total time of heating from o" to 80°. Total time from 0° to 70^ = 7' 40".

It appears, therefore, from thefe Experiments, that the conducing power of common atmofpheric air is to that of the Torricellian vacuum as y^^ to 11-^^ inverfely, or as 1000 to 602 ; which differs but very little from the refult of all the foregoing Experiments.

Notwithftanding that it appeared, from the re- fult of thefe laft Experiments, that any difference there might pofTibly have been in the forms or dimenfions of the inflruments N' i and N" 2 could hardly have produced any fenfible error in the refult of the Experiments in queflion ; I \7as willing, however, to fee how far any con-

F F 2 fiderable

4o6 Of the Propagation of Heat

fiderable alterations of fize in the inftrument would afFed the Experiment : I therefore provided myfelf with another inftrument which I fhall call 'Therrti07neter N°3. different from thofe already de- fcribed in fize, and a little different in its. con- flruclion.

The bulb of the thermometer was of the fame form and fize as in the inftruments N" i and N° 2. that is to fay, it was globular, and half an inch in diameter j but the glafs globe, in the centre of which it was confined, was much larger, being 3 inches 7 1 lines in diameter j and the bore of the tube of the thermometer was much finer, and confequently its length, and the divifions of its fcale, were greater. The divifions were marked upon the tube with threads of filk of different colours at every tenth degree, from o'' to So**, as in the before-mentioned inftruments. The tube or cylinder belonging to the glafs globe was 8 lines in diameter, a little longer than the tube of the thermometer, and perfeftly cylindrical from its upper end to its junftion with the globe, being without any choak ; the thermometer being con- fined in the centre of the globe by a different con- trivance, which was as follows. To the opening of the cylinder was fitted a ftopple of dry wood, covered with a coating of. hard varnifh, through the centre or axis of which palfed the end of the tube of the thermometer : this ftopple confined the tube in the axis of the cylinder at its upper end. To confine it at its lower end, there was fitted to it a fmall fteel fpring, a little below the point 0°; which, being faftened to the tube of the thermo- meter.

in various Subjiances* 407

meter, had three elaftic points proje6lmg outwards, which, preffing againfl the infide of the cylinder, confined the thermometer in its place. The total length of this inftrument, from the bottom of the globe to the upper end of the cylinder, was 18 inches, and the freezing point upon the thermometer fell about 3 inches above the bulb ; confequently this point lay about i| inch above the jundion of the cylinder with the globe, when the thermometer was confined m its place, the centre of its bulb coinciding with the centre of the globe. Through the ftopple which clofed the end of the cyHnder paiTed two fmall glafs tubes, about a line in di- ameter, which being about a Hne longer than the flopple were clofed occafionally with fmall floppies fitted to their bores. Thefe tubes (which were fitted exadly in the holes bored in the great ftopple of the cylinder to receive them, and fixed in their places with cement) ferved to convey air, or any other fluid, into the glafs ball, without its being necelTary to remove the Hopple clofing the end of the cylinder ; which ftopple, in order to prevent the pofition of the thermometer from being eafily deranged, was cemented in its place.

I have been the more particular in the defcription of thefe inflruments, as I conceive it to be ab- folutely neceffary to have a perfeft idea of them in order to judge of the Experiments made with them, and of their refults.

With the inftrument laft defcribed (which I have called Thermometer N° 3.) I made the following

F F 3 Kxperi-

4o8

Of the Propagation of Heat

Experiment. It was upon the i8th of July 1785, in the afternoon, the weather variable, alternate clouds and fun-fhine; wind llrong at S. E. with now and then a fprinkling of rain ; barometer at 27 inches io| lines, thermometer at i8°i, and hygrometer variable from 44° to extreme moillure. In order to compare the refult of the Ex- periment made with this inftrument with thofe made with the thermometer N° 2. I have placed together in the fame Table the different Experi- ments made with them.

(Exp. No 15.)	(Exp. No4and No 5.)
Thertn'jmttei- No 3.	Thermometer No 2.
Its bulb half an inch in dia-	Its bulb half an inch in diameter, (hut
meter, (hut up in the centre of	up in the centre of a glafs globe, i-| inch
a glafs tube, 3 inches 7I lines	in diamtiter, and furrounded hy air.
in diameter, and furrounded by	TaLen out of freezing luater, and plunged into
air.	boiling ivattr.
Taken out of freezing loater, and plunged into boiling -water.
Time elapfed. ^^^^
	acquired.
Time elapfed. Heat acquired.	Exp. No 4. Exp. No 5. Medium.
oo	. 00
M. S. 0	M. S. M. S. M. S. ^
0 33 10	c 30 0 30 0 30 10
0 38 20	0 35 0 37 0 36 20
0 54 30	0 41 0 41 0 41 30
0 51 40	0 49 0 53 0 31 40
1 7 50	11 0 59 10 50
I 28 60	I 24 J 20 I 22 60
2 28 70	2 45 225 2 35 70
50 80 16 59 rr total time of heat-	9 10 9 3^ 9 24 80
16 55 17 3 16 59 zz total
ing from qo to Sqo.	time of heating from 0° to 800.
Time from 0° to 700=7' 59".	Time from qo to 70<^ZZ7' 35".

If the agreement of thefe Experiments with the thermometers N" 2 and N'' 3 furprifed me, I was not lefs furprifed with their difagreement in the Experiment which follows :

Expert'

in various Subjiances^ 409

Experiment y N^ 16.

Taking the thermometer N° 3. out of the boiling water, I immediately fufpended it in the middle of a large room, where the air, which was quiet, was at the temperature of i8°f R. and obferved the times of cooling as follows ;

Time elapfed. Heat loll.

M. S.	0
I S5	70
0 12	60
P ?>?>	50
1 15	40
4 0	30

9 55=total time of cooling from 80° to 30°. Time from 70" to 30*^= 8' o"; but in the Experi- ment No. 12. with the thermometer N" 2, the time employed in cooling from 70° to 30** was only 6' 11". In this Experiment, with the ther- mometer N" 3. the time employed in cooling from 60° to 30*" was 7' 48"; but in the above-mentioned Experiment, with the thermometer N° 2. it was only s' 10", It is true, the air of the room was fomewhat cooler when the former Experiment was made, than when this latter was made, with the thermometer N" 3 ; but this difference of tem- perature, which was only 2°|, (in the former cafe the thermometer in the room flanding at i6<', ' and in the latter at i8°|,) certainly could not have

F F 4 . occa-

41 o Of the Propagation of Heat

occafioned the whole of the apparent difference in the refults of the Experiments.

Does air receive Heat more readily than it parts with it? This is a queftion highly deferving of further invefligation, and I hope to be able to give it a full examination in the courfe of my projeded inquiries ; but leaving it for the prefent, I fhall pro- ceed to give an account of the Experiments which I have already made. Conceiving it to be a ftep- of confiderable importance towards coming at a further knowledge of the nature of Heat, to afcertain, by indifputable evidence, its paifage through the Torricellian vacuum, and to deter- mine, with as much precifion as pofTible, the law of its motions in that medium ; and being appre- henfive that doubts might arife with refpedt to the Experiments before defcribed, on account of the contaft of the tubes of the inclofed thermometers in the inftruments. made ufe of with the containing glafs globes, or rather with their cylinders : by means of which (it might be fufpe6led) that a cer- tain quantity, if not all the Heat acquired, might poffibly be communicated ; to put this matter beyond all doubt, I made the following Experi- ment.

In the middle of a glafs body, of a pear -like form, about 8 inches long, and 2f inches in its greateft diameter, I fufpended a fmall mercurial thermometer, 5^ inches long, by a fine thread of fdk, in fuch a manner that neither the bulb of the thermometer, nor its tube, touched the contain- ing glafs body in any parto The tube of the ther- mometer

t.-

in 'various Subfiances, 411

mometer was graduated, and marked with fine threads of filk of different colours, bound round it, as in the thermometers belonging to the other inflruments already defcribed ; and the thermome- ter was fufpended in its place by means of a fmall fteel fpring, to which the end of the thread of filk which held the thermometer being attached, it (the fpring) was forced into a fmall globular pro- tuberance or cavity, blown in the upper extremity of the glafs body, about half an inch in diameter, where the fpring remaining, the thermom.eter ne- celfarily remained fufpended in the axis of the glafs body. There was an opening at the bottom of the glafs body, through which the thermometer was introduced; and a barometrical tube being foldered to this opening, the infide of the glafs body was voided of air by means of mercury ; and this opening being afterwards fealed herme- tically, and the barometrical tube being taken away, the thermometer was left fufpended in a Tor- ricellian vacuum.

In this inftrument, as the inclofed thermometer did not touch the containing glafs body in any part, on the contrary, being diftant from its internal furface an inch or more in every part, it is clear, that whatever Heat paffed into or out of i\\Q ther- mometer muit have paffed through the furrounding Torricellian vacuum : for it cannot be fuppofed, that the fine thread of filk, by which the ther- mometer was fufpended, was capable of condud:- jng any Heat at all, or at leafh any fenfible quantity. J therefore flattered myfelf with hopes of being

able,

412; Of the 'Propagation of Heat

able, with the affiftance of this inftrumentj to determine pofitively with regard to the paffage ©f Heat in the Torricellian vacuum: and this, I think, I have done, notwithftanding an unfortu- aate accident that put it out of my power to purfue the Experiments fo far as I intended.

This inllrument being fitted to a fmall Hand or jE)Ot of v/ood, in fuch a manner that the glafs body remained in a perpendicular fituation, I placed it in Miy room, by the fide of another inclofed ther^ siometer (N"" 2.), which was furrounded by air, and obferved the effects produced on it by the variation of Heat in the atmofphere. I foon dif- covered, by the motion of the mercury in the in- clbfed thermometer, that the Heat paffed througk ihe TorricelHan vacuum; but it appeared plainly from the iluggifhnefs, or great infenfibility of the thermometer, that the Heat pafled with much greater difficulty in this medium than in common air. I now plunged both the thermometers into a bucket of cold water j and I obferved that th^s mercury in the thermometer furrounded by air defeended much fafter than that in the thermome- ter furrounded by the Torricellian vacuum. I took them out of the cold water, and plunged them mto a veffel of hot water (having no conveniencies at hand to repeat the Experiment in due form with the freezing and with the boiling water) j and the thermometer furrounded by the Torricellian va- cuum appeared ftill to be much more infenfible or Huggifh than that furrounded by air.

Thefe

in various Subjiances, 413

Thefe trials were quite fufficient to convince me of the paffage of Heat in the Torricellian vacuum, and alfo of the greater difficulty of its paffage in that medium than in common air ; but, not fatisfied to reft my inquiries here, I took the firft opportunity that offered, and fet myfelf to repeat the Experiments which I had before made with the inftruments N° i and N° 2. I plunged this inftrument into a mixture of pounded ice and water, where I let it remain till the mercury in the inclofed thermometer had defcended to 0°; when, taking it out of this cold mixture, I plunged it fuddenly into a veffel of boiling water, and pre- pared myfelf to obferve the afcent of the mercury in the inclofed thermometer, as in the foregoing Experiments ; but unfortunately the moment the end of the glafs body touched the boiling water, it cracked with the Heat at the point where it had been hermetically fealed, and the water rufhing into the body, fpoiled the Experiment : and I have not fmce had an opportunity of providing myfelf with another inftrument to repeat it.

It having been my intention from the beginning to examine the conducing powers of the artificial airs or gaffes, the thermometer N° 3. was con- ftru6led with a view to thofe Experiments ; and having now provided myfelf with a ftock of thofe different kinds of airs, I began withjixed air, with which, by means of water, I filled the globe and cylinder .containing the thermometer; and flopping up the two holes in the great ftopple clofing the end of the cylinder, I expofed the inftrument in

freezing

L

\

414 Of i^^ Propagation of Heat

freezing water till the mercury in the inclofed thermometer had defcended to 0°; when, taking it out of the freezing water, I plunged it into a large veflel of boihng water, and prepared myfelf to obferve the times of heating, as in the former cafes ; but an accident happened, which fuddenly put a Hop to the Experiment. Immediately upon plunging the inflrument into the boiling water, the mercury began to rife in the thermometer with fuch uncommon celerity, that it had paffed the firfl divifion upon the tube (which marked the loth degree, according to Reaumur's fcale) before I was aware of its being yet in motion ; and having thus miffed the opportunity of obferving the time elapfed when the mercury arrived at that point, I was preparing to obferve its paffage of the next, when all of a fudden the ftopple clofmg the end of ihe cylinder was blown up the chimney with a o-reat explofion, and the thermometer, which, bet- ing cem.ented to it by its tube, was taken along with it, and was broken to pieces, and dellroyed in its fall.

This unfortunate Experiment, though it put a ftop for the time to the inquiries propofed, opened the way to other refearches not lefs interefling. Sufpefting that the explofion was occafioned by the rarefadion of the water which remained attached to the infide of the globe and cylinder after the operation of filling them with fixed air; and thinking it more than probable, that the uncom- mon celerity with which the mercury rofe in the thermometer was principally owing to the fame

caufe.

in various Subjiances, 415

caufe, I was led to examine the conducing power of moiji air, or air faturated with water.

For this Experiment I provided myfelf with a new thermometer N" 4. the bulb of which, being of the fame form as thofe already defcribed (viz. globular j was alfo of the fame fize, or half an inch in diameter. To receive this thermometer a glafs cylinder was provided, 8 lines in diameter, and about 14 inches long, and terminated at one end by a globe i f inch in diameter. In the centre of this globe the bulb of the thermometer was con- fined, by means of the ftopple which clofed the end of the cylinder; which flopple, being near 2 inches long, received the end of the tube of the thermometer into a hole bored through its centre or axis, and confined the thermometer in its place, without the afliftance of any other appara- tus. Through this flopple two other fmall holes were bored, and lined with thin glafs tubes, as in the thermometer N° 3. opening a paffage into the cylinder, which holes were occafionally flopped up with Hopples of cork ; but to prevent accidents, fuch as I have before experienced from an ex- plofion, great care was taken not to prefs thefe floppies into their places with any confiderable force, that they might the more eafily be blown out by any confiderable effort of the confined air, or vapour.

Though in this inftrument the thermometer was not altogether fo fleady in its place as in the thermometers N° i, N° 2, and N''3. the elafticity of the tube, and the weight of the mercury in the bulb of the thermometer, occafioning a fmali

vibration

41 6 Of the Propagation of Heat

vibration or trembling of the thermometer upon any fudden motion or jar; yet I preferred this method to the others, on account of the lower part of this thermometer being entirely free, or fuf- pended in fuch a manner as not to touch, or have any communication with, the lower part of the cylinder or the globe : for though the quantity of Heat received by the tube of the thermometer at its contact with the cylinder at its choaks, in the inftruments N° i and N° 2. or with the branches of the fteel fpring in N" 3. and from thence com- municated to the bulb, muil have been exceedingly fmall; yet I was defirous to prevent even that, and every other poffible caufe of error or inaccuracy.

Does humidity augment the conducing power of air ?

To determine this queftion I made the following Experiments, the weather being clear and fine, the mercury in the barometer (landing at 27 inches 8 lines, the thermometer at 19°, and the hygrometer at 44°.

t

in various ^uhfiances*

417

(Exp. N'' 17.) Thermometer N° 4. Surrounded by air dry to the 44th degree of the quill hygrometer of the Manheim Academy. Tahen out of freezing 'water, and plunged Into lo'iTmg water.	(Exp. N" 18.) j The fame Thermometer (N''.4.) Surrounded by air rendered! as mofi as pofTible by wetting the infide of the cylinder and globe with water. 1 Taken out of freezing water,' and plunged into loiling water. ■
Time eJapfed. Heat acquired.	Time elapfed. Heat acquired, 1
M. S. 0 34 10 0 39 20 0 44 30 0 51 40 J 6 Si:> 1 35 60 2 40 70 not obferved. 80	M. S. 0 t 06 TO 04 20 • 0 5 30 : 09 40 0 iB 50 0 26 60 0 43 70 ' 7 45 80 i
89:= total time of heating from 0° to 70''^	I 51 r= total time of heating from 0° to 70°^ ;

From thefe Experiments it appears, that the con- ducing power of air is very much increafed by hu- midity. To fee if the fame refult would obtain when the Experiment was reverfed, I now took the thermometer with the moi/^ air out of the boiling water, and plunged it into freezing water; and moving it about continually from place to place in the freezing water, I obferved the times of cooHng, as fet down in the following Table. N. B. To com- pare the refult of this Experimerit with thofe made with dry air, I have placed on one fide in the fol- lowing Table the Experiment in queilion, and on the other fide the Experiment N" 1 9. made with the thermometer N" 2.

4i8

Of the Propagation of Meat

(Exp. N°i9.)	(Exp. N° 10.)
'Thermometer N" 4.	Thermometer N" 2.
Surrounded by motfl air.	Surrounded by dry air.
Taken out of boiling ivater,	Taken out of boiling ivaier.
and plunged into freezing water. Time elapfed. Heat Joft.	and plunged into freezing water.
Time elapfed. Heat loft.
^o""	9n°
M. S.
04 70	0 33 70
0 14 60	0 34 60
0 31 50	0 44 50
0 52 40	0 ss 40
I 22 30	I 18 30
23 20	I 57 20
42 10	3 4° 10
9 8 =: total time of	9 12 = total time of
cooling from 80° to 10".	cooling from 80° to io°.

Though the difference of the whole times of cooling from 80° to 10° in thefe two Experiments appears to have been very fmall, yet the difference of the times taken up by the firil twenty or thirty degrees from the boiling point is very remarkable, and fhows with how much greater facility Heat palfes in moid; air than in dry air. Even the flow- nefs with which the mercury in the thermometer N° 4. defcended in this Experiment from the 30th to the 20th, and from the 20th to the 10th degree, I attribute in fome meafure to the great conducing power of the moifl air with which it was fur- rounded ; for the cylinder containing the thermo- meter and the moid air, being not wholly fubmerged in the freezing water, that part of it which remained put of the water was neceifarily furrounded by thq

air

in various Subjiances, 419

air of the atmofphere ; which being much warmer than the water, communicated of its Heat to the glafs ; which, palling from thence into the con- tained moid air as foon as that air became colder than the external air, was, through that medium. Communicated to the bulb of the inclofed thermo- meter, which prevented its cooling fo fad as it tvould otherwife have done. But when the weather becomes cold, I propofe to repeat this Experiment with variations, in fuch a manner as to put the mat- ter beyond all doubt. In the mean time I cannot help obferving, with what infinite wifdom and good- nefs Divine Providence, appears to hcu^e guarded us againft the evil effeds of exceffive Heat and Cold in the atmofphere ; for if it were poffible for the air to be equally damp during the fevere cold of the winter months as it fometimes is in fummer, its conducing power, and confequently its apparent coldnefs, when applied to our bodies, would be fo much increafed, by fuch an additional degree of moifture, that it would become quite intolerable ; but, happily for us, its power to hold water in fo- lution is diminiihed, and with it its power to rob us of our animal heat, in proportion as its coldnefs is increafed. Every body knows how very dif- agreeable a very moderate degree of cold is when the air is very damp ; and from hence it appears, why the thermometer is not always a juft meafure of the apparent or fenfible Heat of the atmofphere. If colds or catarrhs are occafioned by our bodies being robbed of our animal heat, the reafon is plain why thofe diforders prevail moft during the VOL. II. G G cold

42 o Of the Propagation of Heat

cold autumnal rains, and upon the breaking up of the froft in the *fpring. It is likewife plain from whence it is that fleeping in damp beds, and in- habiting damp houfes, is fo very dangerous ; and why the evening air is fo pernicious in fummer and in autumn, and why it is not fo during the hard frolts of winter. It has puzzled many very able philofophers and phyficians to account for the manner in which the extraordinary degree or rather quantity of Heat is generated which an animal body is fuppofed to lofe, when expofed to the cold of winter, above what it communicates tp the fur- rounding atmofphere in warm fumm^ weather ; but is it not more than probable, that the difference of the quantities of Heat, actually loft or commu- nicated, is infinitely lefs than what they have ima- gined ? Thefe inquiries are certainly very intereft- ing ; and they, are undoubtedly within the reach of well contrived and well conduced Experiments. But taking my leave for the prefent of this curious fubjed of inveftigation, I haften to the fequel of my Experiments.

Finding fo great a difference in the conducing powers of common air and of the Torricellian va- cuum, I was led to examine the conducing powers of common air of different degrees of denfity. For this Experiment I prepared the thermometer N® 4. by flopping up one of the fmall glafs tubes pafling through the ftopple, and opening a paffage into the cylinder, and by fitting a valve to the ex- ternal overture of the other. The inftrument, thus prepared, being put under the receiver of an

air-

in 'various Subjiances.

421

air-pump, the air paiTed freely out of the globe and cylinder upon working the machine, but the valve above defcribed prevented its return upon letting air into the receiver. The gage of the air-pump ihowed the degree of rarity of the air under the re- ceiver, and confequently of that filling the globe and cylinder, and immediately furrounding the ther- mometer.

With this inflrument, the weather being clear and fine, the mercury in the barometer ftanding at 27 inches 9 lines, the thermometer at 15", and the hygrometer at 47°, I made the following Ex- periments.

(£xp. N° 20.)	(£xp. No 21.)	(Exp. No 22.)
Thermometer No 4.	Thermometer No 4.	Thermometer No 4.
Surrounded by com-	Surrounded by air ra-	Surrounded by air ra-
mon air, barometer	refied bypumpingtill the	refied bypumping till the
ftanding at 27 inches	barometer-gage ftood at	barometer- gage ftood at
9 lines.	6 inches \i\ lines.	1 inch 2 lines.
Taken out of freezing	Taken out of freezing	Taken cut of freeing
water, and f lunged into	water, and plunged into	•water, and plunged into
boiling water.	boiling tuater.	boiling ivater.
Time Heat	Time Heat	Time Heat
elapfed. acquired.	elapfed. acquired.	elapfed. acquired.
. 0°	, 0°	00
M.S. 0	M. S. 0	M. S. 0
0 31 10	0 31 10	0 29 10
0 40 20	0 38 20	0 36 20
0 41 30	0 44 30	0 49 30
0 47 40	0 51 40	11 40
I 4 50	1 7 50	11 50
1 25 60	1 19 60	I 24 60
2 28 70	2 27 70	2 31 70
10 17 80	10 21 80	not obferved. 80
7 36 = total time	7 37 n total time	7 5 1 zz total time
of heating from 0° to	of heating from oo to	of heating from 0° to
170°-	700.	700.

The refult of thefe Experiments, I confefs, fur- prifed me not a little ; but the difcovery of truth

G G 2 being

422 Of the Propagation of Heat

being the fole objed of my inquiries (having no favourite theory to defend) it brings no difappoint- ment along with it, under vs^hatever unexpected fhape it may appear. I hope that further Experi- ments may lead to the difcovery of the caufe v^^hy there is fo little difference in the conducing powers of air of fuch very different degrees of rarity, while there is fo great a difference in the condufting powers of air, and of the Torricellian vacuum. At prefent, I fhall not venture any conjeftures upon the fubjeft ; but in the mean time I dare to alfert, that the Experiments I have made may be de- pended on.

The time of my flay at Manheim being expired (having had the honour to attend thither his mofl Serene Highnefs the Eleftor Palatine, reigning Duke of Bavaria, in his late journey), I was pre- vented from purfuing thefe inquiries further at that time ; but I fhall not fail to recommence them the firfl leifure moment I can find, which I fancy will be about the beginning of the month of No- vember. In the mean time, to enable myfelf to purfue them with effed, I am fparing neither la- bour nor expence to provide a complete apparatus neceffary for my purpofe ; and his Electoral High- nefs has been gracioully pleafed to order M. Ar- TARiA (who is in his fervice) to come to Munich to affift me. With fuch a Patron as his mofl Se- rene Highnefs, and with fuch an afliflant as Ar- TARIA, I fhall go on in my purfuits with chearful- nefs. Would to God that my labours might be

as

in various Subjiances.

423

as ufeful to others as they will be pleafant to me!

I fhall conclude this chapter with a fhort account of fome Experiments I have made to determine the conducing powers of water and of mercury ; and -with a table, fhowing at one view the conduct- ing powers of all the different mediums which I have examined.

Having filled the glafs globe inclofmg the bulb of the thermometer N" 4, firfl with water, and then with mercury, I made the following Experiments,- to afcertain the condudling powers of thofe two Fluids.

(Exp. N° 23.) Thermometer N° 4.	(Exp. N° 24, 25, and 26.) Thermometer N" 4.
Surrounded by ivater. Taken out of freezing ivater, and f lunged into boning -water.	Surrounded by mercury. Taken out of free-zing water, and f lunged inlio boiUng ivater.
Time elapfed. Heat acquired.	Time eiapfed. ^^^^ acquired.
	Ex. N''24. Ex. N°25. Ex.N°26.
0°	0°
M S.	M.S. M.S. M.S.
0 19 10 08 20	05 05 05 10 04 02 05 20
09 30 0 11 40 015 50 0 21 60	02 0 2 04 30 04 05 05 40 04 04 07 50 07 04 08 60
0 34 70 a 13 80	0 15 09 0 14 70 Notobfervcd. 0 58 Not obferved. 80
157 = total time of heat- ing from o^ to 70°	0 41 0 31 0 48 zz total times of heating no-ni 0° to 70°.

The total times of heating from o° to 70** in the three Experiments with mercury being 41 feconds,

G G 3 31 feconds.

424 Of the Propagation of Heat

31 feconds, and 48 feconds, the mean of thefe times is 36^ feconds ; and as in the Experiment with water the rime employed in acquiring the fame degree of Heat was i' ^'j" ~ 117 feconds, it appears from thefe Experiments, that the conduct- ing power of mercury to that of water, under the clrcumflances defcribed, is as t,6^ to 117 inverfely, or as 1000 to 313. And hence it is plain, why mercury appears fo much hotter, and fo much colder, to the touch than water, when in fa6l it is of the fame temperature : for the force or violence of the fenfation of what appears hot or cold depends not entirely upon the temperature of the body ex- citing in us thofe fenfations, or upon the degree of Heat it aftually poffelfes, but upon the quantity of Heat it is capable of comm.unicating to us, or re^ ceiving from us, in any given (hort period of time, or as the intenfity of the communication ; and this depends in a great meafure upon the conducing powers of the bodies in queftion.

The fenfation excited in us when we touch any thing that appears to us to be hot is the entrance of Heat into our bodies j that of cold is its exit ; and whatever contributes to facilitate or accelerate this communication adds to the violence of the fenfation. And this is another proof that the ther- mometer cannot be a juft meafure of the intenfity of xhQ.fenf.ble Heat, or Cold, exifting in bodies ; or rather, that the touch does not aiford us a jufl in- dication of their real temperatures.

A Table

in 'various Subjiances.

425

A Table of the conducting Powers of the under-^ mentioned Meqiums as determined by the fore- going Experiments.

Therm. N«i.		Thermometer N" 4.
Taken out of freezing	water, and plunged into boiling water.
Time elapfed.
ci ° —		__„ ^		^^	^^
sz ^	U .. Q		iT-i+H	00	CO	-5^2	-0
Torricellian ' cuum (Exp. 3, 4. and I		1! c a.	•Zi IP* f4 -111 be i ^ -o w	« 0			.'e '5 2 X 0"
				,
M.S.	M.S.	M.S.	M.S.	M.S.	M.S.	M.S.	0
0 52	0 31	0 31	0 29	0 6	0 19	0 5	10
0 58	0 40	0 38	0 36	0 4	0 2	° 3f	20
I 3	0 41	0 44	0 49	0 5	0 9	0 2|	30
I 18	0 47	0 51	I I	0 9	0 11	0 4|	40
1 25	1 4	I 7	I I	0 18	0 15	° . 5	50
I 58	I 25	I 19	I 24	0 26	0 21	0 6|	60
3 19	2 28	2 27	2 31	0 43	0 34	0 I2|	70
II 57	10 17	10 21		7 45	2 13	0 58	80
10 53	7 36	7 37	7 5' I 51	I S7	036!=	total
times of h	eating fror	n qo to 70	0.

. In determining the relative condufting powers of thefe mediums, I have compared the times of the heating of the thermometers from 0° to 70** in (lead of taking the whole times from 0° to 80% and tAis I have done on account of the fmall vari- atig-i in the Heat of the boiling water arifmg from th/ variation of the weight of the atmofphere, and iMo on account of the very flow motion of the /mercury between the 70th and the 80th degrees, / and the difficulty of determining the precife mo-

G G 4 ment

426 Of the Propagation of Heat

ment when the mercury arrives at the 8oth degree.

Taking now the conducing power of mercury = 1000, the conducing powers of the other me- diums, as determined by thefe Experiments, will be as follows, wz.

Mercury - - - 1000

Moift air - - - 330

Water - - - 313

Common air, denfity=:i SotVb-

Rarefied air, denfityzzl 8otVo

Rarefied air, denfity — ^V 78

The Torricellian vacuum ^f^

And in thefe proportions are the quantities of Heat which thefe different mediums are capable of tranf- mitting in any given time ; and confequently thefe numbers exprefs the reh-iiv e fenftble temperatures of the mediums, as well as their conducing powers. How far thefe decifions will hold good under a variation of circumllances experiment only can determine. This is certainly a fubjed of inveftigation not lefs curious in itfelf than it is interefting to mankind ; and I wiih that what I have done may induce others to turn theiir atten- tion to this long neglefted field of experimental inquiry. For my own part, I am determines not to quit it. *

In the future profecution of thefe inquiries, I d<. not mean to confine myfelf folely to the determin- ing

in 'various Siibjiances, 427

Ing of the conducing powers of Fluids ; on the contrary, folids, and particularly fuch bodies as are made ufe of for cloathing, will be principal fubjefts of my future Experiments. I have indeed already begun thefe refearches, and have made fome progrefs in them ; but I forbear to anticipate a matter which will be the fubjedt of a future com- miinication.

C 4^3 3

CHAP. II.

The relative Wannth of various Sub/lances ufed in making artificial Cloathing^ determined by Ex- ■periment. — Relative Warmth of Coverings of the fame Thicknefs, and formed of the fame Subjiance^ but of different Denfities, — Relative Warmth of , Coverings formed of equal ^antities of the fame Subfiance, difpofed in different Ways. — Experi- ments made with^ a View to determining how far the Power which certain Bodies pMefs of confining Heat depends on their chemical Properties. — Ex- periments with Charcoal— w/V/6 Lampblack— -zf/V/^ Woodafhes — Striking Experiments with Semen - -^podii.— ^// thefe Experiments indicate that the ' '^h occupies the Interfiices of Subjiances ,-- - Coverings for confining Heat, ads a '^mt Operation. — Thofe Sub- " Air from conducting ' ' ' Manner in ^'- to a

Of the Fropagaiion of Heat, he. 429

refearches chiefly to tbofe points, conceiving that very great advantages to mankind could not fail to be derived from the difcovery of any new fa^ls re- lative to thefe operations.

If the laws of the communication of Heat from one body to another were known, meafures might be taken with certainty, in all cafes, for confining it, and directing its operations, and this v^ould not only be produftive of great oeconomy in the articles of fuel and clothing, but would like wife greatly increafe the comforts and conveniencies of life ; objefts of which the philofopher fhould never iofc fight.

The route which I have followed in this inquiry is that which I thought bid faireft to lead to ufeful difcoveries. "Without embarraffing myfelf with any particular theory, I have formed to myfelf a plan of experimental invefligation, which I conceived would conduft me to the knowledge of certain fads, of which we are now ignorant, or very imperfectly in- formed, and with which it is of confequence that we fhould be made acquainted.

The firft great objed which I had in view in this enquiry was to afcertain, if pofTible, the caufe of the warmth' of certain bodies ; or the circumftances upon which their power of confining Heat depends. This, in other words, is no other than to determine the caufe of the conducting and non- conducting power of bodies, with regard to Heat.

To this end I began by determining by a6lual experiment the relative conducting powers of va-

lious

430 Of the Propagation of Heat

rious bodies of very different natures, both fluids and folids, of fome of which Experiments I have already given an account in the Paper above men- tioned, which is pubhfhed in the Tranfaclions of the Royal Society for the year 1786; I fhall now, taking up the matter where I left it, give the con- tinuation of the hiftory of my refearches.

Having difcovered that the Torricellian vacuum is a much Worfe conductor of Heat than common air, and having afcertained the relative conducing powers of air, of water, and of mercury, under dif- ferent circumftances, I proceeded to examine the conducing powers of various foUd, bodies^ and par- ticularly of fuch fubitances as are commonly made ufe of for clothing.

The method of making thefe Experiments was as follows : a mercurial thermometer, (fee Fig. 4,) whofe bulb was about -~^ of an inch in diameter, and its tube, about 10 inches in length, was fuf- pended in the axis of a cylindrical glafs tube, about ^ of an inch in diameter, ending with a globe It^ inch in diameter, in fuch a manner that the centre of the bulb of the thermometer occupied the centre of the globe ; and the fpace between the internal furface of the globe and the furface of the bulb of the thermometer being filled with the fubftance whofe conducing power was to be determined, the inflrument was heated in boiling water, and after- wards being plunged into a freezing mixture of pounded ice and water, the times of cooling were obferved, and noted down.

The

in various Subjiances, 431

The tube of the thermometer was divided at every tenth degree from 0°, or the point of freezing, to 80**, that of boiling water, and thefe divifions being marked upon the tube with the point of a diamond, and the cylindrical tube being left empty, the height of the mercury in the tube of the ther- mometer was feen through it.

The thermometer was confined in its place by means of a flopple of cork, about if inch long, fitted to the mouth of the cylindrical tube, through the centre of which ftopple the end of the tube of the thermometer pafled, and in which it was ce- mented.

The operation of introducing into the globe the fubftances whofe conducing powers are to be de- termined, is performed in the following manner ; the thermometer being taken out of the cylindri- cal tube, about two-thirds of the fubftance which is to be the fubjedl of the Experiment are introduced into the globe ; after which, the bulb of the ther- mometer is introduced a few inches into the cylin- der ; and, after it, the remainder of the fubftance being placed round about the tube of the thermo- meter ; and laftly, the thermometer being intro- duced farther into the tube, and being brought into its proper place, that part of the fubftance which, being introduced laft, remains in the cylindrical tube above the bulb of the thermometer, is pufhed down into the globe, and placed equally round the bulb of the thermometer by means of a brafs wire which is pafled through holes made for

15 that

432 Of the Propagation of Heat

that purpofe in the ftopple clofmg the end of the cyhndrical tube.

As this inflrument is calculated merely for mea- furing the paflage of Heat in the fubftance whofe conducing power is examined, I fhall give it the name of pajfage-thermometer, and I fhall apply the fame appellation to all other inflruments conftru£ted upon the fame principles, and for the fame ufe, which I may in future have occafion to mention j and as this inflrument has been fo particularly de- fcribed, both here, and in my former Paper upon the fubjeft of Heat, in fpeaking of any others of the fame kind in future it will not be neceffary to enter into fuch minute details. 1 fhall, therefore, only mention xhdr fizesy or the diameters of their bulbs, the diameters of their globes, the diameters of their cylinders, and the lengths and divifions of their tubes, taking it for granted that this will be quite fufHcient to give a clear idea of the inflrument^

In mofl of my former Experiments, in order to afcertain the conducing power of any body, the body being introduced into the globe of the pafTage- thermometer, the inflrument was cooled to the temperature of freezing water, after which, being taken out of the ice water, it was plunged fuddenly into boiling water, and the times of heating from ten to ten degrees were obferved and noted ; and I faid that thefe times were as the conducing power of the body inverfely ; but in the Experiments of which I am now about to give an account, I have in general reverfed the operation j that is to fay,

inftead

in 'various Subjiances, 433

inftead of obferving the times of heating, I have firfl heated the body in boiling water, and then plunging it into a mixture of pounded ice and ice- cold water, I have noted the times taken up in cooling.

I have preferred this laft method to the former, not only on account of the greater eafe and conveni- ence with which a thermometer, plunged into a mix- ture of water, may be obferved, than when placed in a. veflel of boiling water, and furrounded by hot fteam, but alfo on account of the greater accuracy of the Experiment, the heat of boiling water va- rying with the variations of the preffure of the at- mofphere : confequently the Experiments made upon different days will have different refults, and of courfe, ftrid:ly fpeaking, cannot be compared together j but the temperature of pounded ice and water is ever the fame, and of courfe the refults of the Experiments are uniform.

In heating the thermometer, I did not in general bring it to the temperature of the boiling water, as this temperature, as I have juft obferved, is variable ; but when the mercury had attained the y^'^ of its fcale, I immediately took it out of the boiling water, and plunged it into the ice and water; or, which I take to be ftill more accurate, fuffering the mercury to rife a degree or two above yf, and then taking it out of the boiling water, I held it over the velfel containing the pounded ice and water, ready to plunge it into that mixture the mo- ment the mercury, defcending, paflfes the jf.

Having a watch at my ear which beat half fe- conds (which I counted), t noted the time of the

paflage

434 Of i^^ Propagation of Heat

paflage of the mercury over the divlfions of the thermometer, marking 70® and every tenth degree from it, defcending to i o** of the fcale. I continued the cooling to 0°, or the temperature of the ice and water, in very few inflances, as this took up much time, and was attended with no particular ad" vantage, the determination of the times taken up in cooling 60 degrees of Reaumur*s fcale, that is to fay, from 70" to 10", being quite fufficient to afcer- tain the conducting power of any body whatever.

During the time of cooling in ice and water, the thermometer was conftantly moved about in this mixture from one place to another ; and there was always fo much pounded ice mixed with the water ^ that the ice appeared above the furface of the water j the vefTel, which was a large earthen jar, being firft quite filled with pounded ice, and the water being afterwards poured upon it, and frefh quantities of pounded ice being added as the occafion required.

Having defcribed the apparatus made ufe of in thefe Experiments, and the manner of performing the different operations, I fhall now proceed to give an account of the Experiments themfelves.

My firft attempt was to difcover the relative con- ducing powers of fuch fubftances as are commonly made ufe for clothing; accordingly, having pro- cured a quantity of rawftlk, as fpun by the worm, Jheep^s wool, cotton wool., linen in the form of the fineft lint, being the fcrapings of very fine Irifh linen, the fineft part of the fur of the beaver fepa- rated from the fkin, and from the long hair, the fineft part of the fur^of a white Ruffian hare^ and

Eider

in 'Various Sub/^atices,

435

Eider down ; I introduced fucceffively 1 6 grains in weight of each of thefe fubftances into the globe of the paflage-thermometer, and placing it carefully and equally round the bulb of the thermometer, I heated the thermometer in boiling water, as before defcribed, and taking it out of the boiling water, plunged it into pounded ice and water, and ob- ferved the times of cooling.

But as the interftices of thefe bodies thus placed in the globe were filled with air, I fir ft made the Experiment with air alone, and took the refult of that Experiment, as a ftandard by which to com- pare all the others ; the refults of three Experi- ments with air were as follows :

The bulb of the thermometer furrounded by air.
Heat loft.	Exp. No. I.	Exp. No. 7,.	Heat acquired.	Exp. No. 3.
Time elapfed.	Time elapfed.	Time elapfed.
70" 60" 50" 40° 30° 20° 10°	38" 46 59 80 122 231	38" 46 59 79 122 230	10'' 20" 30^ 40<' 50° 60° 70-	39" 43 53 67 96 175
Total times.	576	574	—	473

VOL. II.

H H

The

43^

Of the Propagation of Heat

The following Table fliows the refults of the Experiments, with the various fiibflances therein mentioned :

			'0	■3		.s	.•^
	u		it	0 . C to	.S ?5		<2 <» 03 feo	1 "5 0 >-
s	<	^^	O.VO	0 vo	c ^	> ^	Ji.o	'c >^
		=5 H	<U H	a "		« M	(^ l-l	-c '-'
ffi		<^	Zn	0 u	(3h	pa	X	3
	Exp. I.	Exp. 4-	Exp. 5.	Exp. 6.	Exp. 7.	Exp. 8.	Exp. 9.	Ex. 10,
70°	ii.i,.,	_	.._, '	»—	-^	».«
60°	38"	94"	79"	83"	80"	99	97"	98"
50°	46	110	95	95	93	116	117	116
40°	59	133	118	"7	"5	153	144	146
30°	80	185	162	152	150	185	193	192
20°	122	273	238	2il	218	26^	270	268
10°	231	489	426	378	376	478	494	485
Total times.	576	1284	iitS	1046	1032	I2q6	131S	1305

Now the warmth of a body, or its power to con- fine Heat, being as its power of refilling the paflage of Heat through it, (which I fhall call its non-condud- ing power,) and the time taken up by any body in cooling, which is furrounded byanymedium through which -the Heat is obliged to pafs, being, cateris pa- ribus, as the refifhance which the medium oppofes to the paflage of the Heat, it appears that the warmth of the bodies mentioned in the foregoing Table are as the times of cooling ; the condu^iing powers being inverfely as thofe times, as I have formerly Ihown.

From the refults of the foregoing Experiments it appears, that of the feven different fubftances made ufe of, hares fur and Eider down were the

warmed ;

in 'various Subjiances. 437

warmeft ; after thefe came beavers fur ; raw filk ; flieep's wool j cotton wool ; and laftly, lint, or the fcrapings of fine linen ; but I acknowledge that the differences in the warmth of thefe fubftances were much lefs than I expected to have found them.

Sufpefting that this might arife from the vo- lumes or folid contents of the fubftances being different, (though their weights were the fame,) arifing from the difference of their fpecific gravities ; and as it was not eafy to determine the fpecific gra- vities of thefe fubftances with accuracy, in order to fee how far any known difference in the volume or quantity of the fame fubftance, confined always in the fame fpace, would add to or diminifh the time of cooling, or the apparent warmth of the covering, I made the three following Experi- ments.

In the firft, the bulb of the thermometer was fur- rounded by 1 6 grains of Eider down ; in the fe- cond by 32 grains ; and in the third by 64 grains j and in all thefe Experiments the fubftance was made to occupy exadly the fame fpace, viz. the whole internal capacity of the glafs globe, in the centre of which the bulb of the thermometer was placed ; confequently the thicknefs of the covering of the thermometer remained the fame, while its denfity was varied in proportion to the numbers I, 2, and 4.

The refults of thefe Experiments were as follow :

H H 2

438

Of the 'Propagation of Heat

The bulb of the thermometer being	[urrounded
	by Elder down.
Heat loft.	16 grains.	32 grains.	64 grains.
	(Exp. No. 11.)	(Exp. No. 12.)	(Exp. No. 13.)
70°	,		-
60^	97"	Ill"	112"
	117	128 157	130 165
30° 20° 10°	192 267 486	207 304	224 326 658
Total times.	1304	1472	1615

Without^ flopping at prefent to draw any par- ticular conclufions from the refults of thefe Ex- periments, I fhall proceed to give an account of fome others, which will afford us a little further infight into the nature of fome of the circumftances upon which the warmth of covering depends.

Finding, by the laft Experiments, that the denfity of the covering added fo confiderably to the warmth of it, its thicknefs remaining the fame, I was now defirous of difcovering.how far the inter- nal ftru£lure of it contributed to render it more or lefs pervious to Heat, its thicknefs and quantity of matter remaining the fame. By internal ftruc- ture, I mean the difpofition of the parts of the fubftance which forms the covering ; thus they may be extremely divided, or very fine, as raw

filk

in various Subjiances, 439

filk as fpun by the worms, and they may be equally diftributed through the whole Ipace they occupy; or they may be coarfer, or in larger mafles, with larger interflices, as the ravelings of cloth, or cuttings of threads.

If Heat pafled through the fubflances made ufe of for covering, and if the warmth of the covering depended folely upon the difficulty which the Heat meets with in its palTage through the fubflances, or folid parts ^ of which they are compofed ; in that cafe, the warmth of covering would be always, azteris paribus, as the quantity of materials of of which it is compofed ; but that this is not the cafe, the following, as well as the foregoing Expe- riments clearly evince.

Having, in the Experiment N° 4, afcertained the warmth of i6 grains of raw filk, I now re- peated the Experiment with the fame quantity, or weight, of the ravelings of white taffety, and afterwards with a like quantity of common fewjing filk, cut into lengths of about two inches.

The following Table fhows the refults of thefe three Experiments ;

H H

44©

Of the Propagation of Heat

		#\ 1
	•	>v
	2	4^
	6a	iS	0 Vi
d	»4	4-> •	4-J
o		to	— * faO L
*^		0 bO	03
EC	1— t	en 0 .S **	.S 0 M
1	Qj	^ fl
	CJ	^	0 —
	p^	Pi	«5
	Exp. 4.	Exp, 14.	Exp. 15.
70°			C—K
60"	94"	ft 90	67"
50°	no	106	79
4o'>	133	128	99
30°	185	172	135
20°	273	246	195
IO«	489	427	342
Total times	1284	, 1 169	917

Here, notwithftanding that the quantities of the filk were the fame in the three Experiments, and though in each of them it was made to occupy the fame fpace, yet the warmth of the coverings which were formed were very different, owing to the dif^ ferent difpofition of the material.

The raw filk was very fine, and was very equally diftributed through the fpace it occupied, and it formed a warm covering.

The ravelings of taffety were alfo fine, but not fo fine as the raw filk, and of courfe the interftices between its threads were greater, and it was lefs warm ; but the cuttings of fewing filk were very coarfe, and confequently it was very unequally dif- tributed in the fpace in which it was confined ; and it made a very bad covering for confining Heat.

It

in various Subjiances,

441

It is clear from the refults of the five lafl Experi- ments, that the air which occupies the interftices of bodies, made ufe of for covering, ads a very- important part in the operation of confining Heat ; yet I fhall poflpone the examination of that circum- flance till I fhall have given an account of feveral other Experiments, which, I think, wiU throw llill more light upon that fubjedl.

But, before I go any further, I will give an ac- count of three Experiments which I made, or ra- ther the fame Experiment which I repeated three times the fame day, in order to fee how far they may be depended on, as being regular in their re- fults.

The glafs globe of the paiTage-thermometer being filled with 16 grains of cotton-wool, the in- strument was heated and cooled three times fuccef- fively, when the times of cooling were obferved as follows '.

Heat loft.	Exp. 16.	Exp. 17.	Exp. 18.
70°		. , ^	,
60"	82"	84"	83"
50°	96	95	95
40°	118	117	116
30-	152	153	151
20°	221	221	220
10°	380	377	377
Total times.	1049	1047	1042

The difference of the times of cooling in thefe three Experiments were extremely fmall j but re-

H H 4 gular

442 Of the Propagation of Heat

gular as thefe Experiments appear to have been in their refults, they were not more fo than the other Experiments made in the fame way, many of which were repeated two or three times, though, for the fake of brevity, I have put them down as fmgle Experiments.

But to proceed in the account of my invefliga- tions relative to the caufes of the yarmth of warm clothing. Having found that the finenefs and equal diflribution of a body or fubftance made ufe of to form a covering to confine Heat, contributes fo much to the warmth of the covering, I was de-r firous, in the next place to fee the effeft of con^ denfmg the covering, its quantity of matter re-e maining the fame, but its thicknefs being diminiflied in proportion to the increafe of its denfity.

The Experiment I made for this purpofe was as follows : — I took 1 6 grains of common fewing filk, neither very fine nor very coarfe, and winding- it about the bulb of the thermometer in fuch a manner that it entirely covered it, and was a§ nearly as poffible of the fame thicknefs in every part, I replaced the thermometer in its cylinder and globe, and heating it in boiling water, cooled it in ice and water, as in the foregoing ExperimentSo The refults of the Experiment were as may be feeii in the following Table ; and in order that it may be compared with thofe made with the fame quan« tity of fiik differently dilpofed of, I have placed thpfe Experiments by the fide of it :

in various Subjiances,

443

	Raw filk, 16 grs.	Fine ravelings of taffety, 16 grs.	Sewing filk cut into lengths, 16 grs.	Sewing filk, 16 grs. wound round the bulb of the thermo- meter.
	Exp. No. 4.	Exp. No. 14.	Exp. No. 15.	Exp. No. 19.
70° 60° 30° 20° 10"	94 1X0 133 185 273 489	90 106 128 172 246 427	"67" 79 99 135 195 342	"46" 62 85 121 T9I 399
Total times.	1214	1169	917	904

It is not a little remarkable, that, though the covering formed of fewing filk wound round the bulb of the thermometer In the 19th Experiment, appeared to have fo little power of confining the Heat when the inftrument was very hot, or when it was firft plunged into the ice and water, yet af- terwards, when the Heat of the thermometer ap- proached much nearer to that of the furrounding medium, its power of confining the Heat which re- mained in the bulb of the thermometer appeared to be even greater than that of the filk in the Experi- ment N" 15, the time of cooling from 20° to lo** being in the one 399", and in the other 342". The fame appearance was obferved in the following Experiments, in which the bulb of the thermome- ter was furrounded by threads of wool^ of cotton^ and of linen^ oxjlax, wound round it, in the like

manner

444

Of the Propagation of Heaf

manner as the fewing filk was wound round It in the laft Experiment. '

The following Table (hows the refults of thefe Experiments, with the threads of various kinds ; and that they may the more eafily be compared with thofe made with the fame quantity of the fame fubflances in a different form, I have placed the ac- counts of thefe Experiments by the fide of each other. I have alfo added the account of an Expe- riment, in which i6 grains of fine linen cloth were wrapped round the bulb of the thermometer, going roung it nine times, and being bound together at the top and bottom of it, fo as completely to cover it.

Heat loft.	Sheep's lueol, 16 grains, furrounding the bulb of the thermometer.	M^oolhn thread, 16 grains, wound round the bulb of the thermometer.	Cotton luool, 16 grains, furrounding the bulb of the thermometer.	Cotton thread, 16 grains, wound round the bulb of the thermometer.	Lint, 16 grains, fur- rounding the bulb of the thermometer.	Linen thread, 16 grains, wound round the bulb of the thermometer.	Linen cloth, 16 grains, wrapped round the bulb of the thermo- meter.
	Exp. 5.	Ex. 20.	Exp. 6.	Ex. 21.	Exp. 7.	Ex. 22. 46" 62 S3 117 180 38s 873	Exp. 23.
70° Sc" 40° 30» 20° 10°	"T9" 95 118 162 238 426	46" 63 89 126 2CO 410	95 117 152 221 378	~5" 60 83 "5 179 370	80'' 93 "5 150 218 376	I2" 56 74 108 168 332
Total times.	1118	934	1046	852 1 1032	783

That thread wound light round the bulb of the thermometer fhould form a covering lefs warm than the fame quantity of wool, or other raw ma=

terials

in various Sub/lances. 445

terials of which the thread is made, furrounding the bulb of the thermometer in a more loofe man- ner, and confequently occupying a greater fpace, is no more than what I expeded, from the idea I had formed of the caufes of the warmth of cover- ing; but I confefs I was much furprifed to find that there is fo great a difference in the relative warmth of thefe two coverings, when they are em- ployed to confine great degrees of Heat, and when the Heat they confine is much lefs in proportion to the temperature of the furrounding medium. This difference was very remarkable ; in the Experi- ments with fheep's wool, and with woollen thread, the warmth of the covering formed of 1 6 grains of the former, was to that formed of 1 6 grains of the latter, when the bulb of the thermometer was heated to 70" and cooled to 60°, as 79 to 46 (the furrounding medium being at o") ; But afterwards, when the thermometer had only fallen from 20° to 10° of Heat, the warmth of the wool was to that of the woollen thread only as 426 to 410 ; and in the Experiments with lint, and with linen thread, when the Heat was much abated, the covering of the thread appeared to be even warmer than that of the lint, though in the beginning of the Experi- ments, when the Heat was much greater, the lint was warmer than the thread, in the proportion of So to 46.

From hence it fliould feem that a covering may, under certain circumflances, be very good for con- fining fmall degrees of warmth, which would be

but

44^ Of the Propagation of Heat

but very indifferent when made ufe of for confining a more intenfe Heat, and vice verfa. This, I be- lieve, is a new faft ; and, I think the knowledge of it may lead to further difcoveries relative to the caufes of the warmth of coverings, or the manner m which Heat makes its pafTage through them. But I forbear to enlarge upon this fubjed, till I ihall have given an account of feveral other Expe- riments, which I think throw more light upon it, and which will confequently render the inveftiga- tion eafier and more fatisfaflory.

With a view to determine how far the power "which certain bodies appear to polfefs of confining Heat, when made ufe of as covering, depends upon the natures of thofe bodies, confidered as chymical fubflances, or upon the chymical prin- ciples of which they are compofed, I made the following Experiments.

As charcoal is fuppofed to be compofed almofl entirely of phlogifton, I thought that, if that prin- ciple was the caufe eittier of the conducing power, or the non-condu6ling power of the bodies which contain it, I fhould difcover it by making the Ex- periment with charcoal, as I had done with various other bodies. Accordingly, having filled the globe of the paffage-thermometer with 176 grains of that fubflance in very fine powder, (it having been pounded in a mortar, and fifted through a fine fieve,) the bulb of the thermometer being fur- rounded by this powder, the inflrument was heated in boiling water, and being afterwards plunged

into

in various Subjiances,

447

into a mixture of pounded Ice and water, the times of cooling were obferved as mentioned in the following Table. I afterwards repeated the Expe- riment with lampblack, and with very pure and very dry wood aflies ; the refults of which Experi- ments were as under-mentioned :

The	3ulb of the thermometer furrounded by
Heat loft.	176 grains of	176 grains of	195 grains of	307 grains of
fine powder	fine powder	lampblack.	pure dry
	of charcoal.	of charcoal.		wood afhes.
	E.Kp. No. 24.	Exp. No. 25.	Exp. No. 26.	Exp. No. 27.
70°	^^^	^_^	^.^
60°	79"	91"	124"	96"
K	95	91	118	92
40-	100	109	134	107
SO'*	139	133	164	136
20°	196	192	237	185
10^	331	321	394	311
Total times.	940	937	1 171 ;■■	927

N

The Experiment No. 25 was fimply a repetition of that numbered 24, and was made immediately after it ; but, in moving the thermometer about in the former Experiment, the powder of charcoal which filled the globe was fliaken a little together, and to this circumflance I attribute the difference in the refults of the two Experiments.

In the Experiments with lampblack and with wood aflies, the times taken up In cooling from 70" to 60° were greater than thofe employed in cooling from 60'^ to 50"* j this mod probably arofe

from

44^ Of the Propagation of Heat

from the confiderable quantity of Heat contained by thefe fubftances, which was firft to be difpofed of, before they could receive and communicate to the furrounding medium that which was contained by the bulb of the thermometer.

The next Experiment I made was with femen lycopodii, commonly called witch -meal, a fub fiance which poflefTes very extraordinary properties. It is almoli impoffible to wet it ; a quantity of it ftrewed upon the furface of a bafin of water, not only fwims upon the water without being wet, but it prevents other bodies from being wet which are plunged into the water through it ; fo that a piece of money, or other folid body, may be taken from the bottom of the bafm by the naked hand, with- out wetting the hand ; which is one of the tricks commonly fhown by the jugglers in the country : this meal covers the hand, and defcending along with it to the bottom of the bafm, defends it from the water. This fubflance has the appearance of an exceeding fine, light, and very moveable yellow powder, and it is very inflammable ; fo much fo, that being blown out of a quill into the flame of a candle, it flaflies like gunpowder, and it is made ufe of in this manner in our theatres for imitating lightning.

Conceiving that there mufl have been a ftrong ■ attraftion between this fubfl:ance and air, and fuf- pefting, from fome circumftances attending fome of the foregoing Experiments, that the warmth of a covering depends not merely upon the finenefs of

the

in various Subjlances.

449

the fubftance of which the covering is formed, and the difpofition of its parts, but that it arifes in fome meafure from a certain attraction between the fub- ftance and the air which fills its interftices, I thought that an Experiment with fernen lycopodii might pof- libly throw fome light upon this matter ; and in this opinion I was not altogether miftaken, as will appear by the refults of the three following Expe- riments.

The bulb of the thermometer furrounded by 256 grs. oifemen lycopodii.
Heat loft.	Cooled.	Cooled.	Heat acquired.	Heated.
Exp. No. z8. Exp. No. 29.	Exp. No. 30.
70° 60° 50" 40° 30° 20°	1 46" 162 175 209 284 502	157" 160 T70 203 288 513	0° 10° 20" 30° 40"^ 60° 70°	^0" 68 63 76 121 316 1585
Total times.	1478	1491		2459

In the lad Experiment (N° 30) the refult of which was fo very extraordinary, the inftrument was cooled to 0° in thawing ice, after which it was plunged fuddenly into boiling water, where it re- mained till the inclofed thermometer had acquired the Heat of 70°, which took up no lefs than 2456 feconds, or above 40 minutes ; and it had remained

3 '^^

45 o Of the Propagation of Heat

in the boiling water full a minute and an half before the mercury in the thermometer fhowed the leaft iign of rifmg. Having at length been put into mo- tion,it rofe very rapidly 40 or 50 degrcesjafter which its motion gradually abating became fo flow, that it took up 1585 feconds, or fomething more than 26 minutes, in rifmg from 60° to 70% though the temperature of the medium in which it was placed during the whole of this time was very nearly 80° j the mercury in the barometer {landing but little [^ Ihort of 27 Paris inches.

All the different fubftances which I had yet made ufe of in thefe Experiments for furrounding or covering the bulb of the thermometer, fluids ex- cepted, had, in a greater, or in a lefs degree confined the Heat, or prevented its pafllng into or out of the thermometer fo rapidly as it would have done, had there been nothing but air in the glafs globe, in the centre of which the bulb of the thermometer was fufpended. But the great queftlon is, how, or in what manner, they produced this effeft ?

And firfl:, it was not in confequence of their own non-condu6ling powers, fimply confidered ; for, if infliead of being only bad conductors of Heat, we fuppofe them to have been totally impervious to Heat, their volumes or folid contents were fo ex- ceedingly fmall in proportion to the capacity of the globe In which they were placed, that, had they had no effeft whatever upon the air filling their in- terftices, that air would have been fufliclent to have conduced all the Heat communicated, in lefs time than was adually taken up in the Experiment.

The

in 'various Subjiances* 4^1

The diameter of the globe being 1,6 inches, its contents amounted to 2,14466 cubic inches ; and the contents of the bulb of the thermometer being only 0,08711 of a cubic inch, (its diameter being 0,55 of an inch,) the fpace between the bulb of the thermometer and the internal furface of the globe amounted to 2,14466 — 0,0871 1 — 2,05755 cubid inches ; the whole of which fpace was occupied by the fubftances by which the bulb of the ther- mometer was furrounded in the Experiments in queflion.

But though thefe fubftances occupied this fpace, they were far from Jilling it ; by much the greater part of it being filled by the air which occupied the interftices of the fubftances in queftion. In the Experiment N 4, this fpace was occupied by 16 grains of raw filk ; and as the fpecific gravity of raw lilk is to that of water as 1734 to 1000, the volume of this filk was equal to the volume of 9,4422 grains of water j and as i cubic inch of water weighs 253,185 grains, its volume was equal to^\'4ir^^ 11:0,037294 of a cubic inch; and, as the fpace it occupied amounted to 2,05755 cubic inches, it appears that the filk filled no more than about ^-^ part of the fpace in which it was confined, the reft of that fpace being filled with air.

In the Experiment N° i , when the fpace between the bulb of the thermometer and the glafs globe, in the centre of which it was confined, was filled with nothing but air, the time taken up by the thermometer in cooling from 70° to 10° was

VOL. 11. II ^'j^

452 Of the Propagation of Heat

^y6 feconds ; but In the Experiment N* 4, when this fame fpace was filled with 54 parts air, and I part raw filk, the time of cooling was 1 284 fe- conds.

Now, fuppofing that the filk had been totally In- capable of conducing any Heat at all, If we fuppofe, at the fame time, that It had no power to prevent the air remaining in the globe from conducing It, In that cafe Its prefence In the globe could only have prolonged the time of cooHng in proportion to the quantity of the air it had difplaced to the quantity remaining, that is to fay, as i Is to 54, or a little more than 10 feconds. But the time of cooling was adlually prolonged 708 feconds (for In the Experiment W i. It was ^y6 feconds, and In the Ex- periment N° 4, It was 1284 feconds, as has juft been obferved) ; and this fhows, that the filk not only did not condu6t the Heat itfelf, but that it pre- vented the air by which Its Interftices were filled from conducing it ; or, at lead. It greatly weakened Its power of conducing it.

The next queftion which arlfes Is, how air can be prevented from conducing Heat? and this neceffarily Involves another, which Is, how does air conduct Heat ? .

If air conducted Heat, as It is probable that the metals and water, and all other folld bodies and unelaftic fluids condud It, that Is to fay, if Its par- ticles remaining in their places, the Heat palfed from one particle to another, through the whole

mals.

in various Sub/Dances. 453

mafs, as there is no reafon to fuppofe that the pro- pagation of Heat is neceflarily in right lines, I can- not conceive how the interpofition of fo fmall a quantity of any foHd body as ^-^ part of the volume of the air could have efFefted fo remarkable a diminution of the conduding power of the air, as appeared in the Experiment (N° 4) with raw lilk, above mentioned.

If air and water conduced Heat in the fame manner, it is more than probable that their con- ducting powers might be impaired by the fame means; but when I made the Experiment with water, by filling the glafs globe, in the centre of which the bulb of the thermometer was fufpended, with that fluid, and afterwards varied the Experi- ment, by adding 16 grains of raw filk to the water, I did not find that the conducing power of the water was fenfibly impaired by the prefence of the filk*.

But we have jufl feen that the fame filk, mixed with an equal volume of air, diminilhed its con- ducing power in a very remarkable degree; con- fequently, there is great reafon to conclude that water and air condud Heat in a different manner.

But the following Experiment, I think, puts the matter beyond all doubt.

* The Experiment here mentioned was made in the year 1787 j but the refult of a more careful inveltigation of the fubjeft has fince ftiown that Heat is not propagated in water in the manner here fuppofed. (SeeEflay VII.)

112 It

454 ^f ^^^ Propagation of Heat

It is well known, that the power which aif poffeffes of holding water in folution is augmented by Heat, and diminiflied by cold, and that, if hot air is faturated with water, and if this air is after- wards cooled, a part of its water is neceflarily de- pofed.

I took a cylindrical bottle of very clear tranf- parent glafs, about 8 inches in diameter, and 12 inches high, with a fliort and narrow neck, and fufpending a fmall piece of linen rag, moderately wet, in the middle of it, I plunged it into a large vefTel of water, warmed to about ioc° of Fahren- heit's thermometer, where I fuffered it to remain till the contained air was not only warm, but tho- roughly faturated with the moifture which it attract- ed from the linen rag, the mouth of the bottle being well flopped up during this time with a good cork 5 this being done, I removed the cork for a moment^ to take away the linen rag, and flopping up the bottle again immediately, I took it out of the warm water, and plunged it into a large cylindrical jar, about 12 inches in diameter, and 16 inches high, containing jufl fo much ice-cold water, that, when the bottle was plunged into it, and quite covered by it, the jar was quite full.

As the jar was of very fine tranfparent glafs, as- well as the bottle, and as the cold water contained in the jar was perfedly clear, I could fee what paiTed in the bottle mofl diflindlyj and having taken care to place the jar upon a table near the

window.

in various Subjiances, 455

window, in a very favourable light, I fet myfelf to obferve the appearances which Ihould take place, with all that anxious expectation which a convidlion that the refult of the Experiment mufl be decifive, naturally infpired.

I was certain, that the air contained in the bottle could not part with its Heat, without at the fame time, that is to fay, at the fame moment, and in the fame place, parting with a portion of its water ; if, therefore, the Heat penetrated the mafs of air from the centre to the furface, or paffed through it from particle to particle, in the fame manner as it is pro- bable that it palfes through water, and all other unelailic fluids *, by far the greateft part of the air contained in the bottle would part with its Heat, when not aSiually in contact with the glafs, and a proportional part of its water being let fall at the fame time, and in \h^ fame place, would neceflarily defcend in the form of rain ; and, though this rain might be too fine to be vifible in its defcent, yet I was fure I Ihould find it at the bottom of the bottle, if not in vifible drops of water, yet in that kind of cloudy covering which cold glafs acquires from a contaft with hot fleam or watery vapour.

But if the particles of air, inftead of communis eating their Heat from one to another, from the centre to the furface of the bottle, each in its turn, and for itfelf, came to the furface of the bottle, and

* This opinion refpe6llng the manner in which Heat is propagated in water, and other iinelaftic fluids, was afterwards found to be erro- peouSj as has been ftiown in the preceding Efl:iy,

I I 3 there

45 6 Of the Propagation of Heat

there depofited its Heat and its water, I concluded that the cloudinefs occalioned by this depofit of water would- appear all over the bottle, or, at lead, not more of it at the bottom than at the fides, but rather lefs ; and this I found to be the cafe in fad.

The cloudinefs firfl made its appearance upon the iides of the bottle, near the top of it ; and from thence it gradually fpread itfelf downwards, till, growing fainter as it defcended lower, it was hardly vifible at the diftance of half an inch from the bottom of the bottle ; and upon the bottom itfelf, which was nearly flat, there was fcarcely the fmalleft appearance of cloudinefs.

Thefe appearances, I think, are eafy to be ac^ counted for. The air immediately in contact with the glafs being cooled, and having depofited a part of its water upon the furface of the glafs, at the fame time that it communicates to it its Heat, flid,es downwards by the fides of the bottle in con- fequence of its increafed fpecific gravity, and, taking its place at the bottom of the bottle, forces the whole mafs of hot air upwards ; which, in its turn coming to the fides of the bottle, there de- pofits its Heat and its water, and afterwards bend- ing its courfe downwards, this circulation is con- tinued till all the air in the bottle has acquired the exaO: temperature of the water in the jar.

From hence it is clear why the firft appearance of condenfed vapour is near the top of the bottle, as alfo why the greateH colleftion of vapour is in

that

in 'various Subjiances. 457

that part, and that fo very fmall a quantity of it is found nearer the bottom of the bottle.

This Experiment confirmed me in an opinion which I had for fome time entertained, that, though the particles of air individually, or each for itfelf, are capable of receiving and tranfporting Heat, yet air in a quiefcent ftate, or as a fluid whofe parts are at reft with refpeft to each other, is not capable of conducting it, or giving it a paflfage ; in fhort, that Heat is incapable of pqjjing through a mafs of air, penetrating from one particle of it to another, and that it is to this circumftance that its non-condud- ing power is principally owing.

It is alfo to this circumftance, in a great meafure, that it is owing that its non-condufting power, or its apparent warmth when employed as a covering for confining Heat, is fo remarkably increafed upon its being mixed with a fmall quantity of any very fine, hght, folid fubftance, fuch as the raw filk, fur. Eider down, &c. in the foregoing Experi- ments : for as I have already obferved, though thefe fubftances, in the very fmall quantities in which they were made ufe of, could hardly have prevented, in any confiderable degree, the air from conducing, or giving a pa/fage to the Heat, had it been capable of pafling through it, yet they might very much impede it in the operation of tranfport- ing it.

But there is another circumftance which it is neceflary to take into the account, and that is the attraction which fubfifts between air and the bodies

I I 4 abovg*

45 8 Of the Propagation of Heat

above-mentioned, and other like fubftances, confli-. tuting natural and artificial clothing. For, though the incapacity of air to give a paflage to Heat in the manner folid bodies permit it to pafs through them, may enable us to account for its warmth under cer- tain circumftances, yet the bare admiffion of this principle does not feem to be fufficient to account for the very extraordinary degrees of warmth which we find in furs and in feathers, and in various other Idnds of natural and artificial clothing ; nor even that which we find in fnow ; for if we fuppofe the particles of air to be at liberty to carry off the. Heat ■which thefe bodies are meant to confine, without any other obflrudion or hindrance than that arifmg from their vis inertia, or the force neceffary to put them in motion, it feems probable that the fuccef^ fion of frefh particles of cold air, and the confe- quent lofs of Heat, would be much more rapi4 than we find it to be in fa6l.

That an attraftion, and a very flrong one, ac- tually fubfifts betv/een the particles of air, and the fine hair or furs of beafls, the feathers of birds, wool, &c. appears by the obftinacy with which thefe fubftances retain the air which adheres to them, even when immerfed in water, and put under the receiver of an air-pump ; and that this attraction is efTential to the warmth of thefe bodies, I think is very eafy to be demonftrated.

In furs, for inftance, the attradion between the particles of air, and the fine hairs in which it i^ concealed, being greater than the increafed elaf-.

ticity^

in 'various Suhflances, 455

ticlty, or repulfion of thofe particles with regard to each other, arifing from the Heat communicated to them by the animal body, the air in the fur, though heated, is not eafily difplaced ; and this coat of confined air is the real barrier which de- fends the animal body from the external cold. This air cannot carry off the Heat of the animal, becaufe it is itfelf confined, by its attraction to the hair or fur ; and it tranfmits it with great difiiculty, if it tranfmits at all, as has been abundantly fliewn by the foregoing Experiments.

Hence it appears why thofe furs which are the fineft, longefl, and thickeft, are likewife the warm* eft ; and how the furs of the beaver, of the otter, and of other like quadrupeds which live much in water, and the feathers of water-fowls, are able to confine the Heat of thofe animals in winter, not- withftanding the extreme coldnefs and great con- ducing power of the water in which they fwim. The attradion between thefe fubftances, and the air vi^hich occupies their interftices, is fo great, that this air is not dillodged even by the contad: of water, but remaining in its place, it defends the body of the animal at the fame time from being wet, and from being robbed of its Heat by the fur-» rounding cold fluid ; and it is poffible that the preifure of this fluid upon the covering of air con- fined in the interftices of the fur, or feathers, may at the fame time increafe its warmth, or non-con- dudting power, in fuch a manner that the animal may not, in faft, lofe more Heat when in water, than

wheA

460 Of the Propagation of Heat

when in air: for we have feen by the foregoing Experiments, that, under certain circumftances, the warmth of a covering is increafed, by bringing its component parts nearer together, or by in- creafmg its denfity even at the expence of its thick- nefs. But this point will be further invelligated hereafter.

Bears, wolves, foxes, hares, and other like quadrupeds, inhabitants of cold countries, which do not often take the water, have their fur much thicker upon their backs than upon their bellies. The heated air occupying the interflices of the hairs of the animal tending naturally to rife up- wards, in confequence of its increafed elaflicity, would efcape vidth much greater eafe from the backs of quadrupeds than from their bellies, had not Providence wifely guarded againft this evil by increafmg the obftrudions in thofe parts, which entangle it and confine it to the body of the animal. And this, I think, amounts almofl to a proof of the principles aifumed relative to the manner in which Heat is carried off by air, and the caufes of the non-condu(5ling power of air, or its apparent warmth, when, being combined with other bodies, it afls as a covering for confining Heat.

The fnows which cover the furface of the earth in winter, in high latitudes, are doubtlefs defigned by an all-provident Creator as a garment to defend it againft the piercing winds from the polar regions, which prevail during the cold feafon,

Thefe

in various Subjlances, 461

Thefe winds, notwithftanding the vafl trafts of continent over which they blow, retain their fharp- nefs as long as the ground they pafs over is covered with fnow ; and it is not till meeting with the ocean they acquire, from a contad; with its waters, the Heat which the fnows prevent their acquiring from the earth, that the edge of their coldnefs is taken off, and they gradually die away and are loft,.

The winds are always found to be much colder when the ground is covered with fnow than when it is bare, and this extraordinary coldnefs is vul- garly fuppofed to be communicated to the air by the fnow ; but this is an erroneous opinion ; for thefe winds are in general much colder than the fnow itfelf.

They retain their coldnefs, becaufe the fnow pre- vents them from being warmed at the expence of the earth ; and this is a ftriking proof of the ufe of the fnows in preferving the Heat of the earth du- ring the winter in cold latitudes.

It is remarkable that thefe winds feldom blow from the poles diredly towards the equator, but from the land towards the fea. Upon the eaftern coaft of North America the cold winds come from the north-weft ; but upon the weftern coaft of Europe they blow from the north-eaft.

That they ftiould blow towards thofe parts where they can moft eafily acquire the Heat they are in fearch of, is not extraordinary ; and that they ihould grad ually ceafe and die away, upon being

warmed

462 Of the Propagation of Heat

warmed by a contad with the waters of the ocean, is likewife agreeable to the nature and caufes of their motion ; and if I might be allowed a conjec- ture refpefting the principal ufe of the feas, or the reafon why the proportion of water upon the fur- face of our globe is fo great, compared to that of the land, it is to maintain a more equal temperature in the different climates, by heating or cooling the winds which at certain periods blow from the great continents.

That cold winds actually grow much milder upon pafling over the fea, and that hot winds are refrefhed by a contact with its waters, is very cer-^ tain 5 and it is equally certain that the winds from the ocean are, in all climates, much more temperate than thofe which blow from the land.

In the iflands of Great Britain and Ireland, there is not the leaft doubt but the great mildnefs of the climate is entirely owing to their feparation from the neighbouring continent by fo large a trad of fea ; and in all fimilar fituations, in every part of the globe, fimilar caufes are found to produce fimilar effe6ls.

The cold north-weft winds, which prevail upon the coaft of North America during the winter, fel- dom extend above 1 00 leagues from the fhore, and they are always found to be lefs violent, and lefs piercing, as they are further from the land.

Thefe periodical winds from the continents of Europe and North America prevail moft towards the end of the month of February, and in the month

of

iri various Suhjlartces, 463

df March ; and I conceive that they contribute very eflentially towards bringing on an early fpring, and a fruitful fummer, particularly when they are Very violent in the month of March, and if at that time the ground is well covered with fnow. The whole atmofphere of the polar regions being, as it were, tranfported into the ocean by thefe winds, is there warmed and faturated with water : and, a great accumulation of air upon the fea being the necelTary confequence of the long continuance of thefe cold winds from the Ihore, upon their ceafmg the warm breezes from the fea neceflarily com- mence, and, fpreading themfelves upon the land far and wide, affifl the returning fun in difmantling the earth of the remains of her winter garment, and in bringing forward into life all the manifold beauties of the new-born year.

This warmed air which comes in from the fea, having acquired its Heat from a contad with the ocean, is, of courfe, faturated with water; and hence the warm fhowers of April and May, fo neceflary to a fruitful feafon.

The ocean may be confidered as the great refer- Voir and equalizer of Heat ; and its benign influ- ences in preferving a proper temperature in the atmofphere operate in all feafons and in all cli- mates.

The parching winds from the land under the torrid zone are cooled by a conta6l with its waters, and, in return, the breezes from the fea, which at certain hours of the day come in to the fhores in almoft all hot countries, bring with them refrefh-

I ment.

464 Of the Propagation of Heat

ment, and, as it were, new life and vigour both to the animal and vegetable creation, fainting and melting under the exceffive Heats of a burning fun. What a vaft trad of country, now the mod: fertile upon the face of the globe, would be abfo- lutely barren and uninhabitable on account of the exceffive Heat, were it not for thefe refrefhing fea-breezes ! And is it not more than probable, that the extremes of heat and of cold in the dif- ferent feafons in the temperate and frigid zones would be quite intolerable, were it not for the in- fluence of the ocean in preferving an equability of temperature ?

And to thefe purpofes the ocean is wonderfully . well adapted not only on account of the great power of water to abforb Heat, and the vaft depth and extent of the different feas (which are fuch that one fummer or one winter could hardly be fuppofed to have any fenfible effeft in heating or cooling this enormous mafs) ; but alfo on account of the continual circulation which is carried on in the ocean itfelf, by means of the currents which prevail in it. The waters under the torrid zone being carried by thefe currents towards the polar regions, are there cooled by a contaft with the cold winds, and, having thus communicated their Heat to thefe inhofpitable regions, return towards the equator, carrying with them refrefhment for thofe parching climates.

The wifdom and goodnefs of Providence have often been called in queftion with regard to the diftribution of land and water upon the furface of

our

in 'various Subjiances. 465

our globe, the vaft extent of the ocean having been confidered as a proof of the little regard that has been paid to man in this diftribution. But, the more light we acquire refpeding the real con- flitution of things, and the various ufes of the different parts of the vifible creation, the lefs we fhall be difpofed to indulge ourfelves in fuch frivo- lous criticifms.

END OF THE EIGHTH ESSAY.

ESSAY IX.

AN

EJKPERIMENTAL INQUIRY

CONCERNING

THE SOURCE OF THE HEAT WHICH IS EXCITED BT FRICTION.

[Read before the Royal Society, January 25, r798.]

VOL. II. K K

C 469 ]

ESSAY IX.

An Inquiry concerning the Source of the Heat which is excited by Friction.

[Read before the Royal Society, January 25, 1798.]

It frequently happens, that in the ordinary affairs and occupations of life, opportunities prefent themfelves of contemplating fome of the mofl curious operations of Nature ; and very interefting philofophical experiments might often be made, al- moft without trouble or expence, by means of ma- chinery contrived for the mere mechanical purpofes of the arts and manufadures.

I have frequently had occafion to make this ob- fervation ; and am perfuaded, that a habit of keep- ing the eyes open to every thing that is going on in the ordinary courfe of the bufmefs of life has oftener led, as it were by accident, or in the play- ful excurfions of the imagination, put into adion by contemplating the mofl common appearances, to ufeful doubts, and fenfible fchemes for invcftigation

K K 2 and

470 Inquiry concerning the Source of

and improvement, than all the more intenfe medi- tations of philofophers, in the hours exprefsly fet apart for fludy.

It was by accident that I was led to make the Ex- periments of which I am about to give an account ; and, though they are not perhaps of fufficient im- portance to merit fo formal an introduftion, I can- not help flattering myfelf that they will be thought curious in feveral refpefts, and worthy of the honour of being made known to the Royal Society.

Being engaged, lately, in fuperintending the bor- ing of cannon, in the worklbops of the military arfe- nal at Munich, I was ftruck with the very confider- able degree of Heat which a brafs gun acquires, in a fhort time, in being bored ; and with the ftill more intenfe Heat (much greater than that of boil- ing water, as I found by experiment) of the metal- lic chips feparated from it by the borer.

The more I meditated on thefe phenomena, the more they appeared to me to be curious and inte- refting. A thorough inveftigation of them feemed even to bid fair to give a farther infight into the hidden nature of Heat ; and to enable us to form fome reafonable conjectures refpe£ting the exiftence, or non-exiflence, of an igneous jiuid : a fubje£t on which the opinion of philofophers have, in all ages, been much divided.

In order that the Society may have clear and diftinft ideas of the fpeculations and reafonings to which thefe appearances gave rife in my mind, and ^Ifo of the fpecific objects of philofophical invefti-

gation

the Heat excited by Fridion* 471

gation they fuggefted to me, I muft beg leave . to ftate them at feme length, and in fuch man- ner as I fhall think bed fuited to anfwer this purpofe.

From whence comes the Heat aftually produced in the mechanical operation above mentioned ?

Is it furnifhed by the metallic chips which are feparated by the borer from the folid mafs of metal ?

If this were the cafe, then, according to th,e modern dodrines of latent Heat, and of caloric, the capacity for Heat of the parts of the metal, fo reduced to chips, ought not only to be changed, but the change undergone by them fhould be fufficiently great to account for all the Heat pro- duced.

But no fuch change had taken place j for I found, upon taking equal quantities, by weight, of thefe chips, and of thin flips of the fame block of metal feparated by means of a fine faw, and put- ting them, at the fame temperature, (that of boiling water,) into equal quantities of cold water, (that is to fay, at the temperature of 59"! F.) the portion of water into which the chips were put was not, to all appearance, heated either lefs or more than the Other portion, in which the flips of metal were put.

This Experiment being repeated feveral times, the refults were always fo nearly the fame, that I could not determine whether any, or what change, had been produced in the metal, in regard to its

K K. 3 capacity

472 Inquiry concerning the Source of

capacity for Heat, by being reduced to chips by the borer *.

From hence It is evident, that the Heat produced could not poffibly have been furniflied at the ex- pence of the latent Heat of the metallic chips. But, not being willing to reft fatisfied vi^ith thefe trials, how^ever conclufive they appeared to me to be, I had recourfe to the follov^^ing ftill more decifive Ex- periment :

Taking a cannon, (a brafs fix-pounder,) call folid, and rough as it came from the foundry, (fee Fig. I. Tab. IV.) and fixing it (horizontally) in

* As thefe Experiments are important, it may perhaps be agreeable to the Society to be made acquainted with them in their details. One of them was as follows :

To 4590 grains of water, at the temperature of 59°iF. (an allow- ance as comp?nfation, reckoned in water, for the capacity for Heat of the containing cylindrical tin veffel, being included,) were added ioi6| grains of gun-metal in thin flips, feparated from the gun by means of a fine faw, being at the temperature of 210° F. When they had remained together i minute, and had been well ftirred about, by means of a fmall rod of light wood, the Heat of the mixture was found to be —63°.

From this Experiment, the jpicijic Heat of the metal, calculated ac- cording to the rule given by Dr. Crawford, turns out to be rizo.iioo, that of water being ~ i.oooo.

An Experiment was afterwards made with the metallic chips, as follows :

To the fame quantity of water as was ufed in the Experiment above mentioned, at the fame temperature, {wis. 59°2,) and in the fame cylindrical tin veflel, were now put 1016^ grains of metallic chips of gun-metal, bored out of the fame gun from which the flips ufed in the foregoing Experiment were taken, and at the fame temperature (iio"). The Heat of the mixture, at the end of i minute, was juft 63", as before} confequently the fpecific Heat of tliefe metallic chips was =0.1100. Each of the above Experiments w?.s repealed three times, and always with nearly the fame refults.

the

the Heat excited by FriBlon, 473

the machine ufed for boring, and at the fame time finifhing the outfide of the cannon by turning, (fee Fig. 2.) I caufed its extremity to be cut off; and, by turning down the metal in that part, a folid cy- linder was formed, 7^ inches in diameter, and 9^ inches long ; which, when finilhed, remained joined to the reft of the metal (that which, properly fpeak- ing, conftituted the cannon) by a fmall cylindrical neck, only 2-^ inches in diameter, and 3^-5- inches long.

This fhort cylinder, which was fupported in its horizontal pofition, and turned round its axis, by means of the neck by which it remained united to the cannon, was now bored with the horizontal borer ufed in boring cannon ; but its bore, which was 'Ti'J inches in diameter, inftead of being con- tinued through its whole length (9.8 inches) was only 7.2 inches in length; fo that a folid bottom was left to this hollow cylinder, which bottom was 2.6 inches in thicknefs.

This cavity is reprefented by dotted lines in Fig. 2 ; as alfo in Fig. 3. where the cylinder is reprefented on an enlarged fcale.

This cylinder being defigned for the exprefs purpofe of generating Heat by fridion, by having a blunt borer forced againft its folid bottom at the fame time that it lliould be turned round its axis by the force of horfes, in order that the Heat accumu- lated in the cylinder might from time to time be meafured, a fmall round hole, (fee^, £■, Fig.3.) 0.37 of an inch only in diameter, and 4.2 inches in

K K 4 depth.

474 Inquiry concerning the Source of

depth, for the purpofe of introducing a fmall cy- lindrical mercurial thermometer, was made in it, on one fide, in a direftion perpendicular to the axis of the cylinder, and ending in the middle of the folid part of the metal which formed the bottom of its bore.

The folid contents of this hollow cylinder, ex- clufive of the cylindrical neck by which it remained ' united to the cannon, were 385! cubic inches, Englilh meafurej and it weighed 113.131b. Avoir- dupois : as I found, on weighing it at the end of the courfe of Experiments made with it, and after it had been feparated from the cannon with which, during the Experiments, it remained connected*.

Experiment^ N° i.

This Experiment was made in order to afcertain how much Heat was aftually generated by friftion, when a blunt fteel borer being fo forcibly flioved (by means of a ftrong fcrew) againft the bottom of

* For fear I fljould be fafpefted of prodigality in the profecution of my philofophical refearches, I think it neceflary to inform the Society, that the cannon I made ufe of in this Experiment was not facrjficed to it. The Ihort hollow cylinder which was formed at the end of it, was turned out of a cylindrical mafs of metal, about 2 feet in length, projefting beyond the muzzle of the gun, called in the German language the -verlomer kopf, (the head of the cannon to be thrown away,) and which is leprefented in Fig. i.

This original projcftion, which is cut oiF before the gun is bored, is always caft with it, in order that, by means of the preffure of its weight on the metal in the lower part of the mould, during the time it is cooling, the gun may be the more compaft in the neighbour- hood of the muzzle; where, without this precaution, the metal would be apt to be peuous, or full of honeycombs.

the

the Heat excited by Fridion, 47^

the bore of the cylinder, that the prefTure againfl It was equal to the weight of about 1 0000 lb. Avoirdupois, the cylinder was turned round on its axis (by the force of horfes) at the rate of about 32 times in a minute.

This machinery, as it was put together for the Experiment, is reprefented by Fig. 2. W is a flrong horizontal iron bar, connected with proper machinery carried round by horfes, by means of which the cannon was made to turn round it axis.

To prevent, as far as poffible, the lofs of any part of the Heat that was generated in the Experi* ment, the cylinder w^as well covered up with a fit coating of thick and warm flannel, which was care- fully wrapped round it, and defended it on every fide from the cold air of the atmofphere. This covering is not reprefented in the drawing of the apparatus. Fig. 2.

I ought to mention, that the borer was a flat piece of hardened fl:eel, 0.63 of an inch thick, 4 inches long, and nearly as wide as the cavity of the bore of the cylinder, namely, 3 \ inches. Its corners were rounded off at its end, fo as to make it fit the hollow bottom of the bore ; and it was firmly fafliened to the iron bar (jn) which kept it in its place. The area of the furface by which its end was in contad with the bottom of the bore of the cylinder was nearly 24 inches. This borer, which is difl:inguiflied by the letter n, is reprefented in mofl; of the figures.

At

47^ Inquiry concerning the Source of

At the beginning of the Experiment, the tempe- rature of the air in the {hade, as alfo that of the cylinder, was juft 60° F.

At the end of 30 minutes, when the cylinder had made 960 revolutions about its axis, thehorfes being flopped, a cylindrical mercurial thermometer, whofe bulb was -j-Vo- of an inch in diameter, and 3 1 inches in length, was introduced into the hole made to receive it, in the fide of the cylinder, when the mercury rofe almofl inflantly to 130".

Though the Heat could not be fuppofed to be quite equally diftributed in every part of the cy- linder, yet, as the length of the bulb of the ther- mometer was fuch that it extended from the axis of the cylinder to near its furface, the Heat indi- cated by it could not be very different from that of the mean temperature of the cylinder ; and it was on this account that a thermometer of that particular form was chofen for this Experiment.

To fee how faft the Heat efcaped out of the cy- linder, (in order to be able to make a probable con- jefture refpeding the quantity given off by it, dur- ing the time the Heat generated by the friftion was accumulating,) the machinery Handing flill, I fuf- fered the thermometer to remain in its place near three quarters of an hour, obferving and noting down, at fmall intervals of time, the height of the temperature indicated by it.

Thus,

the Heat excited by Fri^ion* 477

- The Heat, as fliown by

Thus, at the end or the thermometer, was

4 minutes - ^ - - - 126* after 5 minutes, always reckon- ing from the firfl ob-

fervation, - - - - 125**

at the end of 7 minutes 123'

12 ----- 120

14 ----- 119

16 118^

26 116"

24 ii5«

28 114"

31 ii3»

34- 112

37f -• - - - - Jii

and when 41 minutes had elapfed - no'

Having taken away the borer, I now removed the metallic duft, or rather fcaly matter, which had been detached from the bottom of the oylinder by the blunt fteel borer, in this Experiment ; and, having carefully weighed it, I found its weight to be 837 grains Troy.

Is it poffible that the very confiderable quantity of Heat that was produced in this Experiment (a quantity which ai^lually raifed the temperature of above 1131b. of gun-metal at leaft 70 degrees of Fahrenheit's thermometer, and which, of courfe, would have been capable of melting 6ilb. of ice, or of caufmg near 5 lb. of ice-cold water to boil)

could

47 S Inquiry concerning the Seurce of

could have been furnifhed by fo Inconfiderable a, quantity of metallic dull? and this merely in con- fequence of a change of its capacity for Heat ?

As the weight of this dull (837 grains Troy) amounted to no more than ^^-g-th part of that of the cylinder, it mult have lolt no lefs than 948 de- grees of Heat, to have been able to have raifed the temperature of the cylinder i degree; and confe- quently it mud have given off 66360 degrees of Heat, to have produced the effedts which were aftually found to have been produced in the Ex- periment !

But, without infilling on the improbability of this fuppofition, we have only to recolleft, that from the refults of aftual and decifive Experiments^ made for the exprefs purpofe of afcertaining that faft, the capacity for Heat, of the metal of which great guns are caft, is not fenfihly changed by being reduced to the form of metallic chips, in the ope- ration of boring cannon ; and there does not feem to be any reafon to think that it can be much changed, if it be changed at all, in being reduced to much fmaller pieces, by means of a borer that

is lefs Iharp.

If the Heat, or any confiderable part of it, were produced in confequence of a change in the capa- city for Heat of a part of the metal of the cylinder, as fuch change could only be fuperfcial, the cy- linder would by degrees be exhaujied; or the quan- tities of Heat produced, in any given fliort fpace of time, would be found to diminilh gradually, in

fucceffive

the Heat elicited by Frid:ion, 479

fucceffive Experiments. To find out if this really happened or not, I repeated the laft-mentioned Experiment feveral times, with the utmoft care j but I did not difcover the fmallefl fign of exhauftion in the metal, notwithftanding the large quantities of Heat actually given oiF.

Finding fo much reafon to conclude, that the Heat generated in thefe Experiments, or excited, as I would rather choofe to exprefs it, was not fur- niflied at the expence of the latent Heat or combined caloric of the metal, I puftied my inquiries a Hep farther, and endeavoured to find out whether the air did, or did not, contribute any thing in the generation of it.

Experiment^ N° 2.

As the bore of the cyHnder was cylindrical, and as the iron bar (w), to the end of which the blunt fteel borer was fixed, was fquare, the air had free accefs to the infide of the bore, and even to the bottom of it, where the fridion took place by which the Heat was excited.

As neither the metallic chips produced in the ordinary courfe of the operation of boring brafs cannon, nor the finer fcaly particles produced in the laft-mentioned Experiments by the fridlion of the blunt borer, (howed any figns of calcination, I did not fee how the air could poflibly have been the caufe of the Heat that was produced ; but, in an inveftigation of this kind, I thought that no pains ftiould be fpared to clear away the rubbifh,

I and

480 Inquiry concerning the Source of

and leave the fubjed as naked and open to infpec- tion as poffible.

In order, by one decifive Experiment, to deter- mine whether the air of the atmofphere had any part, or not, in the generation of the Heat, I con- trived to repeat the Experiment, under circum- ftances in which // was evidently impojfihle for it to produce any effed: whatever. By means of a pifton exadly fitted to the mouth of the bore of the cy- linder, through the middle of which pifton the fquare iron bar, to the end of which the blunt fteel borer was fixed, paffed in a fquare hole made perfedly air-tight, the accefs of the external air, to the infide of the bore of the cylinder, was effec- tually prevented. (In Fig. 3. this pifton (/>) is feen in its place j it is likewife fhown in Fig. 7 and 8.).

I did not find, however, by this Experiment, that the exclufion of the air diminifhed, in the fmalleft degree, the quantity of Heat excited by the fric- tion.

There ftill remained one doubt, which, though it appeared to me to be fo flight as hardly to de- ferve any attention, I was however defirous to re- move. The pifton which clofed the mouth of the bore of the cylinder, in order that it might be air- tight, was fitted into it with fo much nicety, by means of its collars of leather, and prefTed againft it with fo much force, that, notwithftanding its being oiled, it occafioned a confiderable degree of fridion, when the hollow cylinder was turned

round

the Heat excited by Fridion, 48 1

round its axis. Was not the Heat produced, or at leaft fome part of it, occafioned by this friftion of the pifton? and, as the external air had free accefs to the extremity of the bore, where it came in con- taft with the pifton, is it not poffible that this air may have had fome fhare in the generation of the Heat produced ?

Experiment^ N" 3.

A quadrangular oblong deal box, (fee Fig. 4.) water-tight, ii| Engliih inches long, 9^^ inches wide, and g^-^ inches deep, (meafured in the clear,) being provided, with holes or flits in the middle of each of its ends, juft large enough to receive, the one, the fquare iron rod to the end of which the blunt fteel borer was faftened, the other, the fmall cylindrical neck which joined the hollow cylinder to the cannon ; when this box (which was oc- cafionally clofed above, by a wooden cover or lid moving on hinges) was put into its place ; that is to fay, when, by means of the two vertical open- ings or flits in its two ends, (the upper parts oF which openings were occafionally clofed, by means of narrow pieces of wood Aiding in vertical grooves,) the box (g, /j, i, k. Fig. 3.) was fixed to the ma- chinery, in fuch a manner that its bottom (/, k,') being in the plane of the horizon, its axis coincided with the axis of the hollow metallic cylinder ; it is evident, from the defcription, that the hollow me- tallic cylinder would occupy the middle of the box, ■yvithout touching it on either fide (as it is repre-

fented

482 Inquiry concerning the Source of

fented in Fig. 3.) ; and that, on pouring water into the box, and filling it to the brim, the cylinder would be completely covered, and furrounded on every fide, by that fluid. And farther, as the box was held faft by the ftrong fquare iron rod {m\ which pafled, in a fquare hole, in the centre of one of its ends, (^, Fig. 4.) while the round or cylindri- cal neck, which joined the hollow cylinder to the end of the cannon, could turn round freely on its axis in the round hole in the centre of the other end of it, it is evident that the machinery could be put in motion, without the leail danger of forcing the box out of its place, throwing the water out of it, or deranging any part of the apparatus.

Every thing being ready, I proceeded to make the Experiment I hadprojeded, in the foil owing manner:

The hollow cylinder having been previoufly cleaned out, and the infide of its bore wiped with a clean towel till it was quite dry, the fquare iron bar, with the blunt fteel borer fixed to the end of it, was put into its place ; the mouth of the bore of the cylinder being clofed at the fame time, by means of the circular piflon, through the centre of wl^ch the iron bar pafled.

This being done, the box was put in its place, and the joinings of the iron rod, and of the neck of the cylinder, with the two ends of the box, having been made water-tight, by means of collars of oiled leather, the box was filled with cold water, (yi%. at the temperature of 60%) and the machine

was put in motiori*

The

the Heat excited by Fridion. 483

The refult of this beautiful Experiment was very ftriking, and the pleafure it afforded me amply re- paid me for all the trouble I had had, in con- triving and arranging the complicated machinery ufed in making it.

The cylinder, revolving at the rate of about 32 times in a minute, had been in motion but a fhoYt time, when I perceived, by putting my hand into the water, and touching the outfide of the cy- linder, that Heat was generated ; and it was not long before the water which furrounded the cy- linder began to be fenfibly warm.

At the end of i hour I found, by plunging a thermometer into the water in the box, (the quan- tity of which fluid amounted to 18.771b. Avoir- dupois, or 2 1 wine gallons,) that its temperature had been raifed no lefs than 47 degrees ; being now 107° of Fahrenheit's fcale.

When 30 minutes more had elapfed, or i hour and 30 minutes after the machinery had been put in motion, the Heat of the water in the box was 142**.

At the end of 2 hours, reckoning from the be- ginning of the Experiment, the temperature of the water was found to be raifed to 178".

At 2 hours 20 minutes it was at 200° 5 and at 2 hours 30 minutes it actually boiled !

It would be difficult to defcrlbe the furprife

and aftonilhment exprefled in the countenances of

VOL. II. L l the

484 Inquiry concerning the Source of

the by-ftanders, on feeing fo large a quantity of cold water heated, and aftually made to boil, without any fire.

Though there was, in fa6l, nothing that could juftly be confidered as furprifing in this event, yet I acknowledge fairly that it afforded me a degree ©f childifli pleafure, which, were I ambitious of the reputation of a grave philofopher, I ought moft certainly rather to hide than to difcover.

The quantity of Heat excited and accumulated in this Experiment was very confiderable ; for, not only the water in the box, but aJfo the box itfelf, (which weighed 15I lb.) and the hollow me- tallic cylinder, and that part of the iron bar which, being fituated within the cavity of the box, was immerfed in the water, were heated 150 de- grees of Fahrenheit's fcale ; viz. from 60'* (which was the temperature of the water, and of the machinery, at the beginning of the Experi- ment) to 210°, the Heat of boiling water at Munich.

The total quantity of Heat generated may be ellimated with fome confiderable degree of pre- cifion, as follows :

Of

the Heat excited by Fridhn, 485

Of the Heat excited Quantity of Ice-cold water which,

there appears to have ^'''^ ^^"^ s^ven quantity of Heat,

tj- might have been heated i8o de-

been actually aCCUmU- grees, or made to boil.

lated. In Avoirdupois weight.

In the water contained in the wooden box, i8| lb. Avoirdupois, heated 150 de- grees, namely, from 60" to ib. 210° F 15.2

In 113.131b. of gun-metal, (the hol- low cylinder,) heated 150 degrees; and, as the capacity for Heat of this metal is to that of water as o.iioo to i.oooo, this quantity of Heat would have heated 1 2 1 lb. of water the fame number of degrees - 10.37

In 2>^-ys cubic Inches of iron, (being that part of the iron bar to which the borer was fixed which entered the box,) heated 150 degrees; which may be reckoned equal in capacity for Heat to 1.2 lib. of water - - - - i.oi

N. B. No eftimate is here made of the Heat accumulated in the wooden box, nor of that difperfed during the Experiment.

Total quantity of ice-cold water which, with the Heat actually generated by fric- tion, and accumulated in 2 hours and 30

minutes, might have been heated 180 de-

grees, or made to boil - - . 26.58

LL 2 From

f' 486 Inquiry concerning the Source of

From the knowledge of the quantity of Heat aftually produced in the foregoing Experiment, and of the ti7?ie in which it was generated, we are enabled to afcertain the velocity of its produ6iion, and to determine how large a fire mufl have been, or how much fuel muil have been confumed, in order that, in burning equably, it Ihould have produced by combuflion the fame quantity of Heat in the fame time.

In one of Dr. Crawford's Experiments, (fee his Treatife on Heat, p. 321,) 371b. 70Z. Troy, zr 1 8 1920 grains, of water, were heated 2^-^ de- grees of Fahrenheit's thermometer, with theHeat generated in the combuflion of 26 grains of wax. This gives 382032 grains of water heated i degree with 26 grains of wax ; or 1469344^ grains of water heated i degree, or '4|-|^ = 81.631 grains heated 180 degrees, with the Heat generated in the combuflion of i grain of wax.

The quantity of ice-cold water which might have been heated 180 degrees, with the Heat ge- nerated by friftion in the before-mentioned Ex- periment, was found to be 26.58 lb. Avoirdupois, — 188060 grains; and, as 81.631 grains of ice- cold water require the Heat generated in the com- buflion of I grain of wax, to heat it 180 degrees, the former quantity of ice-cold water, namely 188060 grains, would require the combuflion of no lefs than 2303.8 grains (~4t-oz. Troy) of wax, to heat it 180 degrees.

As

the Heat excited by Friclion. 487

As the Experiment (N° 3) In which the given quantity of Heat was generated by friftion, lafted 2 hours and 30 minutes, = 150 minutes, it is neceflary, for the purpofe of afcertaining how many wax-candles of any given fize muft burn to- gether, in order that in the combuftion of them the given quantity of Heat may be generated in the given time, and confequently with the fame celerity as that with which the Heat was generated by fridion in the Experiment, that the fize of the candles fhould be determined, and the quantity of wax confumed in a given time by each candle, in burning equably, fhould be known.

Now I found by an Experiment, made on pur- pofe to finiih thefe computations, that when a good wax-candle, of a moderate fize, | of an inch in dia- meter, burns with a clear flame, juft 49 grains of wax are confumed in 30 minutes. Hence it ap- pears, that 245 grains of wax would be confumed by fuch a candle in 150 minutes; and that, to burn the quantity of wax (:= 2303.8 grains) ne- ceflary to produce the quantity of Heat adually obtained by fridtion in the Experiment in quefl:ion, and in the given time, (150 minutes,) nine candles, burning at once, would not be fufficient ; for 9 multipUed into 245 (the number of grains con- fumed by each candle in 150 minutes) amounts to no more than 2205 grains ; whereas the quantity of wax neceflary to be burnt, in order to produce the given quantity of Heat, was found to be ^2>^?>''^ grains.

L L 3 From

488 Inquiry concerning the Source of

From the refult of thefe computations It appears, that the quantity of Heat produced equably, or in a continual llream, (if I may ufe that expreffion,) by the fri£lion of the blunt fteel borer againfl the bottom of the hollow metallic cylinder, in the Experiment under confideration, was greater than that produced equably in the combuflion of nine wax-candles^ each | of an inch in diameter, all burning together, or at the fame time, with clear bright flames.

As the machinery ufed in this Experiment could eafily be carried round by the force of one horfe, (though, to render the work lighter, two horfes ~ were actually employed in doing it,) thefe compu- tations fhow further how large a quantity of Heat might be produced, by proper mechanical con- trivance, merely by the ftrength of a horfe, with- out either fire, light, combuflion, or chemical de- compofition j and, in a cafe of neceffity, the Heat thus produced might be ufed in cooking vidluals.

But no circumflances can be imagined, in which this method of procuring Heat would not be dif- advantageous ; for, more Heat might be obtained by ufmg the fodder neceifary for the fupport of a horfe, as fuel. . ,

As foon as the lafl-mentioned Experiment (N° 3.) was finifhed, the water in the wooden box was let off, and the box removed ; and the borer being taken out of the cylinder, the fcaly metallic powder, which had been produced by the friftion of the borer againfl the bottom of

the

the Heat excited by Fridion. 489

the cylinder, was colleded, and, being carefully weighed, was found to weigh 4145 grains, or about 8|- oz. Troy.

As this quantity was produced in 2| hours, this gives 824 grains for the quantity produced in half an hour.

In the firfl Experiment, which laftedonly half an hour, the quantity produced was 837 grains.

In the Experiment N° i. the quantity of Heat generated, in haf an hour, was found to be equal to that which would be required to heat 5 lb. Avoirdupois of ice-cold water 180 degrees, or caufe it to boil.

According to the refult of the Experiment N'* 3. the Heat generated in haf an hour would have caufed 5.31 lb. of ice-cold water to boil. But, in this I aft-mentioned Experiment, the Heat generated being more effedually confined, lefs of it was loft ; which accounts for the difference of the refults of the two Experiments.

It remains for me to give an account of one Experiment more, which was made with this ap- paratus. I found by the Experiment N° i. how much Heat was generated when the air had free accefs to the metallic furfaces which were rubbed together. By the Experiment N° 2. I found that the quantity of Heat generated was not fen- fibly diminiflied when the free accefs of the air was prevented; and by the refult of N** 3. it ap- peared that the generation of the Heat was not prevented, or retarded, by keeping the apparatus

L L 4 immerfed

490 Inquiry concernhig the Source of

iramerfed in water. But as, in this laft-mentioned Experiment, the water, though it furrounded the hollow metallic cylinder on every fide, externally, was not fufFered to enter the cavity of its bore, (being prevented by the pifton,) and confequently did not come into contadt with the metallic furfaces where the Heat was generated ; to fee what efFeds would be produced by giving the water free accefs to thefe furfaces, I now made the

Experiment^ N° 4.

The pifton which clofed the end of the bore of the cylinder being removed, the blunt borer and the cylinder were once more put together ; and the box being fixed in its place, and filled with water, the machinery was again put in motion.

There was nothing in the refult of this Experi- ment that renders it neceflary for me to be very particular in my account of it. Heat was gene- rated, as in the former Experiments, and, to all appearance, quite as rapidly ; and I have no doubt but the water in the box would have been brought to boil, had the Experiment been continued as long as the laft. The only circumftance that furprifed me was, to find how little difference was occafioned in the noife made by the borer in rubbing againft the bottom of the bore of the cylinder, by filling the bore with water. This noife, which was very grating to the ear, and fometimes almoft infup- portable, was, as nearly as I could judge of it, quite as loud, and as difagreeable, when the fur-

3 . - faces

the Heat excited by FriSlion. 491

faces rubbed together were wet with water, as when they were in contaft with air.

By meditating on the refults of all thefe Expe- riments, we are naturally brought to that great queftion which has fo often been the fubjed of fpeculation among philofophers ; namely,

"What is Heat ? — Is there any fuch thing as an igneous fluid? — Is there any thing that can with propriety be called caloric?

We have feen that a very confiderable quantity of Heat may be excited in the Fridion of two me- tallic furfaces, and given oif in a conftant flream or flux, in all diredions, without interruption or intermiflion, and without any figns of diminution or exhauftion.

From whence came the Heat which was con- ' tinually given off in this manner, in the foregoing Experiments ? Was it furnilhed by the fmall par- ticles of metal, detached from the larger folid maifes, on their being rubbed together ? This, as we have already feen, could not poffibly have been the cafe.

Was it furnifhed by the air? This could not have been the cafe ; for, in three of the Experi- ments, the machinery being kept immerfed in water, the accefs of the air of the atmofphere was completely prevented.

Was it furnifhed by the water which furrounded the machinery ? That this could not have been the cafe is evident : firfl, becaufe this water was con- tinually receiving Heat from the machinery, and

could

492 Inquiry concerning the Source of

could not, at the fame time, be giving to, and r<?- ceiving Heat fro7n, the fame body ; and fecondly, becaufe there was no chemical decompofition of any part of this water. Had any fuch decompofi- tion taken place, (which indeed could not reafon- ably have been expe£led,) one of its component elaitic fluids (mofl probably inflammable air) mufl:, at the fame time, have been fet at liberty, and, in making its efcape into the atmolphere, would have been deteded ; but though I frequently examined the water to fee if any air bubbles rofe up through it, and had even made preparations for catching them, in order to examine them, if any fhould appear, I could perceive none ; nor was there any fign of decompofition of any kind whatever, or other chemical procefs, going on in the water.

Is it pofllble that the Heat could have been fupplied by means of the iron bar to the end of which the blunt fl:eel borer was fixed ? or by the fmall neck of gun-metal by which the hollow cylinder was united to the cannon ? Thefe fuppo- fitions appear more improbable even than either of thofe before mentioned ; for Heat was continually going off, or out of the machinery, by both thefe paflages, during the whole time the Experiment Med.

And, in reafoning on this fubjeft, we mufl not

forget to confider that mofl remarkable circum-

ftance, that the fource of the Heat generated by

friction, in thefe Experiments, appeared evidently

to be inexhaujiible.

It

the Heat excited by Friclion. 493

It is hardly neceflary to add, that any thing which any infulated body, or fyftem of bodies, caa continue to furnifli without /imitation, cannot pof- fibly be a material fuhftance : and it appears to me to be extremely difficult, if not quite impoffible, to form any diftind idea of any thing, capable of being excited and communicated, in the manner the Heat was excited and communicated in thefe Experiments, except it be motion.

I am very far from pretending to know how, or by what means, or mechanical contrivance, that particular kind of motion in bodies, which has been fuppofed to conftitute Heat, is excited, con- tinued, and propagated, and I Ihall not prefume to trouble the Society with mere conjeftures ; par- ticularly on a fubjedl which, during fo many thou- fand years, the mofl enlightened philofophers have endeavoured, but in vain, to comprehend.

But, although the mechanifm of Heat fhould, in fa£l, be one of thofe myfteries of nature which are beyond the reach of human intelligence, this ought by no means to difcourage us, or even leflen our ardour, in our attempts to invefligate the laws of its operations. How far can we advance in any of the paths which fcience has opened to us, be- fore we find ourfelves enveloped in thofe thick mills which, on every fide, bound the horizon of the human intellect ? But how ample, and how interefting, is the field that is given us to ex- plore 1

Nobody,

494 Inquiry concerning the Source of

Nobody, furely, in his fober fenfes, has ever pretended to underftand the mechanifm of gravi- tation ; and yet what fublime difcoveries was our immortal Nev^^ton enabled to make, merely by the inveftigation of the laws of its adion !

The effefts produced in the world by the agency of Heat are probably juji as exteti/ive, and quite as important, as thofe which are owing to the tendency of the particles of matter towards each other ; and there is no doubt but its operations are, in all cafes, determined by laws equally im- mutable.

Before I finifli this Eflay, I would beg leave to obferve, that although, in treating the fubje£l I have endeavoured to inveftigate, I have made no mention of the names of thofe who have gone over the fame ground before me, nor of the fuc- cefs of their labours; this omiflion has not been owing to any want of refpeft for my predeceflbrs, but was merely to avoid prolixity, and to be more at liberty to purfue, without interruption, the na» tural train of my own ideas.

Bescrip-

the Heat excited by Fridion. 495

Description of the Figures.

Fig. 1 fliows the cannon ufed in the foregoing Experiments, in the flate it was in when it came from the foundry.

Fig. 2 fhows the machinery ufed in the Ex- periments N° I and N^ 2. The cannon is feen fixed in the machine ufed for boring cannon. W is a ftrong iron bar, (which, to fave room in the drawing, is reprefented as broken off,) which bar, being united with machinery (not expreffed in the figure) that is carried round by horfes, caufes the cannon to turn round its axis.

372 is a ftrong iron bar, to the end of which the blunt borer is fixed; which, by being forced againft the bottom of the bore of the fhort hollow cylinder that remains connedled by a fmall cylindri- cal neck to the end of the cannon, is ufed in gene- rating Heat by fridion.

Fig. 3 ihows, on an enlarged fcale, the fame hollow cylinder that is reprefented on a fmaller fcale in the foregoing Figure. It is here feen con- nefted with the wooden box (g, b, /, k^) ufed in the Experiments N"* 3 and N° 4. when this hol- low cylinder was immerfed in water.

p, which is marked by dotted lines, is the pifton which clofed the end of the bore of the cylinder.

n is the blunt borer feen fidewife.

df e, is the fmall hole by which the thermo- meter was introduced, that was ufed for afcertain-

ing

4gS hquiry concerning the Source, ^r.

ing the Heat of the cylinder. To fave room in the drawing, the cannon is reprefented broken off near its muzzle ; and the iron bar, to which the blunt borer is fixed, is reprefented broken off at m.

Fig. 4, is a perfpe£live view of the wooden box, a feftion of which is feen in the foregoing Figure. (See g, h, /, k. Fig. 3.)

Fig. 5 and 6, reprefent the blunt borer n, joined to the iron bar 7/z, to which it was faflened.

Fig. 7 and 8, reprefent the fame borer, with its iron bar, together with the pillon which, in the Experiments N° 1 and W 3. was ufed to clofe the mouth of the hollow cylinder.

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