FLUIDITY AND TEMPERATURE 129

changed to a liquid or gas, the time of relaxation becomes smaller
and smaller, and for air Maxwell has given the value

r = 1/5,099,100,000 sec.

With rising temperature, the value of r increases, and according to
Graetz, one may write

n =

where $ is the temperature reckoned from the temperature at
which the viscosity is infinite, i.e., the temperature of solidifica-
tion.

In gases the modulus of rigidity is known to be equal to the
gas pressure at the critical temperature and is of the order of
magnitude of a hundred atmospheres. In solids, where the
modulus of rigidity is known, it has a value from 100,000 to
1,000,000 atmospheres. Since E decreases in passing from the
solid through the liquid into the gaseous condition, its value
approaches the critical pressure P at the critical temperature #0,
and we have according to Graetz

E = P + 61(^0 - 0) + &2(#o ~ #)2 + &a (#o - #)3 + . . -
where 61, 62, 63, - - - are constants.
From Maxwell, we get

= Er = -
n

or

where a, ft, /32, . . . are constants. Since the formula with a
large number of constants is of little practical use, Graetz neg-
lected the constants ft, ft, . . . which are of small magnitude
and thus obtained

or if the temperatures are changed to the absolute scale,
T — T
r, = a±~_ (44)