THE MECHANICAL EQUIVALENT OF HEAT. 
503 
If h, the temperature coefficient of the resistance of the wire, is determined by 
prehminary observations, and we assume yS = pW' where y? is a constant (p. 400), we 
have with (15) sufficient data to determine the values of yS^. Equation (11) then 
gives 
_ {1 - 
Mx-E.y^n 
( 16 ). 
Affain equation (10) gives 
p + 
“ Ml J.E.Mi 
and 
V _ P f wyE" {k + q) 
^ J.E.Mi 
(u). 
( 18 ). 
which are two equations to give p and l-^. 
Similarly, by performing the operations X^i and Xoj, we could determine the value 
of ; and since (p. 480) 
, /Mb + pW, /W, + gW 
1- Wi + W, Wi + M^ 
we can find both f and g, the coefficients of increase in specific heat of water and of 
the calorimeter. 
Appendix II. 
The Regulator for Maintaining the Laboratory at a Constant Temperature. 
A narrow glass tube, several feet in length, was fixed in a horizontal position on 
one of the walls.''' This tube contained chloride of silver, and a stream of dry 
ammonia gas at a low temperature was passed through it until the compound 
AgCbSNHg was formed.! This compound, as pointed out by Isambert, dissociates 
at ordinary temperatures, the pressure of the vapour at about 15° C. altering by more 
than 12 millims. per degree. Care had to be taken to completely saturate the 
AgCl, for the pressure of the vapour AgCl, 2 NH 3 changes at ordinary tempera¬ 
tures by a much smaller amount. The horizontal tube communicated with a gas 
regulator of the ordinary pattern, except that the diameter of the regulator tube 
could be made of any size, since we are dealing with the pressure of a saturated 
vapour, not with that of a gas, and thus the changes in volume caused by a movement 
of the mercury column could be disregarded, the diameter of the smallest pipe in our 
* The exterior of this tube was blackened, 
t This substance was suggested by Mr. C. T. Hetcock. 
