71 
V')(.P\ 458-f-^ ^ 
PiX-P“458+^i ^ 
Now the density of steam has been determined accurately 
for a temperature of 212° Fahrenheit (by the method of 
Dumas), to be such that its volume is 1,670 times that of the 
water which produced it. Hence, if the above law be correct 
for steam, we have for any other pressure, the specific volume 
_t^_1670X15^458+^ ^ 
“ 670 ^ P 
From this formula all the tables of the density of steam 
have been deduced, and all the calculations of the duty of 
steam engines have been founded on it. Up to the present 
time, however, this formula has never been verified by direct 
experiment, nor are the methods hitherto employed in deter- 
mining the density of gases and vapours applicable in this 
case, except at the boiling temperature of the liquid under 
ordinary atmospheric pressure. But, on the other hand, 
theoretical calculations throw considerable doubt on the 
above formulae as applied to steam and other condensable 
vapours. Several years ago Dr. Joule and Professor 
William Thomson announced, as the result of applying the 
new dynamical theory of heat to the law of Carnot, that for 
temperatures higher than 212° there is a very considerable 
deviation from the gaseous laws, in the case of steam. Later, 
in 1855, Professor Macquorn Ranhine has given a new 
formula for the density of steam, independant of Gay Lussac’s 
law, and this confirms Mr. Thomson’s surmise. Still these 
speculations need the confirmation of direct experiment. 
The density of 'steam is ascertained by vapourizing given 
weights of water in a glass globe of known capacity, and 
noting the temperature at which the water disappears and fills 
the vessel in the form of steam. Two difficulties, however, 
have to be overcome. First, the pressure of the steam renders 
it necessary that the glass globe should be heated in a strong, 
and therefore of necessity, opa(}ue vessel. Second, as 
