2 
MR. J. W. CAPSTICK OX THE RATIO OF THE SPECIFIC HEATS 
of expansion. For the explanation of these in the case of the hypothetical perfect 
gas, no knowledge of the special constitution of the molecule is required, but for most 
other properties, and especially thermal properties, the kinetic theory fails to explain 
the facts from want of information concerning the dynamical peculiarities of the 
molecules of different gases. 
From the ratio of the two specific heats of a gas we can calculate the relative rates 
of increase per degree rise of temperature of the energy of translation of the molecule 
as a whole, and the energy due to the motion of the atoms relatively to the centre of 
gravity of the molecule. 
If (i is the ratio of the rate of increase of the internal energy to that of the trans¬ 
lational energy, we have the well-known equation— 
/3+ 1 =2/{3(y- 1)}, 
where y is the ratio of the specific heats of the gas. 
Thus the constant y has a high theoretical value as leading directly to a funda¬ 
mental dynamical property of the molecule, and a knowdedge of its value for a large 
number of gases suitably chosen would not improbably afford material on which to 
base a theory of the configuration and motions of the atoms in a molecule, or would 
at least give valuable data by which to test theories based on other considerations. 
Stated briefly the following is tlie present state of our experimental knowledge of 
the ratio of the specific heats. 
Almost all tlie older work was rendered valueless by Fontgen’s showing 
(Poggendorff’s ‘ Annalen,’ vol. 141, p. 552 and vol. 148, p. 580) how great 
an effect the size of the apparatus lias on the results. His own values for air and 
carbonic acid are probably near the truth, but the difficulty he experienced in finding 
a suitable pressure gauge, and the large size of the apparatus, have caused his method 
to be put out of the field by Kundt’s Dust Figure method (PoGG. ‘Ann.,’ vol. 127, 
p. 497, and vol. 135, pp. 337 and 527). 
The earliest experiments by this latter method are those of Kundt and Warburg 
(PoGG. ‘Ann.,’ vol. 157, p. 353) on Mercury Vapour, by which it was .shown that /3 is 
zero for the mercury molecule, and hence the molecule has no power of absorbing 
internal energy, thus confirming the chemical view that the molecule is monatomic. 
Next we have the work on Carbon Monoxide, Carbon Dioxide, Nitrous Oxide, 
Ethylene, and Ammonia, by Wullner (Wied. ‘Ann.,’ vol. 4, p. 321), who, using 
Kundt’s earliest single-ended form of apparatus, found that with the exception 
of air these gases all have ratios of the specific heats that fall considerably with rise 
of temperature. 
Up to this time it was thought that all diatomic gases have y equal to 1'4. To 
test the point further Strecker (Wied. ‘Ann.,’ vol. 13, p. 20, and vol. 17, p. 85) 
investigated the halogens and their hydracids. He found that hydrochloric, hydro- 
bromic, and hydriodic acids have the value 1‘4, but that the simple halogens and 
