OF THE PARAFFINS AND THEIR MONOHALOGEN DERIVATIVES. 
35 
the mean results conform, affords some justification for regarding the value of y as 
approximately independent of the pressure. 
It is plain that the gases fall into four groups, the members of any one group having 
within the lunits of experimental error the same ratio of the specific heats. These 
groups are 
1. Methane, 
2. Methyl chloride, bromide, and iodide, 
3. Ethane and its derivatives. 
4. Propane and its derivatives. 
So that with the single exception of methane, compounds with similar graphic formulm 
have the same y. 
Methane was almost the last gas that I investigated, and it was in consequence of 
its appearing to fall away from the law, that exceptional trouble was taken to secure 
that it should be pure. All who have worked with this gas know how difficult it is 
to prepare it free from hydrogen, and the presence of hydrogen would raise the value 
of y; but the precaution taken of passing the gas over palladium, the concordance of 
the results for methane prepared by the two different methods, and the evidence of 
the analysis show that there could not be anything approaching enough hydrogen 
present to account for the difference. Nor can we account for the difference by sup¬ 
posing the results for the three methyl compounds to be too low, for, apart from the 
fact that there are three of them, and that their values for y agree fairly well with 
each other, the most likely error in their case is that due to the presence of air and 
moisture, which would make the results too high. Hence we must conclude that 
methane has not the same y as its three substitution products. 
It is strange that it should break through a law that appears to hold for all the 
other gases, but the circumstance is not without parallel, Menschutkin’s etheri¬ 
fication values for the fatty acids, for instance, show a similar feature, as do 
Perkin’s molecular rotation constants, and the viscosity coefficients of the same 
series of acids. In each of these cases a law is found to hold for all the members of 
the series except the first one or two. 
It appears, then, that as a law to which marsh gas is an exception, one hydrogen 
atom of a paraffin can be replaced by a halogen atom, without affecting the y of the 
gas, and consequently without altering the internal energy of the molecule. This 
result is similar to that which Strecker obtained for the hydracids of the halogens, 
for he showed that hydrochloric, hydrobromic, and hydriodic acids have all approxi¬ 
mately the same y as hydrogen. It should however be noticed that he found the 
introduction of a second halogen atom caused a large fall in y, the elementary gases, 
chlorine, bromine, and iodine, having ratios nearly equal, but much lower than those 
of the acids, from which fact we may anticipate a similar feature in the case of the 
paraffins. 
F 2 
