SPECIFIC HEATS OF SOME COMPOUND GASES. 589 
My former conclusion that monohalogen derivatives of the same paraltiin have the 
same y can now be stated in more general terms. The two ally} compounds show 
that it is not restricted to saturated hydrocarbens, and the equality of y of ethylene 
and ethylidene chlorides extends it to isomeric dihalogen derivatives. Hence, so far 
as the experiments have gone, we can say that without any exception corresponding 
halogen derivatives of the same hydrocarbon have the same ratio of the specific heats. 
Similarly it appears that the equality of y for the two propyl chlorides is not an 
isolated fact, for two other pairs of isomeric bodies, viz., ethylene and etbylidene 
chlorides and methyl acetate and ethyl formate prove to have the same y. As I have 
mentioned above, there is some doubt about the results for the two last mentioned, 
but at present the balance of evidence is in favour of the statement that zsomeric com- 
pounds have the same y. 
In the case of the paraffins ethane and propane, it was one in my former paper 
that one H can be replaced by a halogen without altering the value of y, but that the 
introduction of a halogen into rbthans causes a fall in y. We see now that the 
unsaturated gas ethylene behaves in the same way as methane, for the y of vinyl 
bromide is markedly lower than that of ethylene. Thus the possibility of inter- 
change of hydrogen with a halogen without altering y is not general, a fact that is 
brought out still more clearly by a consideration of the higher substitution products. 
It was shown by StrecKER* that hydrochloric, hydrobromic and hydriodie acids 
all have nearly the same y as hydrogen, namely 1:4, whilst the y's of chlorine, bro- 
mine, iodine and iodine chloride all lie near 1°3. Hence in this case one halogen can 
be put in the place of hydrogen without affecting y, but the introduction of a second 
halogen causes a large fall. 
The parafiin derivatives show the same feature. he second Cl introduced into the 
molecule invariably causes a large fall in y whether the first has done so or not. 
Thus, whilst ethyl chloride has the same y as ethane, ethylene and ethylidene 
chlorides are 4 per cent. lower. 
The methane substitution products are volatile enough to allow the whole series to 
be investigated, and here we find that every successive chlorine atom introduced 
causes a fall in y. 
The work done on the last four gases in the table is only the beginning of a line of 
investigation, the completion of which would require an entire remodelling of my 
apparatus, so as to enable it to stand higher temperatures. The object of the experi- 
ments was to find whether other chemically analogous atoms are interchangeable in 
the same way as the halogens. 
Although C and Si replace each other in many compounds, CCl, and 8iCl, are not 
strictly analogous, for they violate the usual rule that the compound with the higher 
molecular weight has the higher boiling-point. They appear however to have the 
same y. 
‘Wied, Ann.’ vol. 13, p. 20, and vol. 17, p. 85. 
