CaCO, 21°09 
ae 21°34 
BaCO,, 21°49 
PbCO,, 21°91. 
The same regularity holds if, instead of taking an average 
of all, we take simply Regnault’s or Kopp’s determinations 
alone. 
The fact that the molecular heat increases with the atomic 
weight holds good for liquids as well as for solids, and is here 
even more striking. Three series will serve my purpose in 
this connection. For the specific heats themselves I will refer 
tables of specific heat soon to be published by the 
Smithsonian Institution, merely stating that I have used Reg- 
nault’s determinations for all the substances except CCl,, for 
which Hirn’s value has been employed. 
Cl,,; 31°91 
SiCl,, 32°42 
= 35°25 
SnCl,, 87°15 
rULs 9 Fee C,H,Br, 23°44. 
AsCl,, 31°95. C,H,I, 24°58. 
It may then be stated as a general rule, to which the present 
evidence offers only a few exceptions, that in any definite series 
of similar solid or liquid compounds the molecular heat increases 
with the atomic weight, although in a very different ratio. I have 
tried to establish a similar relation among the elements, but 
thus far without success. The results here are extremely dis- 
coraant. 
Now what is the meaning of this regularity? Is Dulong and 
Petit’s law set aside by it? Speaking for solid substances 
alone, I should answer, certainly not. ‘The divergencies from 
equality are easily explained. ‘The specific heat of a substance 
varies with the temperature, generally increasing as the te 
ure rises. But the rate of increase is very different for 
ifferent bodies. The specific heat of carbon increases veTy 
rapidly, that of platinum with extreme slowness. So, then, in 
