PROFESSOR KOPP ON THE SPECIFIC HEAT OF SOLID BODIES. 
201 
as great as that of hydrogen. As far as may be judged from its specific heat, chlorine 
may be such a peroxide ; but this consideration shows no necessity for assuming that it 
actually is so. 
In a great number of cases the atomic heat of compounds gives more or less accurately a 
measure for the degree of complexity of their composition*. And this is the case also with 
such compounds as are comparable in their chemical deportment to undecomposed bodies. 
If cyanogen or ammonium had not been decomposed, or could not be so with the means 
at present offered by chemistry, the greater atomic heats of their compounds, compared 
with those of analogous chlorine or potassium compounds (compare § 96), and of cyano- 
gen and ammonium as compared with chlorine and potassium, would indicate the more 
complex nature of those so-called compound radicals. The conclusion appears admis- 
sible that for the so-called elements the directly or indirectly ascertained atomic heats 
are a measure for the complexity of their composition. Carbon and hydrogen, for 
example, if not themselves simple bodies, are more so than silicium or oxygen ; and still 
more complex compounds are the elements which are now considered as following Dulong 
and Petit’s law; with the restriction, however, that for these also the atomic heats 
may be more accurately determined and differences proved in them which justify similar 
conclusions f. One might be tempted, by comparing atomic heats, to form an idea how 
the more complex of the present indecomposable bodies might be composed of more 
simple ones, just as such a comparison has been shown to be possible for chlorine ; but 
it is at once seen that to carry out such an attempt the atomic heats of the elements, 
especially those which can only be indirectly determined, are not settled with adequate 
certainty. 
It may appear surprising, or even improbable, that so-called elements which can 
replace each other in compounds, as, for instance, hydrogen and the metals, or which 
enter into compounds as isomorphous constituents, like silicium and tin, should possess 
unequal atomic heats and unequal complexity of composition. But this is not more 
surprising than that indecomposable bodies, and those which can be proved to be com- 
pound, as, for example, hydrogen and hyponitric acid, or potassium and ammonium, 
should replace one another, preserving the chemical character of the compounds, and 
even be contained as corresponding constituents in isomorphous compounds. 
I have here expressed suppositions in reference to the nature of the so-called elements 
which appear to me based on trustworthy conclusions from well-proved principles. It is 
* The differences in the atomic heats of the elements are of course most distinctly seen in their free state, 
but in their analogous compounds these differences are the less prominent the more complex the compounds, 
that is, the greater the number of atoms of the same kind and the same atomic heat which are united to those 
elementary atoms whose atomic heat is assumed to he unequal. The difference in the atomic heats of G and As, 
for instance (1*8 and 6-4), is relatively far greater than for Ga <3 G 3 and K As0 3 (20-2 and 24-8). 
f It is possible, for example, that certain indecomposable bodies which only approximately obey Dulong and 
Petit’s law, are analogous compounds of simpler substances of essentially different atomic heat : the approximate 
agreement of the atomic heats of such indecomposable bodies would then depend on a similar reason to that for 
the atomic heats of Ga € 0 3 and K As 0 3 . Compare the previous note. 
