PROFESSOR KOPP ON THE SPECIFIC HEAT OF SOLID BODIES. 
199 
PART VI.— CONSIDERATIONS ON THE NATURE OF THE CHEMICAL ELEMENTS. 
112. The proof given in the preceding that Dulong and Petit’s law is not univer- 
sally valid, justifies certain conclusions, in reference to the nature of the so-called 
chemical elements, which may here be developed. 
What bodies are to be regarded as chemical elements X Does the mere fact of inde- 
composability determine this X or may a body be indecomposable in point of fact and yet 
from reasons of analogy be regarded not as an element but as a compound X The history 
of chemistry furnishes numerous examples of cases in which sometimes one and some- 
times another mode of view led to results which at present are regarded as accurate. 
The earths were in 1789 indecomposable in point of fact, when Lavoisier expressed the 
opinion that they were compounds, oxides of unknown metals. Lavoisier’s argumenta- 
tion was based on the fact that the earths enter as bases into salts, and that it was to be 
assumed in regard to all salts, that they contained an oxygen acid and an oxygen base. 
But the view, founded on the same basis, that common salt contains oxygen, and the 
subsequent view that what is now called chlorine contained a further quantity of 
oxygen besides the elements of an oxygen acid, did not find an equally permanent recog- 
nition. On the basis of the actual indecomposability of chlorine, Davy maintained 
from about 1810 its elementary character; and this view has become general, especially 
since Berzelius, after a long struggle against it, adopted it, more I think because he 
was outvoted than because he was convinced. 
Almost all chemists of the present time consider chlorine, and in conformity therewith 
bromine and iodine, as elementary bodies ; but the persistence is known with which 
Schonbein attacks this view, and adheres to the opinion that these bodies are oxygen 
compounds, peroxides of unknown elements. Is there anything which enables us to decide 
with more certainty on the elementary nature of chlorine and the analogous bodies than 
has hitherto been the easel 
No one can maintain that the bodies which chemists regard as elements are abso- 
lutely simple substances. The possibility must be confessed that they may be decomposed 
into still simpler bodies ; how far a body is to be regarded as an element is so far relative, 
that it depends on the development of the means of decomposition which practical che- 
mistry has at its disposal, and on the trustworthiness of the conclusions which theoretical 
chemistry can deduce. A discussion as to whether chlorine or iodine is an elementary 
body can only be taken in the sense whether chlorine is as simple a body as oxygen or 
manganese, or nitrogen ; or whether it is a compound body, as peroxide of manganese or 
peroxide of hydrogen for example. 
If Dulong and Petit’s law were universally valid, it would not merely indicate for 
chemical elements a relation between the atomic weight and the specific heat in the 
solid state, but it could be used as a test for the elementary nature of a body whose 
atomic weight is known. That iodine, from a direct determination of specific heat, and 
chlorine by an indirect determination had atomic heats agreeing with Dulong and 
mdccclxv. 2 E 
