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PROFESSOR KOPP ON THE SPECIFIC HEAT OF SOLID BODIES. 
Petit’s law, would be a proof that iodine and chlorine, if compounds at all, are not more 
so than other so-called elements for which this law is regarded as valid. 
According to Neumann’s law, compounds of analogous atomic composition have 
approximately the same atomic heats. In general, bodies, whose atom consists of a 
greater number of indecomposable atoms, or is of more complicated composition, have 
greater atomic heats. In these compounds, more especially those whose elements all 
follow Dulong and Petit’s law, magnitude of atomic heat is exactly a measure of the com- 
plexity or of the degree of composition (compare § 93). If Dulong and Petit’s law were 
valid, it could be concluded with great positiveness that the so-called elements, if they are 
compounds of unknown and simpler substances, are compounds of the same order. It 
would be a remarkable result that the act of chemical decomposition had everywhere 
found its limit at such bodies as those which, if compound at all, have with every 
difference of chemical deportment the same degree of composition. Imagine the 
simplest bodies, probably as yet unknown to us, the true chemical elements, forming 
a horizontal spreading layer, and piled above them, the simpler and then the more 
complicated compounds ; the universal validity of Dulong and Petit’s law would include 
the proof, that all elements at present assumed by chemists lay in the same layer, and 
that chemistry in recognizing hydrogen, oxygen, sulphur, chlorine, and the different 
metals as indecomposable bodies, had penetrated to the same depth in that field of 
inquiry, and had found at the same depth the limit to its penetration. 
This result I formerly propounded * when I still believed in the validity of Dulong 
and Petit’s law. But with the proof that this law is not universally true, the conclu- 
sion to which this result leads loses its justification. Starting now from the elements 
recognized in chemistry, we must rather admit that the magnitude of the atomic heat 
of a body depends not only on the number of elementary atoms contained in one atom 
of it, or on the complexity of the composition, but also on the atomic heat of the 
elementary atoms entering into its composition ; it appears now possible that a decom- 
posable body may have the same atomic heat as an indecomposable one. 
To assume in chlorine the presence of oxygen, and to consider it as analogous to per. 
oxide of manganese, or in general to the peroxide of a biatomic element f, is less in 
accordance with what is at present considered true in chemistry, than to consider it as 
the peroxide of a monoequivalent element, analogous to peroxide of hydrogen. It is 
remarkable that peroxide of hydrogen, in the solid state or in solid compounds, must 
have almost as great an atomic heat (for H0 2-3+4 = 6 - 3) as those elements which obey 
Dulong and Petit’s law, and especially as iodine, bromine, and chlorine, according to 
the direct and to the indirect determination of their atomic heat ; the same must be the 
case for the analogous peroxides of such still unknown elements as have an atomic heat 
* “ On the Difference of Matter from the Empirical point of view,” an Academical Discourse. Giessen, i860. 
f I will not omit to mention that equivalent weights of iodine and peroxide of manganese have almost equal 
capacity for heat. As regards oxidizing action, 127 of iodine corresponds to 43-5 peroxide of manganese; 
Regnault found the specific heat of the former =0-0541; I found that of the latter =0-159; 
127 x 0-0541 =6-87; 43-5 x 0-159=6-92. 
