EXPERIMENTAL KNOWLEDGE OF THE PROPERTIES OF MATTER. 475 



capacities of bodies, and in -which the principal causes of error incidental 

 to such delicate experiments had been obviated. 



Dulong and Petit — Elements. 



In their great work on the laws of cooling, published in the ' Jour. 

 de I'Ecole Polytech.' tome xi., Dulong and Petit gave determinations 

 of specific heats between 0° and 350° and showed that the specific heats 

 of bodies in general increase with rise of temperature ; examples of 

 this are given also in ' Ann. Chim. Ph.' (2), 7, 1818 ; as an example, 

 among others, the mean heat capacity of iron 



between 0° and 100°= -1098 

 „ „ 200 =-1150 

 „ „ 300 =-1218 

 „ „ 350 =-1255 



This gives a fair idea of the general rate of increase of specific heat of 

 metals and other solid bodies generally with rise of temperature. Dulong 

 and Petit in a subsequent paper ^ enunciate the following law : * The 

 atoms of all simple bodies {elements) have exactly the same cajjacitij for 

 heat.' This law is arrived at by multiplying the specific heats of a num- 

 ber of elements in the solid state, viz., Bi, Pb, An, Pt, Sn, Ag, Zn, Te, 

 Cu, Ni, Fe, Co, S, by their atomic weights ; it is true that the products 

 so obtained were not exactly the same for each element, but this deviation 

 was attributed by these physicists to errors arising from two sources — the 

 determination of the specific heats, and the analyses from the results of 

 which the then received atomic weights were deduced. And this law 

 was not restricted by them to bodies in the solid state alone, but was 

 supposed to apply to bodies in the liquid and gaseous states. This opinion 

 was supported by the specific heats of oxygen and nitrogen gases as deter- 

 mined by de Laroche and Berard, and of hydrogen ; the number for the 

 last gas they find somewhat too small, but consider that the determination 

 of its specific heat was vitiated by an error which de Laroche and Berard 

 had overlooked. In the number given by Dulong and Petit the atomic 

 weights were referred to that of oxygen taken as unit, and the specific 

 heats to that of water. 



Neumann's Laiv/or Compounds. 



In 1831 Neumann published ^ a set of determinations of specific heats 

 of minerals, and attempts to apply to them the principle of Dulong and 

 Petit's law ; to find, in fact, if there exist any simple relations among 

 their molecular heats corresponding to what Dulong and Petit had appa- 

 rently established as to the atomic heats of elements. He includes in his 

 determinations, however, some elements, and finds that arsenic and anti- 

 mony are, or seem to be, exceptions to the law of atomic heats. Noticing 

 that these crystallise in forms not belonging to the regular system, the 

 doubt occurs to him whether the specific heat is independent of crystal- 

 line form, and whether it is the same for substances of the same chemical 

 composition but crystallising in different systems. This doubt is, how- 

 ever, set at rest by his observation that calc spar and aragonite have the 

 same specific heat, and similarly in other cases of dimorphism. 



> Ann. Chim. (2), 10, 1819, p. 405. ^ p^^^. j^,in. 23, 1831, p. 1. 



