EXPERIMENTAL KNOWLEDGE OF THE PROPERTIES OF MATTER. 477 



bodies Regnault discusses the question which Neumann had raised in 

 1831 and applies to it all the resources of his experimental skill, not con- 

 iining himself to natural minerals as Neumann had done, but using a 

 variety of compounds prepared pure in the laboratory. A number of 

 alloys examined by him gave heat-capacities equal to the sum of those of 

 their component parts in most of the twelve cases he gives ; but in some 

 cases in which the alloy was fused or near its fusing- point at the higher 

 temperature, the specific heat calculated on this supposition differed con- 

 siderably from the observed. He then examines a large variety of oxides, 

 sulphides, chlorides, bromides, iodides, fluorides, nitrates, chlorates, phos- 

 phates, metaphosphates, pyrophosphates, arseniates, sulphates, chromates, 

 borates, tungstates, silicates, carbonates ; each of these sets — e.g., oxides 

 — he divides into series, each series having a like chemical formula, that 

 is, corresponding to the same general formula, as RO representing the 

 series of oxides PbO, HgO, ]\InO, CuO, NiO, MgO, MgO ; R2O3 repre- 

 senting the series Fe^Oj, AS2O3, CraOj, SbjO.-,, BijOj, AI2O3, the numbers 

 of each series being compared among themselves as to their specific heats 

 and formula-weights. Among the protoxides the product of the specific 

 heat and the formula- weight is nearly the same as a rule, but there are 

 two exceptions — oxide of magnesium and oxide of zinc. Among the 

 sesquioxides the same general rule holds good for what we will call the 

 molecular heats, with the exception of alumina, in the form of corundum 

 especially, which has a far lower molecular heat than the other sesqui- 

 oxides. These apparently exceptional cases Regnault explains by a differ- 

 ence in the state of aggregation of these oxides. Another difficulty is the 

 case of iron pyrites, which has the same formula RS2 as stannic sulphide 

 but a much lower molecular heat ; this Regnault explains by saying- 

 (p. 191) that there is no analogy between these two sulphides, while bi- 

 sulphide of molybdenum, which has a molecular heat not very different 

 from that of stannic sulphide, presents, he says, some resemblance to it 

 in physical constitution. Regnault here seems to recognise that it is nofe 

 sufficient that the formulse should be similar and the bodies belong to the 

 same series ; it is necessary also that the formuljB should, as it were, not 

 be accidentally similar, but be similar as representing also bodies of the 

 same class. 



Although Regnault finds that calcite and aragonite do not differ in 

 their molecular heats, he finds that the specific heat of chalk and of 

 saccharoid marble are about the sanae and markedly higher than the 

 other forms. Regnault sums up the general result of this memoir 

 thus : ' In all compounds of similar atomic composition and chemical 

 constitution the specific heats are inversely as the [' atomic '] mole- 

 cular heats ' ; and he gives the same reasons for the fact of this law being- 

 only approximately true, as in the case of elements. 



In further communications on the specific heats of solids Regnaulfc 

 finds by direct determination that of potassium, cooled by contact with 

 solid COo and let fall into a calorimeter containing naphtha, and shows 

 that if the then accepted atomic weight of potassium is halved, this new 

 atomic weight is in accordance with D along and Petit's law ; and this 

 new atomic weight separates potassium from the alkaline earth metals 

 and other metals in the state of oxidation RO, with which metals in facte 

 potassium * has no relations of isomorphism ; that ordinary phosphorus 



' Ann. Chim. C%), 2P, 1841, p. 261. 



