194 Petit and Dulong on some important Points " [Sept. 



capacities deduced from experiment by the weight of the corre- 

 sponding atom. These different products are contained in the 

 last column of the table. 



The simple inspection of these numbers exhibits an approxi- 

 mation too remarkable by its simplicity not to immediately 

 recognize in it the existence of a physical law capable of being- 

 generalized and extended to all elementary substances. These 

 products, which express the capacities of the different atoms, 

 approach so near equality that the slight differences must be 

 owing to slight errors either in the measurement of the capaci- 

 ties, or in the chemical analyses ; especially, if we consider that 

 in certain cases these errors derived from these two sources may 

 be on the same side, and consequently be found multiplied in 

 the result. The number and diversity of the substances on 

 which we operated not permitting us to consider the relation 

 thus pointed out, as simply accidental, we are authorized to 

 deduce from them the following law : 



The atoms of all simple bodies have exactly the same capacity 

 for heat. 



If we recollect what has been said above respecting the kind 

 of uncertainty which exists in fixing the specific weight of the 

 atoms, it will be easy to conceive that the law which we have 

 just established will change if we adopt for the density of the 

 particles, a supposition different from that which we have 

 chosen ; but in all cases the law will exhibit a simple ratio 

 between the weights and the specific heats of the elementary 

 atoms ; and it is obvious that when we had to choose among 

 hypotheses equally probable, we were naturally led to decide in 

 favour of that which established the most simple relation between 

 the elements which we compared. 



But whatever opinion be adopted respecting this relation, it 

 will enable us hereafter to control the results of chemical analy- 

 sis ; and in certain cases will give us the most exact method of 

 arriving at the knowledge of the proportions of certain combina- 

 tions ; but if, in the subsequent part of our experiments, no fact 

 occur to invalidate the probability of the opinion, which we 

 entertain at present, we shall find in this method the advantage 

 of fixing in a certain and uniform manner the specific weight of 

 the atoms of all simple bodies that can be subjected to direct 

 observations. 



The law, which we have announced, appears to be indepen- 

 dent of the focm which bodies affect, provided always that we 

 consider them in the same circumstances. 



This at least is a consequence deducible from the experiments 

 of MM. Laroche and Berard on the specific heat of the gases. 

 The numbers which they give for oxygen and azotic gases do not 

 differ from what they ought to be to agree accurately with our 

 law, except by a quantity less than the probable errors of such 

 experiments. The number relative to hydrogen, it is true, is 



