20 M. J. A. Groshans on the 



The density -numbers of the elements increase with their 

 atomic weights, but are not proportional to these. 



The exact nature of these numbers is not yet distinctly 

 made out ; but the following provisional hypothesis, which, 

 I believe, may ultimately prove to be correct, will help in 

 forming a conception of their nature. 



According to this hypothesis, carbon, hydrogen, and oxygen 

 are simple bodies ; but the other elements are compounds of 

 other simple substances, the number of atoms of which is 

 shown by the density-number of each element. 



The law itself may be enunciated in the following simple 

 form : — 



" The densities of substances are proportional to the density- 

 numbers." 



For example, in the case of two compounds of carbon, 



hydrogen, and oxygen (comparable with one another), and 



with the formulae C^ H g O r and Cy By 0,./, if n and n' equal the 



sum ofp + q + r and p f + q' + r' respectively, and the densities 



are 8 and S', 



h n n 7i r _ 



^7 = — , or -^ = -Kj = constant = k. 

 a iv o o 



The law of Avogadro leads in the same way to a constant ; 

 for if the vapour-density of a substance at 0° C. and 760 mil- 

 lim. equals D, and its molecular weight is a, then 



Da, a . 



— == — whence ^ = constant. 



This constant is unique, and is true of all bodies which can 

 be converted into vapour. Such is not the case, however, 

 with the law of molecular specific heats (axe). This constant 

 differs in the various groups of compounds ; each group 

 possesses a single constant w T hich is peculiar to it. The law 

 of density-numbers resembles in this respect that of Dulong 

 and Petit. 



This new law is applicable to substances in every state of 

 aggregation ; but it is necessary that the substances should 

 be compared under similar conditions, such as : — 



(1) as gases at the boiling-point; 



(2) as liquids at the boiling-point ; 



(3) as solids (crystals hydrated or anhydrous); 



(4) in very dilute solutions. 



When dealing with substances which contain only carbon, 

 hydrogen, and oxygen, I shall continue to use the letter n for 

 the sum of the numbers of density ; but when other elements 

 enter into the composition of a body, I shall employ B to 

 indicate the above sum. 



