ANALYTICAL EVIDENCE. 281 



united to form a compound atom or molecule, have com- 

 bined with 3 eq. of oxygen and 3 eq. of water, to form 

 1 eq. of hydrated cyanuric acid. The third expresses the 

 order in which the elements are supposed to be arranged 

 in hydrated cyanic acid, the whole multiplied by 3. Each 

 equivalent of cyanic acid is formed of 1 eq. of cyanogen, 

 1 eq. of oxygen, and 1 eq. of water ; and hence the same 

 number of atoms of each element, which together formed 

 1 eq. of cyanuric acid, is here so divided as to yield 3 eq. 

 of cyanic acid. 



We have here, therefore, the same absolute and relative 

 amount of atoms of each element, arranged in three diffe- 

 rent ways; yet in each of these the proportions of the 

 elements, calculated for 100 parts, must of course be the 

 same. It is easy, therefore, to see the advantage we pos- 

 sess by the use of formulae ; that, namely, of exhibiting 

 the relations existing between compounds of different 

 composition ; and that also of expressing the actual, 

 probable, or possible differences between substances 

 whose composition, in 100 parts, is the same, while their 

 properties, as in the case above quoted, are perfectly 

 distinct. 



It does not come within our province here to explain 

 the method or rule by which the composition of a sub- 

 stance, in 100 parts (as it is always obtained in analysis), 

 is expressed in a formula ; we shall only describe the rule 

 for calculating, from a given formula, the composition in 

 100 parts. For this purpose it must be noted that C, in 

 a chemical formula, signifies a weight of carbon expressed 

 by the number 76*437 (according to the most recent 

 determinations 75'8 or 75'0, a variation which has no 

 effect whatever on the formulae here adduced, all of 

 which are calculated on the number 76'437); that H 

 signifies a weight of hydrogen = 12'478 ; N a weight of 



