158 ARRANGEMENT OF THE ELEMENTS IN COMPOUNDS. 



crystals of these double salts, not only coincide in the propor- 

 tion of their water and other constituents and in the composi- 

 tion of their acids but also in external form, having been ob- 

 served by Mitscherlich to be isomorphous. A nearer approach 

 to identity could scarcely be conceived than is exhibited by 

 these bodies, which are, indeed, the same both in form and 

 composition. The crystallized acids are both modified in an 

 unusual manner by heat, and form three classes of salts, as 

 phosphoric acid does. The formula of both acids in their ordi- 

 nary class of salts is C 8 H 4 O 10 + two atoms of base (Fremy). 

 But by no treatment can the one acid be transmuted into the 

 other. Lastly, every organic acid produces a new acid by des- 

 tructive distillation, which is peculiar to it and is termed its 

 pyr-acid. Now racemic and tartaric acid, when destroyed by 

 heat, agree in giving birth to one and the same pyr-acid. 



ARRANGEMENT OF THE ELEMENTS IN COMPOUNDS. 



The names of some compounds imply that they contain 

 other compounds, and indicate a certain atomic constitution, 

 while the names of other compounds express no particular ar- 

 rangement of their constituent atoms, but leave it to be inferred 

 that the atoms are all directly combined together. Thus sul- 

 phate of soda implies the continued existence of sulphuric acid 

 and soda in the salt, while nitric acid or peroxide of hydrogen, 

 supposes no partition of the compound to which it is applied. 

 But it is to be remembered that the original framers of the no- 

 menclature were guided more by facilities of an etymological 

 nature in constructing such terms, than by views of the consti- 

 tution of compounds. 



Of a binary compound containing single atoms of its con- 

 stituents, there cannot be two modes of representing the con- 

 stitution, but where one of the constituents is present in the 

 proportion of two or more atoms, several hypotheses can always 

 be formed of their mode of aggregation. In a series of binary 

 combinations of the same elements, such as that of nitrogen 

 and oxygen, NO 15 NO 2 , NO 3 , NO 4 , NO 5 , the simplest view 

 has generally been taken, namely that it is the elements them- 

 selves which unite. But in particular cases the chemist is often 

 involuntarily led into another opinion. Thus deutoxide of ni- 

 trogen is so often a product of the decomposition of nitric acid, 



