On Crystalline Form and Chemical Proportions. 199 
time which will haye a great influence on the future state of 
mineralogy, he has conceived it his duty to submit this idea 
to public discussion. 
The result of his trials on the above-mentioned sulphates was 
published in the Memoirs of the Academy of Berlin for 1818 
and 1819, and in the Annales de Chim. et de Phys. for July, 
1820. The researches of M. Berzelius have shewn, says he, 
that the acids of phosphorus and arsenic, that is, the phosphoric, 
phosphorous, arsenic, and arsenious, are analogous in their 
composition, but different from all the other acids; and 
that in their combinations with the bases, they follow the same 
law, which, however, departs from that which all other oxidized 
bodies obey in their combinations *. On account of the extra- 
ordinary identity of chemical composition in these salts, he 
thought it right to employ them in preference, for the examina- 
tion of the idea just announced. _Berzelius found that the quan- 
tity of oxygen with which phosphorus combines to form phos- 
phoric acid, is to that which produces phosphorous acid with 
the same quantity of phosphorus, in the ratio of 5 to 3. M. Du- 
long, without knowing the labours of Berzelius, was led, by his 
own experiments, to the same result. Berzelius has since found, 
by a long series of experiments, that the same relation exists in 
the composition of the arsenious and arsenic acids. M. Mits- 
cherlich’s researches have brought him to the same conclusion. 
The number of proportions which the phosphoric and arsenic 
acids contain determines directly the ratio in which these acids 
combine with the bases ; but it is not merely in this identity of 
composition that the two series of salts formed by these oxides 
resemble each other, for we shall see that they have, under 
every point of view, an almost perfect identity in their compo- 
sition and in their characters. Every arseniate has a phosphate 
which corresponds to it, composed according to the same pro- 
porticns, combined with the same atoms of water of crystalli- 
zation, and which, at the same time, has the same physical 
qualities. Ina word, these two series of salts differ in no re- 
Spect, except that the radical of the acid of one series is 
phosphorus, and that of the other arsenic. If we add phospho- 
ric or arsenic acid, to carbonate of soda, till the solution shews 
a neutral re-action, we obtain no crystalline salt; but if we add 
carbonate of soda in excess, we procure large and beautiful 
erystals, in which the oxygen of the base is to that of the acid 
as 2to5. The neutral combinations of arsenic and phosphoric 
acid with potash, and the double combinations of the same 
acids with potash and soda, are equally uncrystallizable, unless 
they have an excess of base. The arseniate and the phosphate 
of ammonia crystallize only with an excess of base, as well as 
the double salts which they form with the arseniate and the 
* Aj) this we cannot but regard as mere hypothesis. 
