1821.] Strontian, Lime, Magnesia, Phosphoric Acid, S^c, 13 



tions of muriate of soda, and probably in many other saline 

 solutions, has misled experimenters in their attempts to analy:?e 

 the phosphates. 



I intend the first leisure opportunity to correct my old paper 

 on phosphoric acid, and give it to the pubHc. It contains mariy 

 facts still unknown to chemists in general, notwithstanding the 

 experiments of Berzehus on the subject; and now that 1 am 

 aware of what the true composition of the phosphates is, it will 

 not be so difficult to obtain accurate results. 



5. The exact knowledge of the weight of an atom of arsenic 

 acid is of considerable importance towards the perfection of the 

 atomic theory. Hitherto Berzelius is almost the only person who 

 has made direct experiments to determine the atomic weights of 

 arsenious and arsenic acids. He has concluded from his experi- 

 ments, and the conclusion seems to have been acquiesced in by 

 chemists in general, that the oxygen in arsenious and arsenic 

 acids are to each other in the ratio of 3 to 5. It will be seen 

 from the table near the beginning of this paper, that my atomic 

 number for the weight of an atom of arsenic acid very nearly 

 agrees with that of Berzelius. In a paper published in a late 

 volume of the Annals of Philosophy, indeed, I endeavoured to 

 show that the double of my number, or 14'5, which very nearly 

 agrees with the number actually pitched on by Berzelius, is the 

 real weight of the atom of arsenic acid. The object which I 

 had in view was to get rid of certain fractions which disfigure 

 the composition of arsenious and arsenic acids as I represented 

 them in the fifth edition of my System of Chemistry. The expe- 

 riments which I am now going to relate will show how far these 

 views are consistent with matter of fact. 



If arseniate of soda in qrystals be, as I represented it in a pre- 

 ceding paper, a compound of 1 atom arsenic acid = 14*5 and 

 1 atom soda = 4, then its composition must be as follows : 



Arsenic acid 14*5 



Soda 4-0 



Arseniate of soda 18*5 



We have seen already that nitrate of lead is composed of 



Nitric acid 6*75 



Protoxide of lead 14-00 



Nitrate of lead 20-75 



18*6 grs. of arseniate of soda previously kept in a state of 

 igneous fusion for half an hour in a platinum crucible, were dis- 

 solved in distilled water. 20*75 grs. of nitrate of lead were 

 dissolved in another portion of distilled water, and the two solu- 

 tions mixed intimately with each other. After the arseniate of 

 lead had subsided, the clear liquid was drawn off and examined. 

 When mixed with nitrate of lead, a very copious precipitate feU^ 



