14 Dr. Thomson on the true Atomic Weight of [Jan. 



showing that the liquid still contained a great deal of arsenic 

 ■acid ; consequently the supposition that the crystallizable arse- 

 niate of soda is a compound of one atom arsenic acid and one 

 atom soda cannot be well founded. This will appear still more 

 clearly by the following experiment : 



Supposing that the crystallized arseniate of soda contained 

 two atoms of arsenic acid united to one atom of soda, I took a 

 quantity of carbonate of lime, equivalent to two atoms of lime, 

 which is obviously 12*5 grs. ; for carbonate of lime is com- 

 posed of 



Carbonic acid 2*75 



Lime 3*60 



Carbonate of lime 6*25 



And 6*25 x 2 = 12*5. This quantity I dissolved in muriatic 

 acid, evaporated the solution to dryness, and dissolved the dry 

 salt in distilled water. 18*5 grs. of dry arseniate of soda were 

 dissolved in another portion of distilled water. These two 

 liquids were mixed together. I was surprised to find that no 

 precipitate of arseniate of lime, or at least only a very slight one, 

 appeared. The solution had the property of reddening vegetable 

 blues. We see from this, that bmarseniate of soda is incapable 

 of decomposing muriate of lime. When ammonia was poured 

 into the solution, a copious precipitate of arseniate of lime fell 

 In small silky needles. After the precipitate had subsided, the 

 clear liquid was found to precipitate, when mixed with oxalate 

 of ammonia. It, therefore, contained an excess of lime. This 

 experiment shows clearly that the crystallized arseniate of soda 

 is a binarseniate, and that an atom of arsenic acid weighs more 

 than 7*25, the weight which I assigned in the fifth edition of 

 my System of Chemistry. 



After a great many trials, which I consider it as useless to 

 relate, I found that i9'5 grs. of binarseniate of soda and 41*5 

 grs. of nitrate of lead, when separately dissolved in distilled 

 water, and the solutions well mixed together, after all the arse- 

 niate of lead had precipitated, left a clear hquid which contained 

 no sensible quantity of oxide of lead, or of arsenic acid. But if 

 18"5 grs. or 19 grs. of binarseniate of soda were employed, an 

 excess of lead always remained in solution. From this experi- 

 ment, it is obvious that the equivalent number for anhydrous 

 binarseniate of soda is 19*5, and that it is a compound of 

 2 atoms arsenic acid = 15*5, and 1 atom soda = 4. 



Hence an atom of arsenic acid, instead of weighing 7*25, as I 

 supposed, weighs in fact 7*75, and arseniate of lead is a conn 

 pound of 



Arsenic acid 7*75 



Protoxide of lead 14*00 



Arseniate of lead , , . , 21*75 



