130 Dr. Thomson's Experiments to determine the [Aug. 



was unable to combine the acid and the base in definite pro- 

 portions. 



It was shown formerly that the true weight of an atom of 

 arsenic acid is 7*75. Now if the oxygen in arsenious acid be 

 to that in arsenic acid as 2 to 3, then it follows that an atom 

 of arsenious acid weighs 675. To verify this supposition, 1 put 

 6*75 grs. of pure arsenious acid into a small retort, previously 

 weighed, and the weight marked upon it by a diamond. Upon 

 this I poured a mixture of 12 parts nitric acid, of the specific 

 gravity T25, and 1 part muriatic acid, of the specific gravity 

 ' i # 2, in such quantity as I knew from previous trials would be 

 roore than sufficient to convert the arsenious into arsenic acid. 

 The retort being exposed to a moderate heat, the arsenious acid 

 dissolved. The heat was continued very moderate till the whole 

 liquid portion in the retort was expelled, and nothing remained 

 but a white crust consisting of arsenic acid. The retort was now 

 surrounded with hot sand, and exposed to a heat of between 500° 

 and 600°, which was kept up for several hours. It was then 

 allowed to cool and weighed. The weight of the arsenic acid 

 was found exactly 7*75 grs. This experiment may be repeated 

 as often as you please (provided the requisite precautions be 

 taken) without the least variation. 



It follows from it that the weights of arsenious and arsenic 

 acids are respectively as follows : 



Arsenious acid 6*75 



Arsenic acid 7*75 



If we take 4*75 grains of arsenic and convert them into 

 arsenic acid by means of pure nitric acid, we shall find the 

 weight of the arsenic acid formed, exactly 7-75 grains. It is 

 obvious, therefore, that an atom of arsenic weighs 4*75, and that 

 the constituents of the two acids of that base are : 



Arsenic. Oxygen. 



Arsenious acid 1 atom + 2 atoms 



Arsenic acid 1 +3 



So that arsenic exactly agrees with sulphur in the composition 

 of its acids. 



It is very likely that the black powder into which arsenic 

 sometimes falls, when exposed to the air, is a protoxide of arse- 

 nic, or a compound of one atom arsenic + one atom oxygen. 

 But I have been unable to verify this conjecture. The arsenic 

 which I used did not change spontaneously into a black powder; 

 and when I used an acid, whether the muriatic or the nitric, there 

 was always some arsenious acid formed, while a portion of the 

 arsenic retained its metallic state. Accident will probably some 

 time or other make us acquainted with this oxide, if it exist. 



