180 MR. H. A. SMITH ON 



Dr. Roscoe for the use of the appliances of the laboratory 

 at Owens College,, where the experiments were carried out ; 

 and I am also indebted to him for valuable suggestions. 



XXI. On Arsenic from Alkali- Works. 

 By H. A. Smith, F.C.S. 



Read April 30th, 1872. 



Some time ago I laid before this Society the results of 

 several analyses showing the amounts of arsenic contained 

 in the different species of pyrites used in vitriol-manufac- 

 ture, and in several of the products in the manufacture 

 of which the acid was employed. At that time I car- 

 ried my analyses as far as the carbonate of soda, in which 

 no arsenic was found. I should now like to supplement 

 that paper by showing how the arsenic contained in the 

 pyrites escapes in a not inconsiderable amount to the 

 atmosphere. 



When the salt used for the production of hydrochloric 

 acid is treated with sulphuric acid containing arsenic, the 

 arsenic present becomes converted into the trichloride 

 (AsClj) . This compound is said to be completely decom- 

 posed by contact with water, so that, after passing along 

 with the hydrochloric acid gas through the condensing- 

 towers, it would scarcely be expected that any traces of 

 the arsenic originally present would be found in the 

 escaping gas. I find, however, that this really is the case. 

 A considerable quantity of the arsenic trichloride escapes 

 the action of the water in the condensing- towers, and 



