264 Journal of the Asiatic Society of Bengal. [N.S., XV, 
ing from the decomposition of phosphorous iodide by water ; 
phosphorus, being in a colloidal state in the solution of its 
iodide, cannot be removed by filtration. 
trangely enough if phosphorus tri-bromide or phosphorus 
tri-iodide be previously dissolved in water at the room 
completely lose their reactivity if boiled before the addition of 
arsenious oxide solution. The activity also diminishes when 
the solutions are kept for a long time at the ordinary tempera- 
ture. 
The reaction is very delicate. Indeed, with phosphorus 
tri-chloride the presence of arsenic can be detected definitely 
in a solution of arsenious oxide containing only ‘000075 grams 
hydrion 
reaction. When no additional hydrochloric acid is introduced 
[Hypophosphorous acid and hypophosphites, though more 
powerful reducing agents than phosphorous acid and phos- 
phites, are, curiously enough, without any action upon the 
arsenic compounds. | 
It has also been observed that antimonious and antimonic 
compounds are not affected at all by these reagents and there- 
fore the reaction can be employed as a test for distinguishing 
arsenic from antimony. 
