no 



THE ROYAL SOCIETY OF CANADA 



sulphide sol, the particles of which are negatively charged. Varying 

 small amounts of N/1000 aluminium sulphate were added, drop by 

 drop from a standard burette, to 50 cc. samples of the sol and the 

 whole well mixed. Each 50 cc. sample was divided into two equal 

 parts, one part being centrifuged and the other put aside for observa- 

 tioi of signs of coagulation in the ordinary way. Each set of 25 cc. 

 samples to be specially treated was centrifuged for 20 minutes at 

 2750 R.P.M. and the amount of solid precipitate estimated, as follows: 

 The arsenious sulphide precipitated was treated with nitric acid and 

 ammonium persulphate which oxidise the arsenic to arsenic acid; 





36 



32 



2B 



24- 



.2C 



16 







I Z 3 4- 5 6 7 a 9 10 



AMOUNT or ARSENIOUS SULPHJDE COAGULATED 

 (AKBITfiARY SCALE) 



the arsenic was then precipitated as magnesium ammonium arsenate 

 in presence of the phosphate; the arsenate was then dissolved in 

 hydrochloric acid and to this solution potassium iodide was added 

 in excess; iodine is freed quantitatively and estimated ih the ordinary 

 way by titrating with sta;ndard thiosulphate solution. 



It was found that the centrifuging caused a slight deposit to 

 come down even from the original solution; the amount of this 

 deposit gradually increased with increasing amounts of aluminium 

 sulphate added, but when the amount of the latter reached a certain 

 limiting value the whole of the particles of the sol were coagulated. 



