232 Method of separating Small Quantities 



force through the jet, and, on igniting it as it issues (which 

 must be done quickly by an assistant), and then holding hori- 

 zontally a piece of crown or window-glass f^ over it, in such 

 a manner as to retard slightly the combustion, the arsenic (if 

 any be present) will be found deposited in the metallic state 

 on the glass ; the oxygen of the atmosphere being employed in 

 oxydizing the hydrogen only during the process. If no arse- 

 nic be present, then the jet of the flame as it issues has a very 

 different appearance; and, although the glass becomes dulled in 

 the first instance by the deposition of the newly formed water, 

 yet, such is the heat produced, that in a few seconds it becomes 

 perfectly clear, and frequently flies to pieces. 



If the object be to obtain the arsenic in the form of arsenious 

 add, or white arsenic, then a glass tube, from a quarter to half 

 an inch in diameter (or according to the size of the jet of flame), 

 and eight or ten inches in length, is to be held vertically over 

 the burning jet of gas, in such a manner that the gas may un- 

 dergo perfect combustion, and that the arsenic combined with it 

 may become sufficiently oxydized ; the tube will thus, with pro- 

 per care, become lined with arsenious acid in proportion to the 

 quantity originally contained in the mixture. 



When the glass tube is held at an angle of about forty-five 

 degrees over the jet of flame, three very good indications of the 

 presence of arsenic may be obtained at one operation ; viz., me- 

 ralHc arsenic will be found deposited in the tube at the part 

 nearest where the flame impinges, — white arsenic or arsenious 

 acid at a short distance from it, — and the garlic smell can be 

 readily detected at either end of the tube in which the experi- 

 ment has been made. 



As the gas produced during the operation is consumed, the 

 acid mixture falls into the short limb of the tube, and is thus 

 again brought into contact with the zinc, in consequence of which 

 a fresh supply is soon obtained. This gas, if submitted to either 

 of the processes before described, will give fresh indications of 

 the presence of the arsenic which the mixture may have origin- 

 ally contained ; and it will be easily perceived that the process 

 may be repeated as often as may be required, at the will of the 

 operator, till no further proofs can be obtained. 



When certain mixed or compound liquors are operated on in 



