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SELECTED ARTICLES. 
phosphoric acid, nor the sulphuretted hydrogen, contained 
arsenic, and that the arsenic was combined with the phospho- 
rus itself. He also learned from another druggist, that water 
in which phosphorus had been long kept, was also found to 
contain arsenic; this fact was also confirmed by Wittstock. 
Nevertheless, phosphorus which he himself prepared was ex- 
empt from this admixture. 
From other experiments he ascertained that a considerable 
proportion of arsenic might be mixed with phosphorus, with- 
out changing the external appearance of the latter, except that 
it became of a deeper colour especially at the surface. If the 
quantity was very great, the mixture was of a steel gray co- 
lour, but was soft and ductile like wax. 
Mr. Barwald attributed the presence of this metal in the 
phosphorus to the sulphuric acid employed to extract the 
phosphoric acid; and describes in what way a pure phospho- 
ric acid may be obtained from phosphorus containing arsenic; 
for this purpose he advises, that the phosphorus be treated 
with thirteen times its weight of hot nitric acid of a specific 
gravity of 1,200, in large glass vessels having a narrow open- 
ing, till the phosphorus is all dissolved. When the solution 
is evaporated to one half, a violent reaction takes place accom- 
panied with a disengagement of red fumes. Mr. Wittstock 
explains this, by admitting that the diluted nitric acid, at first 
only changes the phosphorus to the state of phosphorous 
acid, and that it only becomes phosphoric acid when the men- 
struum has become suffciently concentrated; at this stage the 
phosphoric acid is formed at the expense of the nitric acid 
that remains. This latter must be added as long as red fumes 
are disengaged. The evaporation is to be pursued in a platina 
or porcelain vessel as long as nitric acid is given off, which 
takes place when the temperature has been raised to about 
400° F. The phosphoric acid is then to be purified by means 
of sulphuretted hydrogen. To get rid of any excess of this 
latter, Mr. Wittstock uses vegetable charcoal, heated to red- 
ness and then treated with nitric acid and well dried. 
