USEFUL PROPERTIES OP BENZOLE, ETC. 333 
addition of cold water,) distillation will cease; for no fluid 
remains in the retort whose vapor is not condensable at the 
temperature of the head, which can never rise above 2 1 2° F. 
The distillate being rectified a second time in such an ap- 
paratus, keeping now the temperature of the head at or a 
little below 176° F., and reserving the part which comes 
over before the temperature in the retort has passed 194° F., 
a large proportion of a very volatile oil will be obtained, 
which, when exposed to a temperature of 4° F., will be- 
come solid to at least half its bulk. Of course, if the appa- 
ratus be double, the same result may be obtained by a 
single operation. A comparatively small quantity of a 
similar product may be obtained by receiving separately 
the first two or three gallons that run off from the common 
stills on the rectification of some hundred gallons of the 
light naphtha. 
This produce should then be agitated with about a quar- 
ter of its bulk of oil of vitriol, or better still, with about 
one-tenth of strong nitric acid, and then, after separation 
from this agent, with oil of vitriol as above. It should 
then be separated from the acid and distilled (or it may be 
distilled from the sulphuric acid,) reserving again the por- 
tion given off below 194°F. The distillate, being tested 
by agitation with oil of vitriol, should remain perfectly 
colorless, and the acid should not now acquire a darker 
color than a pale straw-brown; if it acquires a deeper tint, 
the process should be repeated. When this result is ob- 
tained, the oil should bo well washed with water, and 
finally with an alkaline solution. 
Its further purification may now be entrusted to its con- 
gealing property. It should be exposed to a temperature 
of about 4°F., (which may be conveniently attained by a 
mixture of ice and salt ;) the solid portion being then pressed 
and filtered, the result, after treatment with chloride of cal- 
cium, is fit for use. 
The use of sulphuric acid in this process is to remove all 
