2C)2 POISONS ! THEIR EFFECTS AND DETECTION. [§ 32O. 
brass plate ( b ), fixed to a retort-stand in the usual way, and protected 
from the unequal cooling effects of currents of air by being covered by 
a flask (c), from which the bottom has been removed. The neck of the 
flask conveniently supports a thermometer, which passes through a cork, 
and the bulb of the thermometer is immersed in the bath of liquid metal. 
In the first examination of a substance the temperature is raised some¬ 
what rapidly, taking off the upper disc with a forceps at every 10° and 
exchanging it for a fresh disc, until the substance is destroyed. The 
second examination is conducted much more slowly, and the discs 
exchanged at every 4° or 5°, whilst the final determination is effected by 
raising the temperature with great caution, and exchanging the discs at 
about the points of change (already partially 
determined) at every half degree. All the 
discs are examined microscopically. The 
most convenient definition of a sublimate is 
this—the most minute films, dots, or crystals 
which can be observed by ^-inch power, and 
which are obtained by keeping the subliming 
cell at a definite temperature for 60 seconds. 
The commencement of many sublimates 
assumes the shape of dots of extraordinary 
minuteness, quite invisible to the unaided 
eye ; and, on the other hand, since the 
practical value of sublimation is mainly as 
an aid to other methods for the recognition 
of substances, if we go beyond short intervals 
of time, the operation, otherwise simple and 
speedy, becomes cumbersome, and loses its 
general applicability. 
There is also considerable discrepancy of statement with regard to the 
melting-point of alkaloidal bodies; in many instances a viscous state inter¬ 
venes before the final complete resolution into fluid, and one observer will 
consider the viscous state, the other complete fluidity, as the melting-point. 
In the melting-points given below, the same apparatus was used, but 
the substance was simply placed on a thin disc of glass floating on the 
metallic bath before described (the cell not being completed), and 
examined from time to time microscopically, for by this means alone 
can the first drops formed by the most minute and closely adherent 
crystals to the glass be discovered. 
Cocaine melts at 93°, and gives a faint sublimate at 98° ; if put 
between two watch-glasses on the water-bath, in fifteen minutes there 
is a good cloud on the upper glass. 
Aconitine turns brown, and melts at 179° C. ; it gives no character¬ 
istic sublimate up to 190°. 
Subliming Cell. 
