§ 320 .] SUBLIMATION OF THE ALKALOIDS. 261 
which any change took place was noted. In this way Dr Guy made 
determinations of the subliming points of a large number of substances, 
and the microscopic appearances of the sublimates were described with 
the greatest fidelity and accuracy. On repeating with care Dr Guy’s 
determinations, however, the senior author could in no single instance 
agree with his subliming points, nor with the apparatus he figures and 
describes could two consecutive observations exactly coincide. Further, 
on examining the various subliming temperatures of substances, as 
stated by different authors, the widest discrepancies were found—differ¬ 
ences of 2 or even 3 degrees might be referred to errors of observation, 
a want of exact coincidence in the thermometers employed, and the 
like ; but to what, for example, can we ascribe the irreconcilable state¬ 
ments which have been made with regard to theine ? According to 
Strauch, this substance sublimes at 177°; according to Mulder, at 
184-7°. But that both of these observations deviate more than 70° 
from the truth may be proved by anyone who cares to place a few 
mgrms. of theine, enclosed between two watch-glasses, over the water- 
bath ; in a few minutes a distinct sublimate will condense on the upper 
glass, and, in point of fact, theine will be found to sublime several 
degrees below 100°. 
Since this great divergence of opinion is not found either in the 
specific gravity, or the boiling-points, or any of the like determinations 
of the physical properties of a substance, it is self-evident that the 
processes hitherto used for the determination of subliming points are 
faulty. The sources of errors are chiefly:— 
1. Defects in the apparatus employed—-the temperature read being 
rather that of the metallic surface in the immediate vicinity of the 
thermometer than of the substance itself. 
2. The want of agreement among observers as to what should be 
called a sublimate—one considering a sublimate only that which is 
evident to the naked eye, another taking cognisance of the earliest 
microscopic film. 
3. No two persons employing the same process. 
With regard to the apparatus employed, the senior author adopts 
Dr Guy’s subliming cell; but the cell, instead of resting on a metallic 
solid, floats on a metallic fluid. For any temperature a little above 
100° this fluid, is mercury, but for higher temperatures fusible metal is 
preferable. » 
The exact procedure is as follows :—A porcelain crucible (a in fig.), 
about 3 inches in diameter, is nearly filled with mercury or fusible 
metal, as the case may be ; a minute speck (or two or three crystals of 
the substance to be examined) is placed on a thin disc of microscopic 
covering glass, floated on the liquid, and the cell is completed by the 
glass ring and upper disc. The porcelain crucible is supported on a 
