SUBLIMATION OP ALKALOIDS. 733 



"The principal precaution to be observed in the 

 application of heat is, that it should be moderate and 

 gradual. It is best to act on the assumption that the 

 substance under examination may be one of a considerable 

 group of bodies, some of which sublime at very moderate 

 temperatures. The spirit-lamp should, therefore, be placed 

 at first three or four inches below the slab of porcelain, so 

 that the point of its flame may not touch it ; and if, under 

 this low temperature, the disc of glass is not dimmed, the 

 lamp should be raised by degrees till the mist makes its 

 appearance. Then, as a general rule, the lamp should be 

 withdrawn, the disc removed, and a new one put in its 

 place. It may be well to state that, as the disc has been 

 passed through the flame to drive off moisture, and has in 

 this way been heated, the flame of the spirit lamp should 

 not be allowed to play on the porcelain slab after the mist 

 has appeared on the disc, at least not for any length of 

 time ; for, if this precaution be neglected, it may happen 

 with the alkaloids as with arsenious acid or corrosive sub- 

 limate, that the mist does not form at all, or that it is 

 driven off as soon as it is deposited. Perhaps, too, it may 

 not be quite unnecessary to recommend that each disc of 

 glass, as it is removed, should be placed with the subli- 

 mate upwards against a glass slide or fragment of porcelain ; 

 and that in this position (sublimate upwards) it should be 

 retained. If this very simple precaution be overlooked, it 

 is quite possible that we may mistake one surface for th<3 

 other, and find ourselves applying our re-agents to the 

 wrong one. The chief precaution relating to the ex- 

 amination and disposal of the sublimates consists in 

 measures for preserving their identity during the examina- 

 tion to which we may have to subject them. This is best 

 done by writing the names and that of the reagents on 

 discs of paper, and placing paper and disk together in 

 sunken grooves or circular spaces. 



" As a precaution omitted by an experimenter so prac- 

 tised as Dr. Helwig is very likely to be overlooked by 

 others, it is well to insist upon and to prescribe as the 

 first step to be taken with a crystalline solution which wo 

 are about to use as a test, the determination of its proper 

 crystalline form or forms as evaporated on a flat surface of 



