60 
ON THE SUBLIMATION OF THE ALKALOIDS. 
mist makes its appearance. Then, as a general rule, the lamp should be with¬ 
drawn, the disk removed, and a new one put in its place. It may be well to 
state that, as the disk 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 disk, at 
least not for any length of time; for if this precaution be neglected, it may 
happen with the alkaloids as with arseuious acid or corrosive sublimate, 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 disk of 
glass, as it is removed, should be placed with the sublimate upwards against a 
glass slide or fragment of porcelain ; and that this position (the sublimate up¬ 
wards) should be retained. If this very simple precaution be overlooked, it is 
quite possible that we may mistake one surface for the other, and find ourselves 
applying our reagents to the wrong one. 
2. The chief precaution relating to the examination and disposal of the 
sublimates consists in measures for preserving their identity during the exami¬ 
nations to which we may have to subject them. This is best done by writing 
the names and that of the reagents on disks of paper, and placing paper and 
disk together in sunken grooves or circular spaces. 
3. The choice, use, and value of reagents is a very important matter. In 
ordinary chemical operations, we are satisfied with a test if it gives a precipitate, 
or a colour, which is either peculiar to the substance under examination or to the 
group to which it belongs ; or if, by some subsidiary test, the precipitate is dis¬ 
solved or the colour changed or discharged. We are equally satisfied with our test 
if the exact reverse occurs—if there is no precipitate and no change of colour, or if 
a precipitate or colour actually produced is not dissolved, changed, or discharged. 
But the case is altered when we have to examine the result of our reactions under 
the microscope. VVe have placed a drop of some reagent upon a sublimate; 
and, after watching the immediate effect, we allow it to continue its action till 
it evaporates and dries up. We then examine the dry spot, and find in it what 
we believe to be characteristic crystals, or other equally characteristic forms. 
But we may have overlooked this important fact, that the reagent, if it be a 
saline solution, has a crystalline form of its own, and that what we are taking 
for a characteristic and diagnostic reaction is, after all, nothing but that very 
crystalline form or forms of the reagent. 
To show that I am not merely imagining a difficulty, and insisting on an un¬ 
necessary precaution, I refer the reader, if he have Helwdg’s work at hand, to 
the third photograph of table vi., the fourth of table xii., and the fourth of 
table x. The photograph in table vi., which represents the reaction of a solution 
of nitro-prusside of sodium on an aqueous solution of muriate of morphia, is 
nothing more than the holly-like crystal of the nitro-prusside itself as it is de¬ 
posited from its solution. The fourth photograph of table xii., which represents 
the reaction of the same solution of nitro-prusside of sodium on the muriate of 
veratriue, is also a slight and familiar modification of the more usual crystals 
of the nitro-prusside. Of the fourth photograph of table x. I cannot speak so 
positively ; but I suspect it to be a form of crystal which the solution of carba- 
zotic acid may itself deposit, without the assistance of the aqueous solution of 
the muriate of strychnia which is represented to contribute to its formation. 
As a precaution omitted by an experimenter who shows so much care as Dr. 
Helwig usually exhibits is very likely to be overlooked by others, it is well to 
insist upou it; and to prescribe as the first step to be taken with a crystalline 
solution which we are about to use as a test, the determination of its proper 
crystalline form or forms as evaporated on a flat surface of glass. 
Another mistake, arising out of a similar want of caution, may consist in con¬ 
founding the effect of some saline reagent with that of the water which holds it 
