ON THE PREPARATION OF MICROSCOPIC CRYSTALS. 
267 
and a few drops poured in a full stream on to a clean slide, and immediately 
poured off, sufficient will remain behind for the production of crystals, which 
will form at once ; when they have attained sufficient size (which they will 
have done in a few seconds), the remaining liquor, if any, should be drained 
from them and the slide allowed to dry. The result will generally be a slide, 
evenly covered with crystals having well-defined edges, and but few of which 
are agglomerated. 
If the solutions used are not too concentrated, the right moment to pour 
them on the slides is just before the crystals begin to form in the soluiion 
itself. I may remark here (and this applies to all crystals deposited from hot 
solutions), that if the liquid is too hot, the slide itself becomes warm, and 
the deposition of the crystals being thereby retarded, the result is not satisfac¬ 
tory. Of course this may be easily remedied by allowing the solution to be¬ 
come cooler ; but if, on the other haud, it is too cold, the result is a mass of 
crystals, which, although perfect, are too small to be of value. Sometimes it is 
of great assistance to breathe on the slide before placing the solution on it, as 
that prevents the immediate deposition of the crystals. This process answers 
extremely well with the following salts,—alum, chlorate of potassium (which 
should not be very concentrated), nitrates of barium and strontium, potassio- 
tartrate of antimony, sulphate of copper, sulphate, acid tartrate, acid oxalate, 
and superacid oxalate of potassium, and many others of similar solubility. Of 
course the strength of each solution must be regulated by experience. 
When crystals are not very soluble in cold water, they may be allowed to 
separate in the bulk of the solution itself as it cools,—if necessary, shaking the 
vessel to prevent their attaining too great a size; then, by means of a piece of 
glass tube closed at one end with the finger, a small quantity of liquid and 
crystals may be taken up and transferred to a slide, which should have been 
well breathed on, to allow the fluid to flow over it more easily. It will be ne¬ 
cessary to keep the slide moving for a minute or so, to prevent the aggregation 
of the crystals ; when they have been dispersed pretty evenly, the superfluous 
fluid should be removed by blotting-paper applied to the edges of the slide. 
The liquid must not be poured off, or the crystals will almost certainly run to¬ 
gether. Although this process answers well, I think it is inferior to allowing 
the crystals to deposit on the glass itself. 
Some difficulty will be experienced in obtaining perfect crystals from sub¬ 
stances which are generally met with in long prisms, as nitrate of potassium, 
oxalic and hippuric acids, etc. With the two former I have found that the 
best method is to make a hot solution, containing rather more of the salt than 
would saturate it at ordinary temperatures ; having filtered it and allowed it to 
become nearly cold, a few drops should be placed on a slide, and a very fine 
glass-rod, or the head of a needle, drawn once or twice across it. Of course 
this stirring mars the general appearance of the slide, but I have found it the 
only method of obtaining perfect crystals of these and similar substances. It 
is easy enough to crystallize them in long prisms, but it is the difficulty of pro¬ 
ducing typical crystals which is overcome by this method. For hippuric acid, 
the solution, when just on the point of crystallizing, should be poured on to a 
cold slide (which, if necessary, should have beeu breathed on), and when the 
crystals have formed, the remaining liquid should be poured off and the slide 
allowed to dry. The result is often most successful for the beauty of the crystals 
and their variety of form. 
The greatest difficulty in producing perfect crystals will be met with in those 
substances which are very soluble in water, as sugar, citric and tartaric acids, 
etc. A good method for crystallizing sugar is to make a cold solution contain¬ 
ing about as much sugar as water, and having filtered it, to drop some on to a 
slide, and then pour off so much of it that only a thin layer of liquid remains ; 
