MICROSCOPIC OBJECTS. 



31 



nothing but a confused mass of amorphous 

 forms It is often better, instead of apply- 

 ing heat to a drop of the solution on a glass 

 slide, to make a hot solution in a test tube, 

 and evaporate a few drops slowly on the 

 glass slip. Arborescent crystals, no matter 

 how beautiful in themselves, are a great 

 nuisance when the student wishes to prepare 

 the typical forms for photography, and they 

 may be avoided by allowing the solution to 

 crystallize out slowly. Potassium bichro- 

 mate, if crystallized rapidly, either over the 



White arsenic, As.j O3, may be obtained 

 either from ils aqueous solution, or by sub- 

 limation. The latter mode gives more bril- 

 liant and regular crystals, but is more 

 troublesome. A small portion of arsenic 

 should be put in a short test-tube about one 

 inch long, and a glass slide laid over the 

 top. On applying heat, the crystals will 

 form on the sides of the tube and on the 

 slide. By a little careful manipulation with 

 the spirit lamp, the bulk of the crystals may 

 be driven from the sides of the tube and 

 made to crystallize on the glass slip. They 

 should be covered with a circle of thin glass 



lamp or from a hot solution, usually gives 

 arborescent forms; but when evaporated in 

 the cold, the characteristic prismatic crystals 

 are obtained. All salts crystallized from an 

 alcoholic solution should, as a general rule, 

 be evaporated in the coM. As an example 

 of this, magnesium platino-cyanide, when 

 evaporated from an alcoholic solution rapid- 

 ly, gives only an amorphous red mass. To 

 obtain the prismatic crystals arranged m 

 rosettes, the solution must be evaporated 

 slowly in a cool room free from draughts. 



— no balsam must be used, or they become 

 too transparent to photograph. The pre- 

 ceding cuts, photographed from slides of 

 ■001 grain, and -002 grain of arsenic, show 

 the general form of the crj'stals. 



Strychnine is very insoluble in water, so 

 it should be crystallized from its dilute 

 alcoholic solution. The form of the crystals 

 is the octohedron, or square prism— usually 

 the latter. As it dissolves in balsam it 

 should be mounted dry. With a solution 

 of picric acid strychnine yields hook like 

 crystals, which may easily be photographed 

 with a low power. 



O & 0 ^ 



O ^ ^ e B 



CRTSTALS OF ARSENIC, FROM A DEPOSIT OF '001 GRAIN. 



