OF MICROSCOPIC OBJECTS. 9 
3RD DIVvISION. 
Under the third division of our list may be ranged the 
following agents :—alcohol, solutions of chromic acid, bi- 
chromate of potash, hyperosmic acid, chloride of palladium, 
bichloride of mercury (in Goadby’s solution), and tannin, or 
the substance may be dried in thin layers or small pieces, 
either spontaneously or in vacuo, or by carefully regulated 
heat; in some cases it may be boiled, or it may be frozen. 
Alcohol is, on the whole, the best and most convenient of 
the hardening agents. It acts by abstracting water and 
coagulating albumen, and its uses as a preservative fluid 
per se are well known. It enters also into many of the 
preservative fluids, and is especially convenient and useful 
when it is desired to mount specimens quickly out of watery 
fluids in Canada balsam, without drying them previously. 
After a longer or shorter soaking in it, according to their 
size or thinness, preparations may be at once placed in tur- 
pentine, and then easily and speedily put up for examina- 
tion in balsam. 
Dr. Beale recommends a mixture of alcohol and a solu- 
tion of caustic soda for the preservation of delicate tissues. 
He observes, “‘ that alcohol alone tends to coagulate albu- 
minous textures and render them opaque, at the same time 
that it hardens them. The alkali, on the other hand, will 
render them soft and transparent, and would dissolve them 
if time were allowed. These two fluids, in conjunction, 
harden the texture, and at the same time make it clear and 
transparent.” 
Chromic acid in solution, 0'°25—0'5 to 1 and 2 per cent. 
of distilled water is much used. On account of its deli- 
quescence, it is most conveniently kept in a saturated solu- 
tion, which may be diluted as desired; and very often the 
weaker this solution the better. When it has had the de- 
sired effect on the tissue, the preparation should be removed 
into diluted alcohol, on account of the readiness with which 
fungi and confervoid growths are formed in chromic acid 
