MICROCHEMICAL REAGENTS. 271 
of protoplasm, since it causes the nucleus of the cell to appear 
sharply defined. 
6. Neutral Acetate of Copper dissolved in twenty parts of 
water. If tissues containing resin are allowed to remain for 
some days in this solution, the metal combines with the resin to 
form green clots. 
”. Tannic Acid is always dissolved freshly just before use in 
twenty parts of water, and employed in the search for alkaloids. 
By moistening thin sections with water, to which a slight 
trace of acetic acid has been added, a concentrated liquid extract 
of the substance is prepared, which is tested by the gradual 
addition of a few drops of the solution of tannic acid upon the 
slide. Ifa turbidity is produced, it may be due to the presence 
of alkaloids, but may also be caused by the so-called bitter 
principles, or by albumen. 
8. Solution of Soda of the specific gravity 1.160, containing 
15 per cent of sodium hydroxide. Instead of this solution, an 
equally strong alcoholic solution may serve for many purposes. 
The corresponding solutions of potassium hydroxide have the 
same action. 
The caustic alkali causes the cell-walls to swell, and dissolves 
many of the constituent substances, especially coloring matters, 
whereby the sections are rendered very much clearer; from 
lignified membranes the lignin is extracted by means of warm 
solutions of caustic potassa or soda. In many cases the treat- 
ment of the tissue with alkali permits of the clearer recognition 
of the relations of the strata. The protein crystalloids reveal 
their organic structure by swelling, whereby their plain surfaces 
become for the most part rounded and the angles changed. 
Many yellow coloring substances (chrysophan in Rhubarb, 
frangulin in Cortex Frangule, chrysarobin) become red by 
alkalies, a reaction for which lime-water is well adapted. 
9. Sodium Hydroxide (solid caustic soda) or potassium 
hydroxide may be conveniently kept and employed in the form 
of powder, when the amount to be applied does not require to 
‘be more accurately measured. | 
10. Ammonia Water, of the specific gravity 0.960, is often 
