PRACTICAL MICROSCOPY. 



scopical varnish making, and therefore strong alcohol 

 cannot be used in such cells. Weak spirit has, however, 

 no action upon a cell of gold size, and this is the only 

 varnish the author advises to be used when an object is 

 mounted in weak spirit. 



As to hardening agents, the chromates have been treated 

 of in Chapter VIII. on Section-cutting, and therefore it is 

 not necessary to recapitulate their uses. 



Tannin dissolved either in water or alcohol is a harden- 

 ing agent especially useful for gelatinous tissues ; it is used 

 sometimes in injecting for hardening the walls of blood- 

 vessels to prevent the passage of the coloured gelatine 

 through them. Alcohol itself is a very powerful but safe 

 hardening agent, and when combined with tannin may find 

 many uses. 



Bichloride of mercury solution has been recommended 

 as a hardening agent, and it probably acts by forming 

 insoluble compounds with the albuminoid matters. It is 

 so deadly a poison that the author can scarcely advise the 

 student to use it. 



Osmic acid, easily purchased in a one per cent, solution, 

 is also a splendid hardening agent, and is of great use in 

 studying the lower forms with naked protoplasm. It is, 

 however, very poisonous. By the action of this reagent the 

 currents in the protoplasm of Myxomycetes are instantly 

 suspended, and in a few moments the plasmodium is suffi- 

 ciently hardened to make sections possible. 



Bleaching and Oxidising Agents. Under this class we 

 have : 



Chlorinated soda (eau Labarraque), 



Chloride of lime (bleaching powder), 



Chlorate of potash, 



Bichromate of potash, 



Nitric acid, 



Turpentine, 



all of which have their special uses. 



