J. W. GROVES ON SECTIONS OF ANIMAL TISSUES. 235 



placed in distilled water for a day or two, and the sections should be 

 mounted in a saturated solution of acetate of potash (Schultze). 



Gold chloride hardens and stains at the same time, but as its 

 powers of penetration are very small, it is chiefly used as a colouring- 

 agent. 



Picric acid has considerable properties of hardening tissues, whilst 

 it renders them of a bright primrose yellow, but as it is extremely 

 soluble in both alcohol and water, it wants great care in use if it is 

 desired to retain much of the colour. 



Ranvier states that a concentrated solution will produce excellent 

 results in 24 hours ; neither shrinking nor coagulation of the albu- 

 men occurring, lime salts being removed at the same time. 



Finally one of the hardening agents, capable of furnishing better 

 results than any other, is the freezing method which has fallen into 

 disrepute with some, because they have considered it inapplicable to 

 tissues just removed from the animal. When thus applied they 

 have almost invariably failed, not to obtain thin sections (for these 

 can be cut as thin as may be desired), but to manipulate them when 

 cut, as they tear most readily in every subsequent process, and, when 

 finally mounted, it is found that nought is left but the tattered 

 fragments of what should have been a good preparation. 



The freezing method undoubtedly enables us to put up specimens 

 sooner than any other, but before they are frozen they must be 

 exposed for some few hours (say five to 24) to the action of one of 

 the hardening agents already mentioned, so as to partially coagulate 

 the fibrin and albumen, and thus enable the sections to be put 

 through the after processes without injury. 



The fresh material should be first placed either in : — 



a. An ^ p.c. aqueous solution of chromic acid. 



b. A 2 p c. ,, ,, ,, bichromate of potash. 



c. „ ,, „ ,, ,, of ammonia, or 



d. In alcohol ,, for 8 or 12 hours. It should 



then be washed in clean water to remove the hardening agent, when 

 it is ready for freezing. 



With this it is necessary to pound the ice into small pieces, and to 

 mix it thoroughly with an equal quantity of salt ; it is moreover 

 necessary to have a free exit for the dissolved mixture. 



With material that has been wholly hardened by chromic acid, 

 alcohol, or some other of the re-agents above mentioned, the mass 

 must be embedded in some material for the purpose of supporting 



