408 HISTOLOGICAL TECHNIQUE. 



small, not over 5 mm. thick, they will be hardened by remaining 

 in the absolute alcohol over night, and mounted sections may be 

 ready for examination by the next afternoon. 



8. Fixation by Boiling. Throw small pieces of the tissue, not 

 larger than 1 cm., into boiling 0.75 per cent, salt solution. Keep 

 them at the temperature of boiling for two minutes. Then throw 

 them into cold water. They may then be cut with the freezing- 

 microtome, or may be placed in 70 per cent, alcohol for hardening. 

 This method is excellent for the detection of albuminous exudates 

 within the tissues, but it causes so much shrinkage that it is not 

 useful for general purposes. 



Methods of Hardening. 



Solutions of chromates, as Miiller's fluid, will, after a time, con- 

 fer a pretty firm consistency upon tissues, and even render them 

 brittle. Tissues fixed in corrosive sublimate are also very much 

 hardened. But the usual practice is to harden specimens in alcohol 

 after fixation. To obtain the best results this hardening should be 

 done gradually, since immersion in strong alcohol is apt to produce 

 undesirable shrinkage, affecting the various tissue-elements in dif- 

 ferent degree. 



Seventy per cent, alcohol (736 cc. 95 per cent, alcohol to 264 cc. 

 water) is weak enough to begin with. After the tissues have been in 

 alcohol of that strength for twenty-four to forty-eight hours, accord- 

 ing to the size of the pieces, they are placed in 80 per cent, alcohol 

 (842 cc. 95 per cent, alcohol to 158 cc. water) for an equal length 

 of time, and then in 95 per cent, alcohol. From the 95 per cent, 

 alcohol they are placed in absolute alcohol, if it be desired to embed 

 them in either collodion or paraffin. If they are not intended for 

 immediate use, they may be kept indefinitely in 80 per cent, alcohol. 



During the hardening it is best not to allow the tissues to rest 

 on the bottom of the vessel containing the alcohol, as they are 

 liable to slight maceration in the alcohol, which there becomes 

 diluted with water from the specimen. They can be kept off 

 the bottom by means of a little crumpled filter-paper. Specimens 

 that have been fixed in a chromate solution should be kept in the 

 dark while being hardened ; those that have been fixed in corrosive 

 sublimate should be hardened in alcohols to which a little tincture of 

 iodine (sufficient to give them a sherry color) has been added. When 

 absolute alcohol is used, its strength should be maintained by con- 



