METHODS OF FIXATION. 289 



would like to obtain speedy results from a microscopical examina- 

 tion without running the risk of loss of material or of poor results. 

 When this is the ease he may use absolute alcohol as a fixing-agent, 

 thus taking advantage also of its ability to harden tissues and tit 

 them for rapid embedding in eollodion or paraffin. 



7. Absolute Alcohol. — If fresh tissues are placed in strong alcohol, 

 say 95 per cent., they are hardened ; but during the process there 

 is an opportunity for the albuminous fluids in the tissues to escape 

 to a certain extent, and for shrinkage to take place in consequence. 

 If absolute alcohol be employed, it causes such rapid coagulation 

 that this leaching of the tissues does not take place. It is neces- 

 sary, however, that the alcohol should remain of nearly its original 

 strength, otherwise the water in the tissues will dilute it sufficiently 

 to destroy this coagulating action. 



An excellent means for maintaining the strength of the alcohol 

 is to keep a layer of anhydrous sulphate of copper in the bottom of 

 the vessel. Crystals of cupric sulphate have a deep-blue color and 

 contain 36 per cent, of water of crystallization which can be removed 

 by heat ; the anhydrous salt has a dirty white color. Both the 

 anhydrous and the hydrated salt are insoluble in alcohol. If, then, 

 crystals of cupric sulphate are heated in an oven until the blue color 

 disappears, and the resulting anhydrous salt is brought in contact 

 with alcohol, any water which the latter may contain will be removed. 

 When the sulphate has become hydrated, its deep-blue color will be 

 restored. It may then be removed and the water again expelled by 

 heat. Care should be taken to allow any finely divided sulphate to 

 settle completely to the bottom of the jar before the supernatant 

 alcohol is used, lest particles come in contact with the tissues im- 

 mersed in the alcohol. Take a small jar with a tightly fitting cover 

 (a museum jar, holding six or eight ounces, will answer). Place the 

 anhydrous cupric sulphate in the bottom and then nearly fill with 

 absolute alcohol. When all the sulphate has settled, a few pieces of 

 crumpled filter-paper are placed upon it and overlaid with a smooth 

 piece placed so as to slant a little. The latter should lie near the 

 surface of the alcohol, but be wholly submerged. Small pieces of 

 the tissue to be fixed are placed upon the filter-paper where they 

 will be covered by the alcohol. The alcohol immediately coagulates 

 the albuminous substances on the surface of the pieces and then 

 gradually replaces the water in the specimen, coagulating the deeper- 

 seated albumins as it penetrates the tissues. The expelled water 



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