The Microscope. 129 



this fluid has but small penetrating power, it is well to divide 

 the organ into small pieces not more than a one-fourth to one- 

 half inches square, and if a prolonged immersion is required 

 it is well to add a slight amount of carbolic acid, or a 

 small piece of camphor. As chromic acid hardens by a species 

 of tanning, objects become brittle if left in the solution too 

 long. This difficulty may be obviated by a little glycerine, but 

 it is not very desirable as it increases the time required for 

 thorough hardening. 



Chromic acid is of almost universal application, it may be 

 used for brain and nerve tissues, lung, the organs of the viscera, 

 decalcifying, etc. It is necessary to thoroughly extract all the 

 acid from the preparation, otherwise the staining will not be 

 permanent, and as the acid is much more soluble in the weaker 

 grades of alcohol, alcohol of 50-60 per cent, should be used and 

 changed often, afterwards increasing the strength to complete 

 the hardening. 



Picric Acid. — Picric acid resembles acetic acid in its action 

 more nearly than it does chromic. It comes in straw-yellow 

 crystalline plates which are stable under all ordinary condi- 

 tions. It is best kept as a saturated solution in pure water, and 

 diluted as circumstances indicate. For fixing the karyokinetic 

 figures of dividing nuclei, the above should be diluted with 

 three or four volumes of water. This reagent is however, 

 chiefly employed in combination, except in decalcifying where 

 it is used full strength. It works better than any other fluid 

 with which I am acquainted in preparing developing teeth 

 for sectioning. 



Nitric Acid. — For killing gland tissues, nitric acid of ten 

 per cent, strength may be used, but for embryos, and cell study, 

 it should not exceed three to five per cent. This acid renders 

 the nuclear spindle, the cross striations of muscle fibers, etc., 

 very distinct. In three to four per cent, solutions, the retina of 

 the higher animals is quickly hardened for sections. (Engel- 

 mann). 



Acetic Acid. — This reagent as a fixative is usually employed 

 in connection with others, though by some it has been very 

 successfully used alone. The most useful form is a one or two 

 per cent, solution obtained by adding one or two parts of glacial 



