THE METHODS OF MOUNTING MICROBES 85 



being hardened in Miiller's fluid, should be washed 

 in water, and then placed in dilute spirit (one of 

 water to two of spirit) for about twenty-four 

 hours. Sometimes the treatment with dilute spirit 

 is dispensed with, especially if the sections are to 

 be cut immediately. (3.) Chromic acid solution is 

 really a mixture of chromic acid and spirit. It 

 is prepared by dissolving one gramme of chromic 

 acid in 600 cc. of distilled water. Two parts of this 

 solution is then mixed with one part of methylated 

 spirit. The material to be hardened is placed in this 

 fluid for twenty-four hours ; the fluid is then 

 changed, and again every third day; the material 

 being hardened in from eight to twelve days. The 

 material should not be allowed to become brittle, 

 which it does if it remains too long in this fluid. 

 After hardening the material is washed in water, 

 and the sections cut immediately (i.e. after imbed- 

 ding), or it is placed in dilute spirit for twenty-four 

 hours, and then transferred to strong methylated 

 spirit. In this fluid the material may remain for 

 an indefinite time ; that is, if it is not required for 

 immediate use. (4.) A two per cent, solution of 

 potassium bichromate is sometimes used, especially 

 where tissues require slow hardening. ' This solu- 

 tion takes from three to seven weeks to harden, 

 according to the size of the specimen, and the fre- 

 quency with which the solution is changed.' (5.) 

 A 0'5 per cent, solution of osrnic acid is used for 

 hardening certain preparations such as the in- 

 ternal ear. This solution must be protected from 

 light ; for this purpose the bottle in which it is 



