Fixation and Fixatives 71 



FIXATIVE MIXTURES 



Dichromate Fixatives. Potassium dichromate was one of the earliest 

 substances employed for fixing. It is, however, a very poor protein coagu- 

 lant. In its original use, in the solution of Midler, it was used in combina- 

 tion with sodium sulfate, which is an excellent protein precipitant. Most 

 of the modern dichromate fixative solutions have been derived from 

 Miiller, although in most the addition of mercuric chloride has made the 

 sodium sulfate unnecessary. One of the best known is: 



This is one of the most widely used of the general purpose histological 

 fixatives. It permits very brilliant afterstaining and is in almost universal 

 use in pathological laboratories. Too much should not be made up at one 

 time, for it is not very stable. If large quantities are to be prepared, it is 

 desirable to omit the acetic acid until immediately before use. Pieces 

 should be fixed for about 24 hr and then washed in running water 

 overnight. 



For the fixation of botanical specimens, it is customary to use less 

 mercuric chloride. The following is a well-known mixture: 



Lavdowsky's Fluid: 



The lower concentration of mercuric chloride permits specimens to 

 be left in this for a much longer period without danger of becoming 

 hardened. 



Many people prefer to add formaldehyde to these mixtures. The best 

 known of these fluids probably is: 



