WITH THE MICROSCOPE. 297 



After the pieces of tissue have remained in this acid fluid for three or 

 four days, it will be found that they have regained the volume they 

 occupied when fresh. Even very soft and pulpy tissues will gradually 

 swell out and regain their original volume in the strongest gly- 

 cerine. 



On Chemical Reagents Dissolved in Glycerine. 



It being established as a principle that, for minute investigation, 

 tissues must be immersed and thoroughly saturated with viscid media 

 miscible in all proportions with water, it almost follows that reagents 

 applied to such tissues should be dissolved in media of the same phy- 

 sical properties. For a long time past I have been in the habit of 

 employing solution of potash, acetic acid, and other reagents, dis- 

 solved in glycerine instead of in water. Thus a complete chemical 

 examination may be conducted upon tissues, solutions, or deposits 

 preserved in viscid media. The reactions are most conclusive, but 

 of course take a much longer time for completion than when carried 

 out in the ordinary manner. Ten or twelve hours must be allowed 

 to elapse before the change is complete, and the process is 

 expedited if the slide be placed in a warm place (about 100). 



372. Acetic Acid Syrup. In some cases 1 have found the addi- 

 tion of very strong solutions of certain reagents necessary. For ex- 

 ample, the greatest advantage sometimes results from the application 

 to a tissue of very strong acetic acid. If the acid be added to glyce- 

 rine in quantity, the solution will no longer be viscid, so that another 

 plan must be resorted to. I thicken the strongest acetic acid with 

 sugar, a gentle heat being applied to dissolve the sugar. Thus a very 

 strong acetic acid solution of the consistence of syrup can be most 

 readily prepared. Fungi do not grow in this solution. Specimens 

 impregnated with syrup may be transferred to glycerine, but they 

 must be soaked in different portions of weak glycerine for some time 

 in order to dissolve out the sugar thoroughly before they are trans- 

 ferred to strong glycerine, otherwise crystallisation of the sugar will 

 probably occur, and the preparation will be destroyed. 



373. Solutions of Potash and Soda. Strong solutions of potash, 

 soda, and other reagents, may be added to the strongest glycerine. 



374. Solutions of Chromic Acid and Bichromate of Potash. A 

 most valuable mixture of this kind to the microscopist, is a solution of 

 chromic acid in glycerine, and another solution of bichromate of 

 potash in the same fluid. A few drops of a strong solution of chromic 

 acid may be added, so as to give to the glycerine a pale straw colour. 

 The bichromate of potash solution is prepared by adding from ten to 



