46 METHODS OF EXAMINATION. 



fluids at the margin of the cover-glass, micro-organisms may be kept 

 unaltered ■ and under observation for a considerable period. Small 

 fragments of tissue may be pressed out or teased out in the same 

 fluids, but never in the serum which exudes from the neighbourhood 

 of the incision, for the fluid itself, may contain numerous micro- 

 organisms, which, becoming attached to, or lying on, or under the 

 fragments, may give rise to appearances which would otherwise not 

 be seen, and are therefore often very misleading. 



Cultivations on solid media, peptonised meat jelly, agar-agar, Koch's 

 blood serum, &c., should always in the first instance be treated in 

 the same way, by squeezing out between the cover-glass and shde, 

 and then by adding distilled water or the saline solution. 



Where such specimens are to be kept for comparison or further 

 observation, a ten per cent, solution of acetate of potash forms a 

 capital mounting medium ; or, in place of this, a mixture of gly- 

 cerine and water, or Farrant's mounting fluid, may be used. 



In this fresh condition the various chemical reagents (§ 21, p. 25) 

 are used to great advantage, and the reactions are exceedingly well 

 defined 



His's Method of Staining and Washing. 



32. In many cases it is necessary to stain micro-organisms, as they 

 exist in fluids which contain no albumen, and in which there is 

 nothing that by the application of heat, alcohol, or bichloride of 

 mercury can be rendered insoluble, and which, therefore, have to be 

 stained and washed in such a manner that they are not removed from 

 the shde or cover-glass during the process. His's method of applying 

 the staining fluid at one edge of the cover-glass under which is the 

 material to be stained, and setting up a shght current by sucking up 

 fluid at the opposite margin with a piece of clean blotting-paper, is 

 the best for this purpose. The specimen is washed first with water, 

 and then with alcohol in the same manner, after which it may be 

 mounted in glycerine, run in at the margin; or the glasses are 

 separated, the films are allowed to dry, and then mounted in Canada 

 balsam. The best stains to use with this method are methyl blue and 

 Bismarck brown or vesuvin. 



