464 SOME ZOOLOGICAL METHODS. 



black. Corrosive sublimate in 5 per cent, solution gives good 

 results. Osmic acid causes darkness and opacity in the pre- 

 parations. Nitrate of silver (in ^ to 1 per cent.) solution 

 may be used, the objects being washed out with solution of 

 acid sulphate of soda. Chromic, picric, and picro-sulphuric 

 acids and bichromate of potash are only second-rate fixing 

 agents for this purpose. 



BRASS (Zeit.f. wiss. Mik., i, 1884, p. 39) employs for fixing 

 unicellular organisms the following liquid : 



Chromic acid . . . . .1 part. 



Platinum chloride . . . . 1 



Acetic acid . . . . . . 1 



Water . . . . 400 to 1000 parts. 



For protozoa that are opaque through accumulation of 

 nutritive material, he proceeds as follows : The organisms are 

 treated for three or four minutes with liquid of Kleinenberg, 

 and then for some time with boiling water. They are then 

 brought into water containing a small proportion of ammonia, 

 in which they reassume their natural forms and dimensions. 

 The ammonia is then neutralised by addition of a little acetic 

 acid, and the preparation is stained with borax-carmine or 

 ammonia-carmine. After washing, the objects are mounted 

 in dilute glycerin. This treatment is said to afford extremely 

 transparent preparations. 



Brass also obtained good results with sublimate solution. 

 CEETES (Comptes rend., 1879, 1 sem., p. 433) makes perma- 

 nent preparations as follows : Fix with osmic acid of 2 per 

 cent. (In the case of very contractile Infusoria, place a drop 

 of the solution on the cover-glass, and place it on the drop of 

 water that contains them. But generally speaking it is best 

 to employ only the vapour of the solution, exposing the 

 organisms to its action for not more than from ten to thirty 

 minutes.) 



The objects having been covered, the excess of liquid is 

 removed by means of blotting-paper, and the following stain 

 is allowed to flow in : 



Glycerin . . ... .1 part. 



Water . . . . . . 1 



Picro-carmine . . . . 1 



(Eosin may also be used. Soluble anilin-blue does not give 

 such good results.) The stain should be placed at the edge 



