168 Animal Microloyy 



tile. The chief objections to it, aside from its extremely poison- 

 ous nature, are its poor powers of penetration, and the fact that 

 it becomes reduced in the presence of the least amount of dust 

 containing organic particles. The substance must be handled 

 with the greatest care, as even the vapors are dangerous. It is 

 usually put up in small quantities (0.1 to 1 gram) in hermetically 

 sealed glass tubes. In making up solutions, the wrappings are 

 removed from such a tube and the tube is dropped into a reagent 

 bottle where it may then be broken by means of a glass rod. 

 Aside from its use in mixtures (see 11 and 26 ), the vapor or a 

 0.05 to a 1 per cent, aqueous solution are commonly used. A 

 stock solution of 1 per cent, is usually kept on hand. It mitst be 

 kept free from dust. As the most practical way of preventing 

 reduction, Lee recommends that the osmic acid for ordinary work 

 be kept as a solution in chromic acid (a 2 per cent. sol. of osmic 

 acid in a 1 per cent, aqueous sol. of chromic acid). This solution 

 may be employed in making up Flemming's solution or for the 

 purpose of fixation by means of osmium vapor. For vapor fixa- 

 tion, however, many workers prefer the vapor from the solid 

 crystals. 



To fix by means of the vapor, the tissue is pinned to the lower 

 end of a cork which fits tightly into the bottle containing the osmic 

 acid, or it is suspended by a thread. Objects which will adhere 

 to a slide are fixed by simply inverting the slide over the mouth 

 of the bottle. The time required for such fixation varies from 

 thirty seconds or a few minutes for isolated cells, to several hours 

 for thicker objects, such as the retina. For fixing in the solution, 

 24 hours are required ordinarily. Objects are then washed in 

 running water for the same length of time. Only small or thin 

 pieces can be fixed by means of either the solution or the vapor. 

 The stains which follow osmic acid best are hematoxylin, methyl 

 green (for study in aqueous media), alum-carmine, picro-carmiiie, 

 and sal'i-iiiiin. 



21. Picric Acid. A cold saturated aqueous solution (about 

 1.2 per cent.) of picric acid is commonly used. Small objects 

 are fixed in from a few minutes (infusoria) to hours; objects 



