METHODS. 5 



Fresh specimens, if in the shape of delicate membranes, are 

 spread over the glass slide, while, if in the shape of masses not 

 transparent, they are cut with the razor in a frozen condition. 

 The freezing mixture may be snow or broken ice with salt in one 

 compartment of a metal box, while the other compartment holds 

 the specimen, fixed, if necessary, by mucilage of gum arabic. 

 Numerous freezing microtomes have been invented; in some, 

 rhigolene or ether- spray is produced, by means of which a fresh 

 specimen may in a few minutes be frozen to such a consistence 

 that it can be cut with a razor. Specimens so obtained are 

 useful for temporary examinations or for staining, especially 

 with chloride of gold. Freshly cut specimens may be preserved 

 by the addition of a very dilute solution of bichromate of po- 

 tassa, which is allowed to flow under the covering-glass, and is 

 drained off by strips of filtering paper held against the edge of 

 the cover. 



Preservation of Tissues. The best method of preservation and 

 hardening of normal and morbid specimens is to divide a large 

 mass of the tissue by incisions into small pieces, of one or two 

 inches diameter, and to place these pieces in a wine-yellow solu- 

 tion (one-half per cent.) of chromic acid. The chromic acid is kept 

 ready in strong solution, of which a small quantity is added to the 

 water holding the specimen in a glass jar. It is important that 

 the specimens be placed in a large quantity of liquid, its bulk 

 exceeding that of the specimen at least five or six times. These 

 precautions are necessary, as the hardening action of chromic 

 acid does not penetrate very deeply. In one or two days, the 

 liquid having become cloudy, the chromic acid solution must be 

 renewed, and such renewal is to be repeated every few days until 

 the solution remains clear. Specimens of bone or teeth are 

 treated in the same manner, and the extraction of the lime-salts 

 may be hastened by a very cautious addition of dilute muriatic 

 acid every fourth or fifth day. If the chromic acid be applied in 

 this way, it hardens the tissues in a few days or weeks, with 

 no other change than a slight shrinkage, and renders them fit 

 for cutting with the razor. After the specimens have been 

 hardened, we still may keep them in very dilute solutions of 

 chromic acid, to which we add small quantities of alcohol in 

 order to prevent the growth of mildew, the most unpleasant 

 enemy of a laboratory for microscopic research. 



A dark wine-yellow solution of bichromate of potassa is also 

 suitable for the preservation of specimens, though in such a 



