162 COLLODION AND OTHER IMBEDDING METHODS. 



v, 4, 1888, p. 506). For large objects the method is inferior 

 to the alcohol method, because the rapid hardening of the 

 external layers is an obstacle to the diffusion necessary to the 

 hardening of the inner layers. 



The method consists in bringing the objects into chloroform. 

 " Under the influence of this reagent, the collodion coagulates 

 into a mass having the consistence of wax, but having also 

 an elasticity that renders it unbreakable, and having besides 

 the precious quality of being admirably transparent, and 

 possessing exactly the index of refraction of glass." 



In some cases, a few hours' immersion is sufficient to give 

 the requisite consistence. In. no case did my specimens 

 require more than three days. But the length of time required 

 varies in a very inexplicable way, so that no rule can be given. 

 The collodion frequently becomes opaque on being put into 

 the chloroform, but regains its transparency after a time. 



Small objects may be hardened by chloroform without pre- 

 liminary hardening by evaporation. All that is necessary is 

 to expose the mass to the air for a few seconds until a mem- 

 brane has formed on it, and then bring it into chloroform. If 

 the mass is in a test-tube this may be filled up with chloroform, 

 and left for two or three days. By this time the collodion 

 mass will be considerably hardened, and also somewhat shrunk, 

 so that it can be shaken out of the tube. It is then brought 

 into fresh chloroform in a larger vessel, where it remains for 

 about six days, after which time it is generally ready for 

 cutting. 



Good chloroform is a necessity, as the reaction cannot be 

 obtained with samples of chloroform that are not free from 

 water. 



The more commonly employed hardening method is the 

 alcohol method. The objects are thrown into alcohol and left 

 there until they have attained the right consistency (one day 

 to several weeks). The bottle or other vessel containing the 

 alcohol ought not to be tightly closed, but should be left at least 

 partly open. 



The strength of the alcohol is a point on which the practice 

 of different writers differs greatly. Some take very weak 

 alcohol; so ROLLETT, one-third alcohol (see ROLLETT'S Unters. 

 lib. d. Ban d. quergestr. Muskelf., 1885; or Zeit. f. wiss Mik., 

 iii, 1, 1886, p. 93). Others take equal volumes of absolute 



