198 COLLODION AND-OTHEK IMBEDDING METHODS. 



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 following rapid method) communicated to me by Prof. 

 GriLSON, has the advantage of being the most expeditious of 

 any. The object is dehydrated, soaked in ether, and brought 

 into a test-tube with collodion or thin celloidin solution. 

 The tube is dipped into a bath of melted paraffin, and the col- 

 lodion allowed to boil (which it does at a very low tempera- 

 ture) until it has become of a syrupy consistence. The mass 

 is then turned out, mounted on a block of hardened celloidin, 

 and the whole hardened in chloroform or in a mixture of chlo- 

 roform and cedar oil for about an hour. It is then cleared in 

 cedar oil (if hardened in pure chloroform ; special clearing 

 will not be necessary if it has been hardened in the mixture). 

 It may now be fixed in the microtome and cut, using cedar oil 

 to wet the knife, and cover the exposed surface of the object 

 after each cut. 



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 KOLLETT, one third alcohol (see ROLLETT'S Unters. 



