504 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



They should be soaked first in a thin solution, until thoroughly 

 impregnated with it, for days, even for small objects ; weeks or 

 months for large ones. When well saturated with this they should 

 be brought into a thick solution, and soaked in it for a long time, the 

 longer the better. 



When it is deemed that they are saturated, they may be imbedded, 

 In many cases this may be efficiently done by simply gumming the 

 object by means of a drop of thick collodion to a cork, or, better, a 

 piece of soft wood, adapted to be afterwards fitted to the microtome. 

 But for the purpose of accurate orientation it is preferable to imbed 

 in a mould. This is done in the manner described for paraffin. A 

 convenient mould for celloidin is made by taking a cork, and winding 

 a strip of paper several times round one end of it, so as to form a 

 projecting collar, which is fixed with a pin. Before using this, or 

 any paper tray, it should be dressed by having the inside painted 

 with collodion, which is allowed to dry before the imbedding mass is 

 poured into it. The object of this is to prevent bubbles of air 

 coming in through the bottom or sides of the mould. Watch-glasses, 

 deep water-colour moulds, and/ the like, also make convenient 

 imbedding receptacles. Care should be taken to have them perfectly 

 dry. 



If bubbles should appear after the mass has been poured in, they 

 should be got rid of before proceeding further by exposing the whole 

 to the vapour of ether for an hour or two in a closed vessel. 



The next step consists in the hardening of the mass. One of the 

 best ways of doing this is as follows : 



' Put the preparation into a desiccator or other suitable closed 

 vessel, on the bottom of which a teaspoonful of chloroform has been 

 poured. As soon as the mass has attained sufficient superficial hard- 

 ness, it is, of course, well to turn it out of its recipient and turn it- 

 over from time to time, in order that it may be equally exposed on all 

 sides to the action of the vapour. Small objects may be sufficiently 

 hardened in from one hour to overnight. When fairly hard (it is not 

 necessary to wait till the mass has attained all the hardness of which it 

 is susceptible), throw it into a mixture of one part of chloroform with 

 one or two parts of cedar oil. From time to time more cedar oil should 

 be added, so as to bring the mixture up gradually to nearly pure cedar 

 oil. As soon as the object is cleared throughout, the mass may be 

 exposed to the air, and the rest of the chloroform will evaporate 

 gradually. The block may now be mounted on the holder of the 

 microtome with a drop of thick collodion (which may be allowed to 

 dry, or may be hardened by putting back into chloroform vapour), 

 and may either be cut at once, or may be preserved indefinitely 

 without change in a stoppered bottle. Cut with a dry knife, the cut 

 surface will not dry injuriously under several hours. The cutting 

 quality of the mass is often improved by allowing it to evaporate in 

 the air for some hours. 



4 The hardening may be done at once in the chloroform and cedar- 

 wood mixture, instead of the chloroform vapour, but the latter 

 process is preferable as giving a better hardening. And clearing may 

 be done in pure cedar oil instead of the mixture, but then it will be 



