HARDENING 505 



very slow, whereas in the mixture it is extremely rapid.' (From 

 Mr. Lee's * Microtomist's Vade-mecum.') 



Instead of cedar oil, white oil of thyme may be employed ; and 

 some workers use glycerin. 



The above process is recommended as giving good results with 

 small objects. For large ones the alcohol process is more generally 

 employed. 



In this the mass is first subjected to a preliminary hardening. 

 The mass, with the imbedded object, is set under a glass shade or 

 put into a loosely closed vessel, so as to allow of just enough com- 

 munication with the air to set up a slow evaporation. It is some- 

 times a good plan to set it under a bell-jar with a dish containing 

 alcohol, so that the evaporation is gone through in an atmosphere of 

 alcohol. As soon as the mass (of which only enough to just cover 

 the object should have been taken) has so far sunk down that the 

 object begins to lie dry, fresh thick solution is added, and the whole 

 is left as before. The process is repeated every few hours for, if 

 need be, two or three days. 



When the mass has attained a consistency such that the ball of 

 a finger (not the nail) no longer leaves an impress on it, it should 

 be scooped out of the dish or mould, or have the paper removed if 

 it has been imbedded in paper, and be submitted to the next stage 

 of the hardening process. 



This, the definitive hardening, consists in putting the preparation 

 into alcohol, and leaving it till it has attained the right consistency 

 (one day to several w r eeks). The strength of alcohol used by 

 different workers varies between 70 per cent, and 85 per cent., the 

 latter strength being probably the best. The vessel containing the 

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

 open. 



' To fix the hardened preparation to the microtome, proceed as 

 follows. Take a piece of soft wood, or, for very small objects, pith, 

 of a size and shape adapted to fit the holder of the microtome. 

 Cover it with a layer of collodion, which you allow to dry. Take the 

 block of collodion, or the impregnated and hardened but not 

 imbedded object ; cut a slice off the bottom, so as to get a clean 

 surface ; wet this surface first with absolute alcohol, then with ether 

 (or allow it to dry), place one drop of very thick collodion on the 

 prepared wood or pith, and press down tightly on to it the wetted 

 or dried surface of the block of collodion. Then throw the whole 

 into weak (70 per cent.) alcohol for a few hours (or even less), or 

 into chloroform, or vapour of chloroform, for a few minutes, in 

 order that the joint may harden.' (From Mr. Lee's ' Microtomist's 

 Vade-mecum.') 



Sections of material prepared in this way are cut with a knife 

 kept abundantly wetted with alcohol (of 50 to 85 or even 95 per 

 cent.). Some kind of drip arrangement may be found very useful 

 here. The knife is set in as oblique a position as possible. These 

 two points are illustrated in fig. 398. 



Another method of definitive hardening and cutting is the 

 freezing method. ' After preliminary hardening by alcohol the mass 



