58 MATERIALS AND PROCESSES OF SLIDE-MAKING 



out the whole oven, is infinitely to be preferred. It is the high cost of such 

 circulating air ovens which leads the author to believe that much more use 

 should be made of the very simple radiant heat embedding device just discussed. 



Assuming that the material has been passed through dehydrating and clear- 

 ing agents and is now awaiting embedding, there are two main methods by 

 which this may be done. Either the object may be transferred directly to a bath of 

 molten wax, or it may be passed through a graded series of wax-solvent mix- 

 tures. The author is strongly in favor of the latter course. Let us suppose that 

 we have selected benzene as the clearing agent, and that the object is in a vial 

 containing a few milliliters of this solvent. Take a block of whatever medium 

 is to be used for embedding and shave a few chips from it with a knife. Add 

 these to the solvent. The chips usually dissolve very slowly and form a thick- 

 ened layer at the bottom of the tube through which the object to be embedded 

 sinks. The average object will be satisfactory if left overnight. Then place the 

 tube itself in the embedding oven maintained at a temperature somewhat 

 above that of the melting point of the wax and add as many further shavings 

 as can be crammed into the tube. When these are completely melted and a 

 large quantity of the volatile solvent has evaporated, remove the object with a 

 pipette or forceps and place it in a dish of pure wax for an hour or two before 

 transferring it to a second dish of pure wax for the time necessary to secure 

 complete impregnation. 



There is no method of forecasting how long an object will take to become 

 completely impregnated with wax. It is very easy to find out (see Table 1) when 

 one has started to cut sections whether or not the impregnation is complete, 

 but there is no basis save experience on which to establish the timing in the 

 different baths of wax. If the object is to be transferred directly from solvent 

 to wax, at least three baths should be employed since nothing is more destruc- 

 tive to a good section than the presence of a small quantity of the clearing 

 agent in the embedding medium. To an absolute beginner seeking a rough 

 guess, it may be said that a block of liver tissue, 3 to 5 mm. in size, will be 

 satisfactorily impregnated with wax after 30 minutes in each of three baths, 

 while a 96-hour chick embryo will require at least two hours in each of three 

 baths for its successful impregnation. 



While the object is being impregnated with the wax, it is necessary to 

 decide what type of vessel will be used to cast the final block. This will depend 

 to a far greater extent on the size of the object than on the preference of the 

 worker. Very small objects may be embedded most satisfactorily in ordinary 

 watch glasses (that is to say, ordinary thin-walled watch glasses, not Syracuse 

 watch glasses of the laboratory type) or in any other thin-walled glass vessel. 



