ioo MICROSCOPIC METHODS. 



on a freezing microtome (of which Cathcart's is a good 

 example) and placed for a few hours in a bowl of water so 

 that the gum and syrup may dissolve out. They are then 

 stained or they may be stored in methylated spirit. 



2. Embedding and Cutting in Solid Paraffin. This 

 method gives by far the finest results, and should always be 

 adopted when practicable. The principle is the impregna- 

 tion of the tissue with paraffin in the melted state. This 

 paraffin when it solidifies gives support to all the tissue 

 elements. The method involves that, after hardening, the 

 tissue shall be thoroughly dehydrated, and then thoroughly 

 permeated by some solvent of paraffin which will expel 

 the dehydrating fluid and prepare for the entrance of the 

 paraffin. The solvents most in use are chloroform, 

 cedar oil, xylol, and turpentine ; of these chloroform and 

 cedar oil are the best, the former being preferred as it 

 permeates the tissue more rapidly. The more gradually the 

 tissues are changed from reagent to reagent in the processes 

 to be gone through, the more successful is the result. A 

 necessity of the process is an oven with hot-water jacket, 

 in which the paraffin can be kept at a constant temperature 

 just above its melting-point, a gas regulator, e.g., Reichert's, 

 being of course necessary. The tissues occurring in patho- 

 logical work have a tendency to become brittle if overheated, 

 and therefore the best results are not obtained by using 

 paraffin melting about 58 C, such as is employed in most 

 biological laboratories. We have used for some years a 

 mixture of one part of paraffin, melting at 48, and two 

 parts of paraffin melting at 54 C. This mixture has a melt- 

 ing-point between 52 and 53 C., and it serves all ordinary 

 purposes well. An excellent quality of paraffin is that 

 known as the "Cambridge paraffin," but many scientific- 

 instrument makers supply paraffins which, for ordinary 

 purposes, are quite as good, and much cheaper. The 

 successive steps in the process of paraffin embedding are as 

 follows : 



i. Pieces of tissue, however hardened, are placed in fresh absolute 

 alcohol for twenty-four hours in order to their complete dehydration. 



