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55. Imbedding. It is best to use fresh paraffin for imbed- 

 ding and sometimes with a melting point higher than that of the in- 

 filtration paraffin, 52 to 54 C. paraffin, answers well in a room of 19 

 to 20 C., and will be generally used. If the cutting is to be done in 

 a room of lower temperature, a softer grade of paraffin may be used 

 for imbedding; if at a higher temperature, a harder paraffin should be 

 chosen, as when summer work is necessary. 



As a general rule, hard tissues require a harder imbedding par- 

 affin which is also better when very thin sections are desired. Large 

 sections which usually must also be relatively thicker need a softer 

 paraffin. It is better to work with a paraffin harder than the room 

 temperature itself would call for and then regulate the cutting tem- 

 perature by placing a source of heat such as an electric light nearer 

 or farther away from the microtome knife. 



Make a small paper box, fill it with the melted imbedding paraffin; 

 float the box on a dish of cold water; transfer to it the tissue from the 

 paraffin oven, arrange it carefully in the box in the way you wish it 

 for cutting, and let the mass cool. 



56. In imbedding in paraffin observe the following rules: 

 (1) Take no more paraffin (no larger box) than is needed to form a 

 mass of convenient size around the specimen. The aim is to have 

 as homogeneous a mass as possible; paraffin tends to crystallize if 

 it cools slowly, hence the smaller the mass the more rapidly may it 

 be cooled. (2) Let the imbedding paraffin when poured into the 

 box be several degrees above its melting point, and the tissue like- 

 wise should have an equal temperature. Should the imbedding 

 paraffin (or the tissue) be too cool it will not set well around the 

 specimen, and a film of air may be enclosed. On the other hand, 

 take care that the paraffin is not hot enough to "cook" the tissue, 

 thereby shrinking it and rendering it hard and tough or ruining it 

 altogether. (3) Cool the paraffin by floating the box on cold water. 

 A homogeneous, translucent, paraffin mass can only be secured if it 

 is quickly cooled. When a film has formed on the surface strong 

 enough to resist rupture, immerse the block, or drop 95% alcohol 

 upon the surface. Ice is an advantage in summer imbedding if cold 

 water is not available. When ice or cold water are not available, 

 good results have been secured by floating the box on a shallow dish 

 of (used) ether-alcohol ( 63). A homogeneous paraffin is only 

 secured if the paraffin is allowed to shrink in cooling; it is therefore 

 well to make the boxes as shallow as possible, that is, much broader 



