414 HISTOLOGICAL TECHNIQUE. 



with a mere trace of glycerin, and metallic right-angles, similarly 

 smeared on the inside, are placed around the specimen in such a way 

 as to form a box with a clear space at least an eighth of an inch broad 

 between its sides and the specimen. Melted paraffin, at a temperature 

 only slightly exceeding that necessary to keep it fluid, is then poured 

 into the box, filling it. The paraffin should now be made to cool 

 as rapidly as possible, in order to prevent its becoming crystalline. 

 For this reason it is well to prepare the box formed by the plate of 

 glass and the metallic right-angles in the bottom of a deep soup- 

 plate or some similar vessel. After the box has been filled with 

 melted paraffin cold water may be poured into the plate until its 

 surface is nearly on a level with the top of the box, and when the 

 top of the paraffin has congealed the plate may be filled with cold 

 water. After a few minutes the box may be taken apart and the 

 block of paraffin left in the water to become cold. 



These paraffin-blocks may be labelled with a needle and kept 

 indefinitely in the dry condition, at a temperature below that at 

 which the paraffin softens. When they are to be used the bottom 

 of the block should be trimmed parallel with the top, sufficient 

 paraffin being removed to obliterate the hollow which formed when 

 the paraffin solidified. This trimmed surface is then made to ad- 

 here to the paraffin-support of the microtome, or a block of hard 

 wood, by means of a heated scalpel. 



It often happens that little air-bubbles are present in the paraffin 

 close to the specimen, or that cracks exist between the specimen 

 and the surrounding paraffin, owing to the retention of a little air 

 at the time of embedding. These defects can be remedied by melt- 

 ing the paraffin with a heated needle. It is important that the 

 paraffin should everywhere be in perfect contact with the specimen. 

 When this repairing, if necessary, has been done and the paraffin 

 has become cold again, the block should be trimmed so that the 

 specimen, or at least its upper part, is contained in a little cubical 

 mass resting on the main block, with a margin of paraffin, about 

 1 mm. thick at the places where the edges of the cube are nearest 

 to the specimen. Those edges should be straight and at right angles 

 to each other, and the sides of the trimmed cube should be vertical. 

 In trimming the block only thin slices should be removed at a time, 

 in order to avoid cracking the paraffin forming the small cubical 

 mass enclosing the specimen. 



These manipulations prepare the specimen for cutting. 



