146 IMBEDDING METHODS. 



that it is important to melt as little paraffin as possible 

 at one time in order that that which is melted may cool again 

 as rapidly as possible. 



If the watch-glass method be adopted, float the watch-glass 

 with the paraffin and objects on to cold water. Do not let it 

 sink till all the paraffin has solidified. When cool, cut out 

 blocks containing the objects ; do this with a slightly warmed 

 scalpel. 



If paper trays be taken, cool them on water, holding them 

 above the surface with only the bottom immersed until all the 

 paraffin has solidified, as if you let them go to the bottom at 

 once you will probably get cavities filled with water formed 

 in your paraffin. Or, you may put them to cool on a block of 

 cold metal or stone. 



SELENKA recommends cooling the mass by passing a stream 

 of cold water through the imbedding tube described above 

 (266). 



It is well (KOEOTNEFE) to let the mass containing the 

 objects lie for a few hours exposed to the sun before cutting ; 

 the paraffin thus acquires a more favorable consistency for 

 cutting. 



The objects having been mounted on the carrier of the 

 microtome in position for cutting, pare the blocks square to 

 the knife, and sufficiently close down to the objects, and go 

 round them with a lens. If any bubbles or cavities or opaque 

 spots be present, prick with a heated needle till all is smooth 

 and homogeneous. Minutes spent in this way are well invested. 



It is well to cut within a few hours of imbedding if the 

 structures be at all delicate, as paraffin may continue to 

 crystallise slowly to a certain extent even after rapid cooling. 

 But this danger is very greatly diminished if the mass Imvi- 

 been properly cooled. 



273. Cutting and Section-stretching. Paraffin sections are 

 cut dry, that is, with a knife not moistened with alcohol or 

 other liquid. By this means better sections' are obtained, but 

 a difficulty generally arises owing to the tendency of sections 

 so cut to curl up on the blade of the knife. It is often im- 

 possible by any means to unroll a thin section that has curled. 

 To prevent sections from rolling, the following points should 

 be attended to : 



