IMBEDDING MUTHODS. 97 



the opposite edge shoald also be parallel to this. It is by 

 no means necessary to have recourse to special mechanical 

 contrivances^ as in the so-called ribbon microtomes ; the 

 Thoma microtome is sufficient. But the automatic microtomeSj 

 and amongst them the Cambridge Rocking Microtome and 

 the Minot, are certainly most advantageous for this purpose. 

 If the paraffin is very hard, it is necessary for sections of 

 10 fi, and advisable for thinner ones, to coat the block with 

 softer paraffin. To do this, take paraffin of about 40 0. 

 melting-point, melt it, heat it to about 80° on the water- 

 bath, dip the block into it for an instant, and rapidly turn 

 it over so that the fluid paraffin may run down away from 

 the top part as much as possible. Allow it to cool, and pare 

 away again the soft paraffin from the two sides that are not 

 to be arranged parallel to the knife. Or, as I frequently 

 prefer, simply plaster a wall of soft paraffin (superheated) 

 on to the fore and aft faces of the block with a small 

 spatula. Large blocks may have two coatings given them. 



It sometimes happens that the ribbon becomes electrified during the 

 cutting, and twists and curls about in the air in a most fantastic and 

 undesirable manner. It may be got flat by warming slightly. 



149. Section Flattening. — The sections having been ob- 

 tained may be cleared and mounted at once if they are 

 quite perfect, that is, neither rolled nor creased nor com- 

 pressed. But should they in the least degree show any 

 of these defects, they must first be unrolled or smoothed, oi 

 expanded to their proper dimensions. 



The most efficacious plan is combined treatment with fluid 

 and heat. The sections are either floated on to the surface 

 of warm water or warm alcohol contained in a suitable dish, 

 which causes them to flatten out perfectly, and are then 

 transfei-red to a slide, by floating them into position, or 

 otherwise. Or the slide has a layer of water spread over it, 

 the sections are laid on the water, and the slide is heated 

 (to somewhat below the melting-point of the paraffin) until 

 the sections flatten out, which happens in a few seconds. 



A special water-bath for flattening sections is described by N owak 

 in Zeit. wiss. Mih., xii, 1896, p. 447. 



150. Clearing and Mounting. — The sections having been 

 duly smoothed by one of these processes, and duly fixed to 



7 



