90 IMBEDDING METHODS. 



without breaking up, thus greatly lightening the labour of mounting 

 a series. For the production of a ribbon, the paraffin must be of a 

 melting-point having the right relation to the temperature of the 

 laboratory, see 151. Secondly, the knife should be set square, 

 Thirdly, the block of paraffin should be trimmed so as to present a 

 straight edge parallel to the knife-edge ; and the opposite edge 

 should 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 microtomes, 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 /x, and 

 advisable for thinner ones, to coat the block with softer paraffin. To 

 do this, take paraffin of about 40 C. 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 obtained 

 may be cleared and mounted at once if they are quite perfect, that 

 is, neither rolled nor creased nor compressed. But should they in 

 the least degree show any of these defects, they must first be unrolled 

 or smoothed, or 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 transferred 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 NOWAK 

 in Zeit. wiss. Mik., xir, 1896, p. 447. 



