FLATTENING SECTIONS AND MOUNTING 501 



the temperature of the laboratory lies between 15 and 17 C. (59 

 and 62 F.) ; though many workers prefer, even with this instrument, 

 a much harder mass. For microtomes tuith fixed knives, such as 

 the Cambridge rocker, harder paraffins may be used than with sliding 

 microtomes, paraffins of from 55 to 60 C. (131 to 140 F.) being 

 used by many workers. For cutting ribbons with these hard 

 masses it is frequently necessary to coat the face of the block 

 nearest to the knife with a softer paraffin, in order that the sections 

 may cohere. 



Masses of intermediate consistency may be made by mixing a 

 hard and a soft paraffin. Two parts of paraffin of 50 C. (122 F.) 

 with one of 36 C. (97 F.) meltjng-point, give a mass melting at 

 48 C. (119 F.). 



Mixtures of paraffin with vaseline and with various fatty and 

 other substances have been recommended. They are now generally 

 abandoned. 



5. Flattening the sections, and mounting. If the sections have 

 <3ome off either rolled or creased, they must be flattened before the 

 paraffin is removed. 



If they are large sections, float them on to warm water in a 

 suitable dish. They will flatten out perfectly in a few seconds, and 

 they may then be lifted out 011 a slide or cover-glass slid under 

 them. The water must not be ivarm enough to melt the paraffin, which 

 must only be warmed, not melted, till the sections have been 

 securely fixed to the slide or cover. A temperature of about 40 C. 

 (104 F.) is about right. 



Or take a clean slide, free from grease, spread on it with a 

 brush enough water to float the sections, lay the sections on it, and 

 warm, either on the water-bath, or on a hot plate, or over a small 

 flame, taking care not to melt the paraffin. 



If the sections are numerous and small, take a perfectly clean 

 slide, so clean that water will readily spread on it. Breathe on it, 

 and smear on it with a brush a streak of water as wide as the 

 sections and of the length of the first intended row. Lay the first 

 row of sections on this streak. Breathe on the slide again, and 

 draw on it another streak of water under the first one. Lay a 

 second row of sections on this ; and so on until the slide is full. 

 Then warm as before. 



The chief difficulty connected with this process lies in the diffi- 

 culty of getting the water to spread evenly on the slide. The slide 

 should be w r ell freed from grease, by means of xylol or some good 

 solvent of fats, and then cleaned with alcohol. The test for suffi- 

 cient freedom from grease is, that on breathing on the slide the 

 moisture of the breath should condense on it evenly, and evaporate 

 evenly. The slide should also be well rubbed with a clean cloth 

 wetted, or rather moistened, with water, before the water is defi- 

 nitely spread on it with the brush. Some sorts of slides cannot be 

 got to spread the water evenly by any means. 



The following is said by De Groot (' Zeitschrift f. wiss. Mikro- 

 skopie,' xv. 1, p. 62) to be infallible. Wrap the corner of a clean 

 -cloth round two fingers and rub it with a piece of chalk. Moisten 



