PARAFFIN METHOD 95 



177. Pure Paraffin. It is now almost universally admitted that 

 pure paraffin is superior for ordinary work to any of the many 

 mixtures with wax and the like that used to be recommended. 

 Paraffin varies enormously in hardness according to the tem- 

 perature of its surroundings. It should therefore be taken of a 

 melting-point suitable to the temperature of the laboratory. A 

 paraffin melting at 50° C. or a little harder, is that which in our 

 experience gives the best results so long as the temperature of the 

 laboratory is hetiveen 15° and 17° C. For higher temperatures a 

 harder paraffin is required, and for lower temperatures a softer 

 one. 



INIany workers of undoubted competence prefer masses some- 

 what harder than this ; so, for instance, Heidenhain (58°), Apathy 

 (55°), Rabl (56°), Mayer (58° to 60° in summer ; in winter about 

 56°, but never less than 50°). Mayer points out that at Naples 

 the temperature during five months of the summer and 

 autumn is over 22° C. in the laboratory, sometimes over 

 30°. Temperatures such as these are seldom realised in 

 the British Isles, and, whilst we quite admit that such hard 

 paraffin may have its raison d'etre for Naples, we hold that for 

 that very reason it is in general unnecessarily hard for cooler 

 climates. 



Our recommendation of a relatively soft paraffin refers to work 

 with the Thoma sliding microtome. Microtomes with fxed 

 knives, such as the Cambridge, the Minot, or the Reinhold-Giltay, 

 will give good results with much harder paraffin, and, in fact, 

 require such. 



Stout knives of hard steel will take a harder paraffin than thin 

 ones of soft steel ; but the latter may be preferable for soft 

 masses. 



For thin sections a harder paraffin is required than for thick 

 ones. 



Hard objects require a harder paraffin than soft ones. 



Brass {Zeit. iciss. Mik., ii, 1885, p. 300) recommends paraffin 

 that has been kept for some years, as it has less tendency to 

 crystallise than new paraffin. 



Paraffin of various melting-points is easily obtainable. Inter- 

 mediate sorts may be made by mixing hard and soft paraffin. 

 Lee finds that 2 parts of paraffin melting at 50° with 1 of paraffin 

 melting at 36° C. give a mass melting at 48° C, and a mixture of 

 1 part of that melting at 53° with 1 part of that melting at 45° 

 gives a mass melting at 50° C. 



According to E. Burchardt {Jena Zeit. Naturw., xxxiv, 1900, 

 p. 719) mixtures of paraffins of different melting-points give better 

 results than an unmixed paraffin of the same melting-points as 

 the mixture. He recommends 10 parts of 40° paraffin + 1 of 

 45° + 1 of 52° + 1 of 58° + 6 of 60°. 



