IMBEDDING METHOD?. 99 



these are seldom realised in the British Isles, and, whilst I 

 quite admit that such hard paraffin may have its raisoii 

 d'etre for Naples, I hold that for that very reason it is 

 in general unnecessarily hard for cooler climates. 



My recommendation of a relatively soft paraffin refers to 

 work with the Thoma sliding microtome. Microtomes with 

 Jid-ed 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. 



BuAss {Zeit. luiss. Mih., 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 found in com- 

 merce. Intermediate sorts may be made by mixing hard 

 and soft paraffin. I find that two parts of paraffin melting 

 at 50° with one of paraffin melting at 36 0. give a mass 

 melting at 48° C, and a mixture of one part of that melting 

 at 53° with one part of that melting at 45° gives a mass 

 melting at 50 C. 



According to B. Buechaedt [Jena Zcit. Naturw., xxxiv, 

 1900, p. 719) mixtures of paraffins of different melting-points 

 give better results than an unmixed paraffin of the same 

 melting-point as the mixture. He recommends 10 pnrts of 

 40° paraffin + 1 of 45° + 1 of 52° -f- 1 of 58° 4- 6 of 60°. 



For methods for ascertaining melting-points see Kissling, 

 Cheni. Centralb. ii, 1901, p. 507. 



152. Overheated FarafBn. — Speb (Zeit. wiss. Mile, ii, 1885, p. 8) takes 

 pai-afEn of about 50° 0. melting-point and heats it in a porcelain capsule 

 by means of a lamp until it has become brownish-yellow, and after 

 cooliag shows an unctuous or soapy surface on being cut. This mass 

 may be obtained ready prepared from Griibler. The object of this pre- 

 paration is to make the mass stickier, in view of cutting ribbons. 



Van Walsem {Verh. Akad. Wetensch. Amsterdam, 1899, p. 132) still 

 recommends the addition of 5 per cent, of yellow wax to paraffin of 52° 

 to 57° melting-point (for large sections of central nervous system). 



Johnston (Joum. Appl. Micr., vi, 1903, p. 2662) adds 1 per cent, of 



