CHAPTER VIII. 79 



paraffin. He moves the holder up in the tube at intervals, and the 

 infiltration proceeds gradually with the minimum risk of shrinkage. 

 Lastly, he removes the objects, on the holder, to the top of a tube of 

 pure paraffin. 



The practice of giving successive baths first of soft and then of 

 hard paraffin, which has been frequently advised, appears to me 

 entirely illusory. 



It is important to keep the paraffin dry that is, protected from 

 vapour of water during the bath. 



It is still more important to keep it as nearly as possible at melting- 

 point. If it be heated for some time to a point much over its normal 

 melting-point, the melting-point will rise, and you will end by having 

 a harder paraffin than you set out with. And as regards the preser- 

 vation of tissues, of course, the less they are heated the better. 

 Overheating, as well as prolonged heating', tends, amongst other 

 things, to make tissues brittle. 



The duration of the bath must, of course, vary according to the 

 size and nature of the object. An embryo of 2 to 3 millimetres in 

 thickness ought to be thoroughly saturated after an hour's bath, or 

 often less. Many workers habitually give much longer baths, I 

 think often longer than necessary. But some objects, such as ova 

 of Crustacea, may require three or four days (HEIDECKE, Jena. 

 Zeit., xxxviii, 1904, p. 506 ; MAYER, Grundzilge, LEE and MAYER, 

 1910, p. 85 ; BRINKMANN, Mitth. Zool. Stat. Neapel, xvi, 1903, 

 p. 367, three to five days for uterus of Selachians ; MULLER, Arch, 

 mikr. Anat., Ixix, 1906, p. 3, for lungs of mammals ; Poso, Esperienze 

 microtechniche, Napoli, 1910, p. 29, five to twelve days for uterus 

 and placenta of Homo). I take as a guide, generally, the length of 

 time the object has taken to clear in the cedar oil, assuming that the 

 warm melted paraffin ought to penetrate at least as quickly as the 

 cold oil ; and then allowing somewhat longer, say as much again, in 

 order to be on the right side. 



140. Water-baths and Ovens. It is important that the paraffin 

 should not be exposed to a moist atmosphere whilst it is in the liquid 

 state. If a water-bath be used for keeping it at the required tempera- 

 ture provision should be made for protecting the paraffin from the steam 

 of the heated water. 



A very convenient apparatus for this purpose is that of Paul Mayer, 

 or " Naples water-bath," which will be found described at p. 146 of 

 Journ. Eoij. Mic. Soc., 1883, or CARPENTER'S The Microscope, p. 452. 

 An extremely simple stove, which any one can make for himself, is 

 described in Centmlbl. Bakt., xlv, 1907, p. 191 (see Journ. Roy. Mic. Soc., 

 1908, p. 109). Tor others, see the price-lists of the instrument makers, 



