^^ PARAFFIN METHOD 



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

 inelting.potnt If it be heated for some time to a point much over 

 Its normal meltmg-point, the melting-point zvill rise, and you will 

 end by havmg a harder paraffin than you set out. with. And as 

 regards the preservation 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 mm. in thick- 

 ness ought to be thoroughly saturated aft<^r an hour's bath, or 

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

 thmk often longer than necessary. But some objects, such as 

 ova ot Crustacea, may require three or four days (Heidecke 

 Jena Zeit., xxxviii, 1904, p. 506 ; Mayer, Grundzuge, 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 ; 

 ^oso Esperienze microtechniche, Napoli, 1910, p. 29, five to twelve 

 days for uterus and placenta of Homo). Lee takes 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. *= 



160. Water-baths and Ovens. Li practically all laboratories 

 rather expensive metal imbedding baths are used. These are 

 thermostatically controlled in various ways. A. Craig-Bennett 

 {J. R. M. S., 1930) describes an admirable imbedding apparatus 

 consisting essentially of a wooden box containing a 100 watt 

 electric lamp (with a dome shade). Beneath the shade is a tray 

 ot wide tubes of wax, so placed that only the top layer of the wax 

 melts. The possibility of overheating the objects is eliminated. 

 Craig-Bennett states that the wax melts quickly and the bath 

 need only be turned on a short interval before use. The box has 

 a hft-up front, and takes up little space. It should be excellent 

 tor research workers, and could be made by laboratory attendants. 



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 



^TJ!S- fu ''^JP^r'^ temperature provision should be made for 



protecting the paraffin from the steam of the heated water. 



». ''1?'"^,''°''^'^"'^'^* ^PP^^at^s for this purpose is that of Paul Mayer, 

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

 Journ.Roy. Mic. Sac, 1883, or Carpenter's The Microscope, p. 452 

 An ex^tremely simple stove, which any one can make for himself, is 



?Q0«' i^o^^'T'^*^^^^■'•' ^'^'' ^^^' ' P- 191 ^«^« '^«^^^«- ^oy. Mic. SoZ 

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

 especially Jung, and Grubler and Hollborn ; and the descriptions in 

 the technical journals. 



