THE PARAFFIN BATH. 177 



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 inelting-point, the melting-point will rise, and 

 you will end by having a harder paraffin than you set out 

 with. And as regards the preservation of tissues, of course 

 the less they are heated the better. 



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

 the size and nature of the object. An embryo of the size of a 

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

 often less, if cedar oil has been used for clearing. In any 

 case, the preparations should be cooled (see below, 279) as 

 soon as saturated. If left for many hours in a warm bath, as 

 is sometimes done, delicate structures may be seriously 

 injured. Indeed, the great point to be attended to in paraffin 

 work of the finer order is to minimise the action of heat. It 

 is therefore important both to employ a paraffin of the lowest 

 melting-point that will give good sections (see below, 280, 

 285), and to abbreviate the warm bath as much as possible. 



If chloroform or other volatile agent be taken, choice may 

 be made of two methods : either, as in Griesbrecht's method, 

 the chloroform containing the objects is heated to the melting- 

 point of the paraffin, and the paraffin gradually added, and 

 the mass kept at the melting-point of the pure paraffin until 

 all the chloroform is driven off ; or, as in Butschli' s method, 

 the objects are simply passed direct from chloroform into a 

 solution of paraffin in chloroform, in which they remain until 

 thoroughly impregnated (half to one hour), and which is then 

 evaporated at the melting-point of the paraffin. Butschli 

 recommends a paraffin solution melting at 35. (Such a solu- 

 tion is made of about equal parts of chloroform and paraffin 

 of 50 melting-point.) Or, in the case of larger objects, 

 instead of evaporating the chloroform (which is often a very 

 long process, as the chloroform must be completely driven off, 

 or the mass will remain too soft for cutting), Butschli simply 

 transfers them from the bath of paraffin solution to a bath of 

 pure paraffin. 



Giesbrecht's method (Zool. Anz., 1881, p. 484), more fully 

 stated, is as follows : 



Objects to be imbedded are saturated with absolute alcohol, 



12 



