98 CHAPTER VIII. 



129. The Paraffin Bath. -The objects having been duly 

 saturated with a solvent, the next step is to substitute 

 melted paraffin for the saturating medium. 



Some authors lay great stress on the necessity of making 

 the passage from the saturating agent to the paraffin as 

 gradual as possible, by means of successive baths of mixtures 

 of solvent and paraffin kept melted at a low temperature, 

 say 35 C. With oil of cedar or toluol, at all events, this is 

 not necessary. All that is necessary is to bring the objects 

 into melted paraffin kept just at its melting-point, and keep 

 them there till they are thoroughly saturated ; the paraffin 

 being changed once or twice for fresh only if the objects are 

 sufficiently voluminous to have brought over with them a 

 notable quantity of clearing agent. 



The practice of giving successive baths first of soft and then 

 of hard paraffin appears to me entirely illusory. 



It is important to keep the paraffin dry tliat 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 ivill 

 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. 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, if cedar oil has been used 

 for clearing. Many workers habitually give much longer 

 baths, I think often longer than necessary. 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 ; ami 

 then allowing somewhat longer, say as much again, in order 

 to be on the right side. 



In any case the preparations should be cooled (see below, 

 1:>2) a.v von as saturated. If left for very many hours in 

 a warm bath, as is sometimes done, delicate structures mav 

 be seriously injured. \Ylrn can say what is not dissolved 



