PARAFFIN BATH. 143 



penetrates very rapidly ; preserves delicate structure better 

 than any clearing agent known to me ; mixes readily with 

 paraffin ; and does not make tissues brittle even though they 

 be kept for weeks or months in it. 



269. The Paraffin Bath. The objects having been duly 

 " penetrated " or " cleared/' the next step is to substitute 

 melted paraffin for the penetrating or clearing medium. 



Some authors lay great stress on the necessity of making 

 the passage from the clearing agent to the paraffin as gradual 

 as possible, by means of successive baths of mixtures of clearing 

 agent 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 paraf- 

 fin 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, 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 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. 



If chloroform be preferred, choice may be made of two 

 methods ; either, as in Giesbrecht'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 chloro- 

 form is driven off ; or, as in Biitschli's method, the objects 

 are simply passed direct from chloroform into a solution of 

 paraffin in chloroform, in which they remain until thoroughly 



