CHAPTER VIII. 77 



bath, or by successive baths of paraffin, as if the least trace of it 

 remains in the paraffin used for cutting it will make it soft. The 

 process of removal requires a very long time, in some cases days. 

 It ought therefore to be reserved for small and easily penetrable 

 objects. 



Naphtha has been recommended by WEBSTER (Journ. Anat. and 

 Physiol, xxv, 1891, p. 278). 



FIELD and MARTIN (Zeit. wiss. Mik., xi, 1894, p. 10) recommend a 

 light petroleum known as " petroleum -aether." It is highly volatile, 

 and thus a cause of shrinkage. 



Sulphide of carbon has been recommended by HEIDENHAIN (Zeit. 

 wiss. Mik., xviii, 1901, p. 166) as being a very powerful solvent of 

 paraffin. Most workers have found it to be much too disagreeable and 

 dangerous a reagent for ordinary work, and not necessary even for 

 delicate work. See under "Teeth" and "Chitin." 



Carbon tetrachloride has been recommended by PLECNIK (op. cit., xix, 

 1903, p. 328) and PRANTER (ibid., p. 329) on the ground of not dissolving 

 out osmium -blackened fats. 



MAYER finds it no better than benzol. 



As a general thesis, the best of all these are cedar oil, benzol, and 

 chloroform. 



Cedar-wood oil is, according to my continued experience, for the 

 reasons stated by me in Zool. Anz., 1885, p. 563, for general work 

 the very best clearing agent for paraffin imbedding. . It penetrates 

 rapidly, preserves delicate structure better than any clearing agent 

 known to me, does not make tissues brittle, even though they may 

 be kept for weeks or months in it, and has the great advantage that 

 if it be not entirely removed from the tissues in the paraffin bath it 

 will not seriously impair the cutting consistency of the mass ; 

 indeed, I fancy it sometimes improves it by rendering it less brittle. 



139. 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, at all events, this is not necessary. I simply put 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. If the objects have been for a very long time months or 



