214 CLINICAL BACTERIOLOGY AND H/EMATOLOGY 



necessary for special work, and for ordinary purposes cannot 

 compare with paraffin for beauty of results and facility of 

 application. 



In the paraffin process the tissue is infiltrated throughout 

 with hard paraffin (such as is used for the better varieties of 

 paraffin candles), so that every cell and every fibre is per- 

 meated and supported on every side. To do this requires a 

 number of processes. It would be of no use to immerse the 

 block of tissue in the paraffin just as it is, for the paraffin 

 would not wet it, much less soak into it. The water is first 

 removed, and this is done by soaking the material in absolute 

 alcohol. But alcohol does not dissolve paraffin or mix with 

 it; it is therefore necessary to remove it by means of some 

 fluid which will mix with it on the one hand and paraffin on 

 the other. Of these there are many : xylol, chloroform, ben- 

 zine, cedar oil, and many more, are in use for special purposes. 

 Chloroform answers most purposes, and is to be generally 

 recommended, though ligroin is perhaps the best of all. The 

 block of tissue is now ready to be soaked in melted paraffin; 

 it is kept in a bath of this substance until the chloroform has 

 been entirely driven off and replaced by the paraffin. The 

 whole is then allowed to cool, is shaped into suitable blocks, 

 and is then ready for cutting. 



We shall now describe the processes in fuller detail. 



FIXING MATERIAL FOR CUTTING 



These processes must be understood by all practitioners, 

 even although they do not intend to cut sections for them- 

 selves. It happens to every medical man to find it necessary 

 to send tumours, etc., to a laboratory to obtain a pathological 

 diagnosis ; and in very many cases the materials are treated 

 in a way which absolutely prevents good sections being 

 obtained. Many fixing fluids are in use, and any of them may 

 be selected, but it is absolutely necessary that the material 

 to be investigated should be cut into small pieces and put 

 into a large bulk of the fluid at once. This is especially 

 necessary in the case of material removed at a post-mortem 

 examination, where the tissues and organs have already 

 undergone alteration. 



As regards the size of the slices which are to be placed in 



