104 CELLOIDIN 



with it. wSee also Halle and Born {Zeit. iviss. Mikr., xii, 1896, 



p. 364). 



189. Concentration by Heating. Increased concentration of the 

 celloidin, above 16 per cent., can be obtained by infiltrating by the 

 hot method up to 16 per cent, and then adding a few chips of dry 

 celloidin, replacing the cork and incubating until the new celloidin 

 is dissolved. The solution then becomes very viscous but should 

 not be allowed to become solid. The tissue can be removed from 

 the bottle with a spoon or spatula, taking a liberal amount of 

 celloidin with it to form the block. If the celloidin is moulded 

 with the fingers and the tissue pushed into the centre of the mass, 

 it needs only a short exposure to the air before it sets on the 

 surface and is ready for the hardening process (see next section). 

 This rapid imbedding method is very suitable for small pieces of 

 tissue and particularly for tough tissues like tendon where a very 

 dense block is essential. 



190. Hardening of the Solid Celloidin Block. Celloidin hardens 

 when placed in reagents such as alcohol, chloroform, benzol and 

 cedar wood oil. 



(a) Alcohol hardening. The older method of placmg the 

 celloidin block, after evaporation of the solvent, into 70 per cent, 

 or 80 per cent, alcohol for one day to several weeks, has been 

 largely superseded by the chloroform hardening first introduced 

 by ViALLANES {Rech. sur VHist. et le Dev. des Insectes, 1883, p. 129). 



(6) Chloroform hardening. The block is placed in a large 

 receptacle or a desiccator containing sufficient chloroform to 

 saturate the air in the vessel. The block is allowed to remain 

 overnight or longer and is then immersed in pure chloroform 

 until it sinks. Celloidin blocks prepared by heat concentration 

 (§ 189) need not be placed in chloroform vapour but transferred 

 direct to pure chloroform. It is usually sufficient for thick 

 sections of soft tissues to imbed in 16 per cent, celloidin and, 

 without allowing evaporation of the solvent, to transfer the paper 

 boat to chloroform vapour. Additional hardening of these blocks 

 may be obtained by paraffin wax infiltration. Gilson's mixture 

 of equal parts of chloroform and cedarwood oil (1892) can also be 

 used for final hardening and, after twenty-four hours in this 

 mixture, the blocks are immersed in pure cedarwood oil for several 



days (see § 195). .^. . a ■ 



(c) Paraffin wax hardening. A further variation of the hardening 

 process is generally described as the celloidin-paraffin imbedding 

 technique, although the paraffin wax is used simply to make the 

 solid celloidin block more rigid. The celloidin block, hardened 

 in chloroform vapour and then in pure chloroform, is placed in 

 benzol until it is transparent. It is then iiffiltrated until saturated 

 with a low melting-point paraffin wax at a temperature not above 

 38° C. A suitable low melting wax mixture can be made by 



