IMBEDDING AND COOLING. 145 



way, even after hours or days in the bath. For such objects the method of 

 imbedding in a vacuum renders the greatest services. It not only ensures 

 complete penetration in a very short time a few minutes, but it has the 

 further advantage of preventing any falling-in of the tissues such as may 

 easily happen with objects possessing internal cavities if it be attempted to 

 imbed them in the ordinary way. 



The principle of this method is, that the objects are put through the 

 paraffin bath in vacuo. In practice this may be realised by means of any 

 arrangement that will allow of maintaining paraffin at the necessary tempe- 

 rature for keeping it fluid under a vacuum. 



The apparatus of HOFFMANN will be found described and figured at p. 230 

 of Zool. Anz., 1884. In this arrangement, the vacuum is produced by 

 means of a pneumatic water aspiration pump, the vessel containing the 

 paraffin being placed in a desiccator heated by a water-bath and furnished 

 with a tube that brings it into communication with the suction apparatus. 

 This arrangement is very efficacious and very simple, if the laboratory pos- 

 sesses a supply of water under sufficient pressure. 



In order to obtain the requisite vacuum without the aid of water under 

 pressure, a simple little apparatus has been designed by FEANCOTTE (Bull. 

 Soc. Belg. Micr., 1884, p. 45). 



In this the vacuum is produced by the condensation of steam. 



FOL (Lehrb., p. 121) employs the vacuum apparatus of Hoffmann, but 

 simplifies the arrangement for containing the paraffin. The paraffin is con- 

 tained in a stout test-tube furnished with a rubber stopper traversed by a 

 tube that puts it into communication with the pump. The lower end of the 

 test-tube dips into a water-bath. You pump out the air once or twice, wait 

 a few minutes to make sure that no more bubbles rise, then let the air in, 

 turn out the object with the paraffin (which by this time will have become 

 abnormally hard), and re-imbed in fresh paraffin. 



272. Imbedding and Cooling. As soon as the objects are 

 thoroughly saturated with paraffin they should be imbedded 

 by one of the methods given above ( 266). If the watch- 

 glass method be followed the paraffin bath will naturally 

 have been given in the watch-glass used for imbedding, and 

 no special imbedding manipulation will be necessary. In any 

 case the important point now to be attended to is that the 

 paraffin be cooled as rapidly as possible. The object of this is 

 to prevent crystallisation of the paraffin, which may happen 

 if it be allowed to cool slowly, and to get as homogeneous a 

 mass as possible. 



Very small objects may be taken out of the paraffin with a 

 needle or small spatula and put to cool on a block of glass, 

 then imbedded in position for cutting on a cone of paraffin 

 by means of a heated needle in the manner described above 

 ( 266). In the use of the needle it should be noted 



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