FIXING AND EMBEDDING. 



445 



objects should be carefully washed in running water, so that all 

 the fixing fluid is removed from them. For this purpose a glass 

 crystallising dish with vertical walls may be used with success. 

 This is covered with a sufficiently close-fitting glass disk, perfor- 

 ated at its centre by a circular hole. Through this central opening 

 a thin stream of water must be conducted from the tap, and the 

 overflow water passes through the same hole. 1 The inflowing 

 water washes the objects about violently, but in spite of their 

 energetic bath they do not leave the sides of the dish. 



As an embedding medium, paraffin is of most value. As 

 before embedding the objects must 

 be quite free of water, it is neces- 

 sary first to dehydrate them. For 

 this purpose, after complete wash- 

 ing, it is recommended to place 

 the objects first for two hours in 

 50 percent, alcohol, then a similar 

 time in 70 per cent., the same in 

 80 per cent., and finally for as 

 long in 95 or 96 per cent, alcohol, 

 in which latter they can, if de- 

 sired, be preserved. 2 If we are 

 dealing with very small objects, it 

 is desirable, in order to avoid 

 losing them, to fix, wash and 

 dehydrate in Fairchild's "perforated pail" (Fig. 156), which can 

 be obtained from Gerhard's at Bonn, price from 4d. to 6d. It 



Fie. 156. Fair 



perforated pail 



for small objects, closed with a cork. 



1 In a somewhat similar appliance I have found it convenient to slip a 

 couple of india-rubber bands over disk and cover, close on each side of the 

 central hole, whereby the cover is kept in place quite securely. This method 

 is not so safe with filamentous algae. [En.] 



2 Up to 80 per cent, these percentage alcohols can be prepared with strong 

 methylated spirit; the 95 or 96 per cent, should be prepared from absolute 

 alcohol. To avoid transferring specimens from one quantity of spirit to an- 

 other, I have used the following plan, assuming, for illustration, that 25 c.c. 

 of 50 per cent, alcohol is needed to start with, though any proportion can be 

 made on a similar scale: (1) Place in 14 c.c. strong methylated alcohol + 11 

 water = 50 per cent, alcohol, approximately ; (2) after two hours draw off one- 

 half of the fluid and add 12 c.c. more spirit (approximately 70 per cent.) ; after 

 two hours draw off one-half, and again add 18 c.c. alcohol; after two hours 

 again draw off as completely as possible, and add 25 c.c. absolute alcohol. 

 For this purpose the methylated spirit must be " old style," or it will not be 

 freely miscible with water. [Eo.] 



