MOUNTING OBJECTS IN CELLS. 241 



up with blotting-paper ; and particular care should be taken 

 thoroughly to dry the surface of the cell and the edge of the 

 cover, since the varnish will not hold to them if they be in the 

 least damp with water. Care must also be taken, however, that 

 the fluid be not drawn away from between the cover and the 

 edge of the cell on which it rests ; since any deficiency here is 

 sure to be filled by varnish, the running in of which is particu- 

 larly objectionable. These minutiae having been attended to, 

 the closure of the cell may be at once effected, by carrying a thin 

 layer of gold-size or asphalte around and upon the edge of the 

 glass cover, taking care that it touches every point of it, and fills 

 the angular channel which is left around its margin. If the wall 

 of the cell be very thin, it will be advantageous to include it in 

 the ring of varnish ; so that this shall hold down the cover, not 

 only on the cell, but on the slide beneath. The Author has 

 found it advantageous, however, to delay closing the cell for 

 some little time after the superfluous fluid has been drawn off; 

 for as soon as evaporation beneath the edges of the cover begins 

 to diminish the quantity of fluid in the cell, air-bubbles often 

 begin to make their appearance, which were previously hidden 

 in the recesses of the object; and in the course of half an hour, 

 a considerable number are often collected. The cover should 

 then be slipped aside, fresh fluid be introduced, the air-bubbles 

 removed, and the cover put on again ; and this operation should 

 be repeated, until it fails to draw forth any more air-bubbles. 

 It will, of course, be observed, that, if the evaporation of fluid 

 should proceed far, air-bubbles will enter beneath the cover ; but 

 these will show themselves on the surface of the fluid ; whereas 

 those which arise from the object itself, are found in the deeper 

 parts of the cell. Much time may be saved, however, and the 

 freedom of the preparation from air-bubbles may be most effec- 

 tually secured, by placing the cell, after it has been filled in the 

 first instance, in the vacuum of an air-pump ; and if several ob- 

 jects are being mounted at once, they may all be subjected to the 

 exhausting process at the same time. The application of the 

 varnish should be repeated after the lapse of a few hours, and 

 may be again renewed with advantage several times in the 

 course of a week or two ; care being taken that each layer 

 covers the edges, as well as the whole surface, of that which pre- 

 ceded it. 



139. The presence of air-bubbles, in any preparation mounted 

 in fluid, is to be particularly avoided, not merely on account of 

 its interference with the view of the object, but also because, 

 when air-spaces, however small, once exist, they are almost cer- 

 tain to increase, until at last they take the place of the entire 

 fluid, and the object remains dry. Even with the most ex- 

 perienced manipulators, however, this misfortune not unfre- 

 quently occurs ; being sometimes due to the obstinate entangle- 

 ment of air-bubbles in the object, when it was originally mounted ; 



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