MOUNTING OBJECTS IN CELLS. 233 



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 effectually secured, by placing the cell, after it has been 

 filled in the first instance, in the vacuum of an Air-Pump (§ 160); 

 and if several objects 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 preceded it. Even when 

 a considerable length of time has elapsed without the appearance 

 of air-bubbles, the mounting should not be considered secure ; for 

 a crack may form in the varnish, through which air may find 

 its way : and thus any one who has a large collection of objects 

 mounted in fluid is pretty sure to find, on examining them from 

 time to time, that some of them have undergone deterioration from 

 this cause. It is well, therefore, to adopt the precautionary mea- 

 sure of re- varnishing the entire collection periodically (say, once a 

 year), the slight trouble which this occasions being amply compen- 

 sated by the preservation of valuable specimens that might other- 

 wise go to ruin. 



172. 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 certain to 

 increase, until at last they take the place of the entire fluid, and 

 the object remains dry. Even in the hands of the most experienced 

 manipulators this misfortune not unfrequently occurs ; being 

 sometimes due to the obstinate entanglement of air-bubbles in the 

 object when it was originally mounted, and sometimes to the 

 perviousness of some part of the cement, which has allowed a 

 portion of the contained fluid to escape, and air to find admission. 

 In either case, so soon as an air-bubble is seen in such a prepara- 

 tion, the attempt should be made to prevent its increase by laying 

 on an additional coat of varnish ; but if this should not be 

 successful, the cover should be taken off and \ the specimen 



