MOUNTING OBJECTS IN FLUID. 235 



face of the slide), the glass cover is then to be laid upon it, one 

 side being first brought into contact with the slide, and the other 

 being gradually lowered, in such a manner that the air shall be 

 all displaced before the fluid. If any air-bubbles remain in the 

 central part of the space between the cover and the slide, the 

 former must be raised again, and more fluid should be introduced ; 

 but if the bubbles be near the edge, a slight pressure on that 

 part of the cover will often suffice to expel them, or the cover 

 may be a little shifted so as to bring them to its edge. There 

 are some objects, however, whose parts are liable to be displaced 

 by the slightest shifting of this kind ; and it is more easy to avoid 

 making air-bubbles, by watching the extension of the fluid as the 

 cover is lowered, and by introducing an additional supply when 

 and where it may be needed, than it is to get rid of them after- 

 wards without injury to the object. When this end has been 

 satisfactorily accomplished, all that is needed is first to remove 

 all superfluous fluid from the surface of the slide, and from around 

 the edge of the cover, with a piece of blotting-paper, taking care 

 not to draw away any of the fluid from beneath the cover, or (if 

 any have been removed accidentally) to replace what may be 

 deficient; and then to make a circle of asphalte or gold- size 

 around the cover, taking care that it "wets" its edges, and ad- 

 vances a little way upon its upper surface. When this first coat 

 is dry, another should be applied, particular care being taken 

 that the cement shall fill the angular furrow at the margin of the 

 cover. In laying on the second coat, it will be convenient, if the 

 cover be round, to make use of the whirling-table (Fig. 63) ; and 

 if the slide be so carefully laid upon ' it, that the glass cover is 

 exactly concentric with its axis, the whirling-table may be used 

 even for the first application of the varnish ; a slight error in this 

 respect, however, may occasion the displacement of the cover. 

 By far the greater number of preparations which are to be pre- 

 served in liquid, however, should be mounted in a " Cell" of 

 some kind, which forms a well of suitable depth, wherein the 

 preservative liquid may be retained. This is absolutely necessary 

 in the case of all objects, whose thickness is such as to prevent 

 the glass cover from coming into close approximation with the 

 slide ; and it is desirable, wherever that approximation is not 

 such as to cause the cover to be drawn to the glass slide by ca- 

 pillary attraction, or wherever the cover is sensibly kept apart 

 from the slide by the thickness of any portion of the object. 

 Hence it is only in the case of objects of the most extreme tenuity, 

 that the "cell" can be advantageously dispensed with; the dan- 

 ger of not employing it, in many cases in which there is no diffi- 

 culty in mounting the object without it, being that after a time 

 the cement is apt to run in beneath the cover, which process is 

 pretty sure to continue, when it may have once commenced. 



134. Cement- Cells. When the Cells are required for mounting 

 very thin objects, they may be advantageously made of varnish 



