216 THE MICROSCOPE AND ITS REVELATIONS. 



the glass-cover from coming into close approximation with the slide; and 

 it is desirable whenever that approximation is not such as to cause the 

 cover to be drawn to the glass-slide by capillary attraction, or whenever 

 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 

 danger of not employing it, in many cases in which there is no difficulty 

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

 apt to run-in beneath the cover, which process is jpretty sure to continue 

 when it may have once commenced. When Cement-cells ( 170) are em- 

 ployed for this purpose, care must be taken that the surface of theringis 

 perfectly flat, so that when the cover-glass is laid-on, no tilting is pro- 

 duced by pressure on any part of its margin. As a general rule it is 

 desirable that the object to be mounted should be steeped for a little time 

 previously in the preservative fluid employed. A sufficient quantity of" 

 this fluid being deposited from the Syringe or Dropping-bottle to over-fill 

 the cell, the object is to be introduced into it either with the Forceps or 

 the Dipping-tube ( 126); and the slide should then be examined on the 

 Dissecting Microscope, that its entire freedom from foreign particles and 

 from air bubbles may be assured, and that its disposition may be corrected 

 if necessary. The cover should then be laid on very cautiously, so as not 

 to displace the object; which in this case is best done by keeping the drop 

 highest in the centre, and keeping the cover parallel to the slide whilst 

 it is being lowered, so as to expel the superfluous fluid all around. This 

 being taken up by the syringe, the cement ring and the margin of the 

 cover are to be dried with blotting-paper, especial care being taken to 

 avoid drawing-off too much liquid, which will cause the gold-size to run- 

 in. It is generally best to apply the first coat of Gold-size thin, with a 

 very small and flexible brush worked with the hand; this will dry suffi- 

 ciently in an hour or two, to hold the cover whilst being * ringed ' on the 

 Turn-table. And it is safer to apply a third coat a day or two afterwards: 

 old Gold- size, which lies thickly, being then applied so as to raise the 

 the ring to the level of the surface of the cover. As experience shows 

 that preparations thus mounted, which have remained in perfectly good 

 order for many years, may be afterwards spoiled by leakage, the Author- 

 strongly recommends that to prevent the loss of valuable specimens, an 

 additional coating of gold-size be laid-on from time to time. 



212. Mounting of Objects in Deep Cells. The objects which require* 

 deep cells are, as a rule, such are as to be viewed by reflected light; and 

 are usually of sufficient size and substance to allow of air being entangled 

 in their tissues. This is especially liable to occur where they have under- 

 gone the process of decalcification ( 198); which will very probably leave 

 behind it bubbles of Carbonic acid. For the extraction of such bubbles, 

 the use of an Air-pump is commonly recommended; but the Author has 

 seldom found this answer the purpose satisfactorily, and is much disposed 

 to place confidence in a method lately recommended steeping the speci- 

 men in a stoppered jar filled with freshly boiled water, which has great 

 power of drawing into itself either Air or Carbonic acid. Where the 

 structure is one which is not injured by Alcohol, prolonged steeping in 

 this will often have the same eifect. The next point of importance is to 

 select a cover of a size exactly suitable to that of the ring, of whose 

 breadth it should cover about two-thirds, leaving an adequate margin 

 uncovered for the attachment of the cement. And the perfect flatness 

 of that ring should then be carefully tested, since on this mainly depends. 



