MOUNTING IX DEEP CELLS 483 



of the cement. And the perfect flatness of that ring should then be 

 carefully tested, since on this mainly depends the security of the 

 mounting. It is to secure this that we prefer rings of tin or bone, 

 to those of glass, for cells of moderate depth ; for their surface can 

 be easily made perfectly flat by grinding with water, first on a piece 

 of grit, and then on a Water-of- Ayr stone, these stones having been 

 previously reduced to a plane surface, or still better with a good flat 

 file. If glass rings are not found to be ' true,' they must be ground 

 down with fine emery on a plate of lead. When the cell has been 

 thus finished off, it must be carefully cleaned out by dropping into it 

 some of the mounting fluid ; and should be then examined under the 

 dissecting microscope for minute fur-bubbles, which often cling to 

 the bottom or sides. These having "been got rid of by the needle, 

 the cell should be finally filled with the preservative liquid, and the 

 object immersed in it, care being taken that no air-bubbles are 

 carried down beneath it. The cell being completely filled so that the 

 liquid is running over its side, the cover may then be lowered down 

 upon it as in the preceding case ; or, if the cell be quadrangular, 

 the cover may be sloped so as to rest one margin on its wall, and' 

 fresh liquid may be thrown in by the syringe, while the other edge- 

 is lowered. When the cover is in place, and the liquid expelled from 

 it has been taken up by the syringe, it should again be examined 

 under a lens for air-bubbles ; and if any of these troublesome 

 intruders should present themselves beneath the cover, the slide 

 should be inclined, so as to cause them to rise towards the highest 

 part of its circumference, and the cover slipped away from that part, so 

 as to admit of the introduction of a little additional fluid by the pipette 

 or syringe ; and when this has taken the place of the air-bubbles the 

 cover may be slipped back into its place. The surface of the ring and 

 the edge of the cover must then be thoroughly dried with blotting- 

 paper, care being taken that the fluid be not drawn away from 

 between the cover and the edge of the cell on which it rests. 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 dammar 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 along its 

 margin. 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 from beneath the edge 

 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 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. When all thesa 



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