INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 865 



the slide to stand till the superfluous fluid has evaporated, or, where 

 the cell is thick enough, blotting-paper may be inserted under the 

 margin of the cover to absorb it. If this plan be adopted, care must 

 be taken, after the fluid is removed, to allow the slide to stand for a 

 minute or two until the slide and the under surface of the cover 

 margin are quite dry, otherwise the cement will not adhere. Two or 

 three drops of thin balsam or gold size are then to be applied at 

 different points of the edge of the cover, when it will run in by 

 capillary attraction and fill the space outside the cell and beneath the 

 cover. Directly this cavity is filled, any superfluous cement remaining 

 on the slide must be removed, otherwise the running-in process will 

 extend too far, and the cement will enter the cell. The slide may then 

 be put aside to harden. It will often be found after a day or two, 

 especially with cells of considerable thickness, that the cement will 

 be so shrunk fi'om evaporation as no longer to quite fill the space 

 destined for it, when a little more may be applied at the edges till 

 the space is refilled, care of course being taken to scrupulously remove 

 the superfluous cement as soon as the requisite amount has run in. 

 It occasionally happens that some of the fluid is forced out of the cell 

 in process of drying, and occupies part of the space which should be 

 filled only by the cement. This " running out " is no doubt caused 

 by the shrinkage of the cement drawing the cover down more closely, 

 and if the fluid extends only a very slight distance beyond the outer 

 margin of the cell no injury is done, but if enough is expelled to make 

 a passage nearly or quite through the cement wall, there will be a 

 liability of leakage. The best safeguard against this mishap is to be 

 cautious that the cement is not run in till the whole of the fluid has 

 evaporated from outside the cell, or even till the thin film between 

 the covering glass and the upper surface of the cell has commenced to 

 dry. When this occurs, the cover will generally be drawn down as 

 closely as is necessary, and the cement may be applied with reasonable 

 security. 



The result of this operation is to secure a double cell, the inner 

 part consisting of the paper or wliatcver material may be used, and 

 the outer of a solid wall of cement firmly uniting the slide and cover, 

 and as wide as may be required. The author uses a cell about one- 

 eighth of an inch less in diameter than the cover, giving a margin of 

 -jJ^ all round. Care must be taken in finishing slides mounted in this 

 manner, as ho has fomid one commence to run in on the application 

 of varnish, after being mounted some months, the fresh varnisli having 

 softened the original cement. This difiiculty would probably be 

 obviated by using a rajjidly drying varnish, and only applying a thin 

 layer at once, or by making a narrow circle of gum round the margin 

 of the cover and aHowing it todrybefitre using the finishing material; 

 or by using jiaper covers, and tlius dis2)cnsing with varnish entirely. 

 Thei-o can bo no doubt that slides mounted in this way will have 

 almost tlio permanency of balsam mountings, so far as freedom from 

 external influences can secure it. 



A modification of the above process may be used with media which 

 will not evaporate to dryness, such as glycerine and castor oil. In 



VOL. III. 3 L 



