56 PBEPARATION AND MOUNTING 



other substance preferred (as mentioned in Chapter II.). 

 Cardboard is easily procured of almost any thickness; but 

 sometimes it is convenient to find a thinner substance even, 

 than this. When thin glass is laid upon a drop of any 

 liquid upon a slide, every one must have observed how 

 readily the liquid spreads betwixt the two : just so when 

 any thin varnish is used to surround an object of little sub- 

 stance, excessive care is needed lest the varnish should 

 extend betwixt the cover and slide, and so render it worth- 

 less. The slightest wall, however, prevents this from taking 

 place, so that a ring of common paper may be used, and 

 serve a double purpose where the objects require no deeper 

 cell than this forms. 



Many objects, however, are of such tenuity as the leaves 

 of many mosses, some of the Diatomacese, scales of insects, 

 &c. that no cell is requisite excepting that which is neces- 

 sarily formed by the medium used to attach the thin glass 

 cover to the slide; and where the slide is covered by the 

 ornamental papers mentioned in Chapter II., and pressure 

 does not injure the object, even this is omitted, the thin 

 glass being kept in position by the cover ; but slides mounted 

 in this manner are frequently injured by dampness, which 

 soon condenses upon the inner surfaces and interferes both 

 with the object and the clearness of its appearance. 



The thin glass, then, is to be united to the slide, so as to 

 form a perfect protection from dust, dampness, or other 

 injurious matter, and yet allow a thoroughly distinct view 

 of the object. This is to be done by applying to the glass 

 slide round the object some adhesive substance, and with the 

 forceps placing the thin glass cover (quite dry and clean) 

 upon it. A gentle pressure round the edge will then 

 ensure a perfect adhesion, and with ordinary care there will 

 be little or no danger of breakage. For this purpose gold- 

 size is frequently used. The asphalt and india-rubber 

 varnish also will be found both durable and serviceable. 

 Whatever cement may be used, it is well to allow it to 

 become in some measure fixed and dried ; but where no 



