30 PREPARATION AND MOUNTING 



upon the slide, and by means of any small instrument a long 

 needle will serve the purpose very well spread over the surface 

 which will be required. The quantity thus extended will be very 

 small, but by breathing upon it may be prevented drying whilst 

 being dispersed. This, like the forementioned, should be then al- 

 lowed to dry; and whilst the objects are being placed on the pre- 

 pared surface, breathing upon it as before will restore the power of 

 adherence. 



When gum or other liquid cement has been used to fix the 

 objects to the glass, the thin covers must not be applied until the 

 slide has been thoroughly dried, and all fear of dampness arising 

 from the use of the solution done away with. "Warmth may be 

 safely applied for the purpose, as objects fastened by this method 

 are seldom, if ever, found to be loosened by it. As objects are 

 met with of every thickness, the cells will be required of different 

 depths. There is no difficulty in accommodating ourselves in 

 this the deeper cells may be readily cut out of thick leather, 

 card, or other substance preferred (as mentioned in Chapter I.). 

 Cardboard is easily procured of almost any thickness ; but some- 

 times 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 be- 

 twixt the two : just so when any thin varnish is used to surround 

 an object of little substance, 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 DiatomaceaB, scales of insects, &c. 

 that no cell is requisite excepting that which is necessarily 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 I., 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. 



