216 SELECTION AND USE 



slide by nand, the rings beneath serving as a guide. Very good 

 cells may be thus made, but the process is of course more 

 tedious than that with the turn-table, and does not give as neat 

 results. 



A few precautions are necessary in order to insure the per- 

 manent adhesion of the cells to the glass. In addition to pro- 

 viding cement of good quality, we must see- that the slide is 

 dry and recently heated. It is difficult, with most cements, to 

 use hot slides, as the cement is apt to flow; but 'the slide should 

 have been recently heated, and after the cement has partially 

 hardened, the cells should be baked by exposure to a temper- 

 ature as high as they will stand. This is easily done by placing 

 them on a board or plate, and leaving the latter for a short 

 time in an oven. 



Where cells of greater depth are required, rings of various 

 materials are cemented to the slide. For objects mounted dry, 

 rings of leather or cardboard answer well, provided they are 

 carefully varnished so as to be impervious to air and moisture. 

 For liquids, rings of glass, tin, ebonite, etc., are used. Kings 

 of rubber and gutta-percha have been suggested, but they do 

 not answer, as they soon become rotten. Full directions for 

 making and using deep cells may be found in the works of 

 Quekett, Carpenter, Beale, Frey, etc. 



HOT-PLATE. This is simply a stout plate of brass or iron, 

 which is supported over a lamp by suitable means. The com- 

 mon plan is to insert four stout wires to serve as legs, but a 

 better mode is to support the plate on the ring of a retort 

 stand, as its distance from the lamp can thus be readily a*d- 

 justed and the temperature regulated. The hot-plate serves to 

 distribute the heat, and thus to prevent the slides from becom- 

 ing suddenly and unequally heated. Moreover, by means of 

 it several slides can be heated at once, and thus much time 

 may be saved. It should be tolerably heavy. The one we use 

 is of cast iron, six inches long and three inches wide. The 

 upper surface has been ground so as to be tolerably smooth, 

 When a hot-plate is not at hand, a good substitute may be 

 found in a smooth brick, or, better still, a plate of soapstone. 

 These may be heated in the fire and will retain their heat for a 

 long time. 



