ZOOLOGY AND BOTANY, MICROSCOPY, ETC. G77 



backed by dark-coloured wax. When filled with the solution, the deposition 

 of silver crystals on the inner surface of the cell will immediately com- 

 mence and proceed slowly toward, but should not be permitted to reach, 

 the centre. When the crystals have approached so near the centre as to 

 leave a clear space of about 1/8 in. in diameter, the solution should be 

 removed by means of a small piece of blotting-paper placed on top of the 

 cell and allowed to remain for a moment. The strength of the solution is 

 not important, but shi)uld not be very weak, as the feathery masses of 

 crystals that add greatly to the beauty and depth of the mount do not then 

 appear. 



If the crystals, when forming, appear white and brilliant, or darken 

 slightly, or appear to be very fine or small at the sides of the cells, while 

 those at the bottom are spray-like and quite large, the result will usually 

 be successful, although the best conditions are when the bottom of tlie cell 

 is occupied by several large feathery sprays of crystals, and the sides by 

 shorter sprays or spine-like crystals, the whole being white and brilliant. 

 Sometimes, after the solution has been removed, a deposition of copper on 

 the silver will be found, or crystals of copper salts will intermingle with 

 the silver, and mar its appearance, in which case it is necessary to reprepare 

 the mount. If the silver be permitted to reach the centre, a black pre- 

 cipitate will form and spoil the preparation as a permanent mount, but as 

 the fluid is then filled with a mass of minute sparkling crystals in constant 

 motion, the effect is both interesting and beautiful when viewed with a 

 power of about twenty-five or fifty diameters. 



The time usually occupied in preparing a silver mount is about five 

 minutes, the preparation being completed when the solution is removed 

 from the cell by the blotting-paper. 



If the crystallization of the silver be unsatisfactory, the cell may be 

 readily cleaned and another layer of wax applied. In order to apply the 

 wax to the cell, a sheet is placed on the cell, pressed slightly with the 

 finger, and a disc of wax forced into the cell by means of a cork that will 

 snugly fit it, sufficient pressure being applied to cause the wax to adhere to 

 the glass slide or to the wax already in the cell. 



There seems to be no rule by which the deposition of the crystals can 

 be regulated, as under apparently the same conditions one preparation will 

 be successful and the next one will be a failure. It would seem that a 

 small quantity of gum in the solution would cause the crystals to adhere, 

 and prevent them from breaking or shaking loose when the slide is handled 

 roughly. Gum arable has been tried without success, as it causes the 

 crystals to turn black. However, the crystals usually adhere firmly enough 

 to the cell and to each other to stand all ordinary usage. A greater mass 

 of crystals may be obtained by repeating the deposition in the same cell, 

 and allowing one mass of crystals to form on the top of the other. When 

 forming in the solution, the crystals seem to almost completely fill the cell, 

 standing out latex-ally, but when the fluid is removed they fall to the bottom 

 and appear to the eye to form a thin layer, but under the Microscope they 

 stand out in bold relief. 



Preparing Crystals of Silicon Fluoride.* — Beautiful objects for polar- 

 ized light are produced by the action of undiluted fluoric acid on an 

 ordinary glass slide, the results varying with the composition of the glass 

 acted upon. The best results are to be obtained by using slips of thin 

 polished plate and the following process : — Cut a circular hole in a piece 

 of sheet modelling wax ; warm the slide slightly, and make the wax adhere 



» Sckntif. Enquirer, ii. (1887) pp. 128-9, from 'Dental Rocoid.' 



