178 THE MICROSCOPE AND ITS REVELATIONS. 



168. Varnishes and Cements. There are three very distinct purposes 

 for which Cements that possess the power of holding firmly to Glass, and 

 of resisting not merely water but other preservative liquids, are required 

 by the Microscopist; these being (1) the attachment of the glass covers to- 

 the slides or cells containing the object, (2) the formation of thin ' cells/ 

 of cement only, and (3) the attachment of the ' glass-plate ' or < tube-cells * 

 to the slides. The two former of these purposes are answered by liquid 

 cements or varnishes, which may be applied without heat; the last re- 

 quires a solid cement of greater tenacity, which can only be used in the- 

 melted state. Among the many such Cements that have been recom- 

 mended by different workers, the following may be specially named as- 

 having stood the test of a large experience, both as to general utility and 

 permanent value : 



a. Japanners 1 Gold size. This, which may be obtained at every Color-shop, 

 is (according to the Author's experience) the most trustworthy of all cements for 

 closing-in mounted objects of almost any description. It takes a peculiarly firm 

 hold of glass; and when dry it becomes extremely tough, without brittleness. 

 When new, it is very liquid and ' runs ' rather too freely; so that it is often advan- 

 tageous to leave open for a time the bottle containing it, until the varnish is- 

 somewhat thickened. By keeping it still longer with occasional exposure to air, 

 It is rendered much more viscid: and though such * old ' Gold-size is not fit for 

 ordinary use, yet one or two coats of it may be advantageously laid over the films, 

 of newer varnish, for securing the thicker covers of large cells ( 171 4). When- 

 ever any other varnish or cement is used, either in making a cell or in closing it 

 in, the rings of these should be covered with one or two layers of Gold-size ex- 

 tending beyond it on either side, so as to form a continuous film extending from 

 the marginal ring of the cover to the adjacent portion of the glass slide. ! 



b. Asphalte Varnish. This is a black varnish made by dissolving half a 

 drachm Caoutchouc in mineral naphtha, and then adding 4 oz. of Asphaltum, 

 using heat if necessary for its solution. It is very important that the Asphaltum 

 should be genuine, and the other materials of the best quality. Some use Asphalte 

 as a substitute for gold-size; but the Author's experience leads him to recommend 

 that it should only be employed either for making shallow * cement-cells ' ( 170), 

 or for finishing-off preparations already secured with gold-size. For the former 

 purpose it may advantageously be slightly thickened by evaporation. 



c. Black Japan. The varnish sold at the Color-shops under this name, may 

 be used for the same purposes as the preceding. When it is used for making 

 * cement-cells,' the slides to which it has been applied should be exposed for a time 

 to the heat of an oven, not raised so high as to cause it to blister; this will increase 

 its adhesion to the glass slide, and will flatten the surface of the rings. 



d. Dammar Cement, which is made by dissolving gum dammar in benzole, and 

 adding about one-third of gold-size, has the advantage of drying very quickly; 

 and may be preferably used for a first coat when glycerine is used as the material 

 for mounting. 



e. Bell's Cement may be recommended on the same grounds; but it ' runs ' so 

 freely, that for ordinary purposes the Author much prefers gold-size or dam- 

 mar. 



/. Canada Balsam is so brittle when hardened by time, that it cannot be safe- 

 ly used as a cement, except for the special purpose of attaching hard specimens 

 to glass, in order that they may be reduced by grinding, etc. Although fresh 

 soft balsam may be hardened by heating it on the slide to which the object is to 

 be attached, yet it may be preferably hardened en masse by exposing it in a shal- 

 low vessel to the prolonged but moderate heat of an oven, until so much of its 

 volatile oil has been driven off that it becomes almost (but not quite) resinous on 

 cooling. If, when a drop is spread out on a glass and allowed to become quite 

 cold, it is found to be so hard as not to be readily idented by the thumb-nail, and 



Sands, the leather is not cemented to the bottom of the plug, but merely strained 

 over it, so as to be easily renewable. 



1 The Author has fluid preparations mounted with Gold-size nearly forty years 

 ago, which have remained perfectly free from leakage; the precaution having 

 been taken to lay on a fresh coat every two or three years. 



