TARNISHES AND CEMENTS. 205 



stand well for a few weeks or months, they became porous after a 

 greater lapse of time, allowing the evaporation of the liquid and 

 admission of air. He has himself found none more durable than 

 that known as Japanners' Gold-Size, which may be obtained at 

 almost every colour shop. * When this is new and Liquid, it dries very 

 quickly, provided a thin layer only be laid on at once ; and its dis- 

 position to run in is thus kept in check. "When the first coat has 

 completely set, a second may be applied ; and it may be advan- 

 tageous to lay a third over this, or the slide may be finished off 

 with Brunswick Black or Asphalte. There are few preservative 

 liquids with which Gold-Size may not be employed ; since it is not 

 acted on by any Aqueous solution, and resists moderately diluted 

 Spirit ; Oil of Turpentine being its only true solvent. The solution 

 of Shell-Lac in Naphtha, which is sold under the name of Liquid 

 Glue, dries more quickly than gold-size, but is more brittle when 

 completely hardened, and does not adhere so firmly and enduringly 

 to glass ; and it is, moreover, more easily acted on by diluted 

 alcohol than the preceding. Its chief use is in mounting objects 

 dry (§ 156). Bell's Microscojiic Cement, which is made by dis- 

 solving Shell-Lac in strong Alcohol, is said by Dr. Beale to resist 

 Glycerine better than ordinary cements. A solution of Asphalte 

 in drying oil or turpentine, known under the name of Brv/nswick 

 Black, has come much into use. It is extremely easy and pleasant 

 to work with, and dries quickly, so that it may be conveniently 

 used as a 'finish' over Gold-Size, to improve the appearance 

 of the slide ; but it is brittle when dry, and is disposed to 

 crack, not merely when subject to any 'jar,' but also (after 

 some time) spontaneously. This evil may be corrected by 

 adding to it a little solution of Caoutchouc in Mineral Naphtha ; 

 or, still better, by dissolving half a drachm of Caoutchouc in 

 10 oz. of Mineral Naphtha, and then adding 4 oz. of Asphaltum, 

 which must be dissolved by the aid of heat if necessary. It is 

 requisite to the goodness of this A sphalte varnish, that the Asphal- 

 tum should be of the best quality. This cement answers well for 

 making Cement-cells (§ 166) ; as does also the Varnish termed 

 Black Japan, provided that the glasses to which it has been applied 

 be exposed to the heat of an oven, not raised so high as to cause the 

 varnish to ' blister. ' — Brushes which have been used either with Gold- 

 Size or Asphalte maybe cleansed by Oil of Turpentine ; those which 

 have been used with Liquid Glue may be cleansed with Naphtha. 



153. Although Canada Balsam has been sometimes used as a 

 Cement, and has the advantage of being worked with extreme 

 convenience, yet it is so apt to crack when hardened by time, that 

 a slight ' jar ' will cause the cell to spring away from the glass to 



* The Author has preparations mounted with Gold-Size more than 

 twenty years ago, which have remained perfectly free from leakage ; the 

 precaution having been taken to lay on a thin coat of varnish every two 

 or three years. 



