82 THE MICROSCOPE. 



either of these, which yield a tough varnish above the lac, which is 

 otherwise liable to become brittle. 



The glycerine may be used in its concentrated form or treacly 

 state, or it may be diluted with distilled water to any required extent, 

 according to the object of the operator, and the subject to be mounted; 

 if there be extremely fine markings on the subject, it is better to add 

 about four or five times its volume of water, as otherwise the thick 

 fluid may prevent these from being so sharply defined as may be de- 

 sired. " I have," adds Mr. Warington, " a number of slides of Desmi- 

 diece, which have been mounted from four to ten months by this means, 

 and they have kept excellently. The glycerine may also be used with 

 the addition of a small portion of culinary salt, corrosive sublimate, 

 creosote, or spirit of wine, if considered desirable." 



Castor-oil may be used as a medium for mounting crystals of salts 

 and other objects. The object required to be mounted is placed on 

 the slider in its dry state, or deposited wet and allowed to dry ; or if 

 in solution, a drop of the liquid is to be placed on the slip of glass, and 

 allowed to crystallise by spontaneous evaporation ; in the latter case 

 I prefer taking a drop of a warm saturated solution of the salt re- 

 quired, and when a good group of well-defined crystals has been ob- 

 tained, to break through the marginal ring of crystalline deposit with 

 a small point of wood, and carefully conduct off the uncrystallised 

 portion or mother-liquor to the extremity of the slide, at the same time 

 placing it in a vertical position to drain until it is dry. A small quan- 

 tity of castor-oil is to be next carefully dropped on the subject, and 

 guided over the field with the point of a needle ; in this way it readily 

 displaces the air and occupies the most minute cavities. After a short 

 time the upper glass is to be placed on the surface, taking care to 

 lower it gradually so as to exclude the air ; if the field is too full of oil, 

 the excess may be removed by a small piece of bibulous paper ; and if, on 

 the contrary, sufficient oil has not been used, an additional portion can 

 be readily introduced by the capillary action between the glasses. I 

 mention these points of practical manipulation, because on them the 

 success of the operation may often depend, as it must be evident, that 

 in the first case the excess would prevent the cell from being properly 

 sealed by the varnish, and in the latter it would be drawn into the field, 

 and the whole operation spoiled. The shell-lac varnish is then to be 

 used as the cementing medium in the same way as has been described, 

 and with the same precautions. I may also observe, that this varnish 

 cannot be replaced by either of the others, as it is actually necessary 

 (and this should always be borne in mind) that there should exist no 



