SCIENCE. 



385 



the right hand, and that hand testing upon the stand of 

 the table. The brush is then moistened a little more 

 with the cement and again applied to the edge of the 

 cover, without, however, allowing the hair of the brush 

 to touch the glass; the small drop at the point of the 

 brush only should be in contact with the glass and be 

 carried around by the rapid spinning of the turn-table. 

 The slide is then set aside so as to allow the ring to be- 

 come thick by evaporation of the benzine and turpen- 

 tine, when the applications of cement may be repeated 

 until the desired thickness is obtained. 



If colored or white zinc cement is to be used it should 

 not be applied until after the first rings of clear cement 

 have become hard, as otherwise the colored cement will 

 run in under the cover and be disseminated among the 

 mounting medium. If white zinc cement has been used, 

 it may be still further improved by running one or two 

 fine lines of asphaltum varnish around it, but not before 

 the cement has thoroughly hardene d. 



The making of a neat ring around the edge of the 

 cover is an art which can only be acquired by practice and 

 experience, and therefore a few hints in regard to the 

 causes of failure will greatly help the beginner. 



If the ring, when finished, shows irregularties both at 

 its inner and outer edge, the cement used is coo thick and 

 should be diluted with turpentine. If the ring is too 

 broad — wider than about one-thirty-second of an inch, 

 unless intentionally widened — the brush has been pressed 

 down too hard upon the glass, which causes it to spread, 

 or too much of the cement has been applied at once. 

 This is especially the cause when irregularities or bulging 

 in the edges of the ring are noticed. 



If the ring is filled with minute air bubbles, the 

 brush has been kept too long in contact with the glass in 

 making the first ring, or its point has been brought in 

 contact with the first ring in making the second applica- 

 tion, when only the minute drop should have touched the 

 glass ; or, finally, the solution may be too thick. 



MOUNTING IN GLYCERINE. 



When a preparation is to be mounted in glycerine, it 

 should, after having been stained, be placed in dilute 

 glycerine for twenty-four hours, and then for the same 

 length of time in strong glycerine (Bower's), in order to 

 make it transparent. The section is then carefully 

 spread upon the slide ; a clean cover, which has been 

 wiped dry, is placed upon it and pressed down, to re- 

 move the excess of glycerine from under the cover, and 

 a small spring-clip is applied, so as to hold the cover in 

 position during the subsequent manipulation of washing. 

 The excess ot glycerine must now be removed as care- 

 fully as possible by washing it off with a stream of water, 

 either from a syringe or from a tap, taking care not to dis- 

 place the cover in doing so. The slide is then stood on 

 edge to dry, the spring-clip still holding the cover, and 

 when all the water has evaporated it is ready for ringing. 

 In order to do this the spring-clip must be removed, the 

 slide placed upon the turn-table, the cover centered and 

 a ring of some water-proof cement applied, in the man- 

 ner described above. 



The best cements for this purpose are, first, the so- 

 called Bell's cement, which may be obtained from any 

 dealer in microscopical supplies, and the composition of 

 which is a secret with the makers ; and second, the au- 

 thor's gelatine cement, which is prepared as follows : 



Take of— 



Coxe's gelatine, 2 drachms. 



Gum ammoniac, 10 grains, 



Acetic acid, No. 8, 1 ounce. 



Dissolve the gum ammoniac in the acetic acid and filter 

 through absorbent cotton ; then warm the acid and gum 

 solution by placing the vessel containing it in a water 

 bath and add the gelatine, stirring until it is dissolved, 



when the resulting solution should be filtered or strained 

 through muslin. After a ring of this cement has been 

 made around the edge of the cover, and has become set, 

 it should be painted over with a solution of bichromate 

 of potash in water (ten grains to the ounce) and exposed 

 to either sunlight 01 ordinary daylight. The action of 

 the light is to make the chromate of gelatine which has 

 been formed insoluble and thus perfectly waterproof. 

 After this gelatine cement ring has become hard, it 

 should be covered with a ring of white zinc cement, 

 when it will be found that none of the glycerine will 

 leak out, even after the specimen has been kept lor 

 years. 



If thicker pieces than thin sections, such as pieces of 

 the mucous membrane of the intestine or bladder of ani- 

 mals, are to be mounted in glycerine or other watery 

 medium, a cell must be employed. This consists of a 

 ring made of either glass, rubber, metal or cement, and 

 which is high enough to prevent the cover-glass from 

 pressing upon the specimen when mcunted. In order 

 to do so, the ring, if it be of glass, rubber cr metal, is 

 first cemented upon the slide with marine glue or the 

 gelatine cement, and is accurately centered with the 

 turn-table. If the cell is to be made of cement a ring 

 of the required diameter (which must be a little smaller 

 than the diameter of the cover-glass) is spun upon a 

 slide in the same manner as was described for applying 

 the ring to the edge of the cover in finishing slides, and 

 is built up to the required height by repeated applica- 

 tions of the brush. It should then be set aside to dry 

 and harden. Any of the cements may be used for this 

 purpose, provided they will stick well to the glass. 



Just before mounting the top of the ring forming the 

 cell must be moistened with gelatine cement ; the speci- 

 men, which has been made transparent by soaking in 

 glycerine, is then placed in the centre of the ring, enough 

 glycerine is added to fill the cell and the cover is applied. 

 It an air bubble is left under the cover the latter must be 

 lifted up and more glycerine must be added; if, on the 

 other hand, too much liquid has been used, the surplus 

 must be washed off, as described above. A ring of 

 gelatine or Bell's cement is next spun around the edge 

 of the cover, in order to seal up the cell, and it is then 

 finished with white zinc cement. 



An excellent substitute for glycerine is Farrant's so- 

 lution, which combines all the advantages of glycerine 

 and some of those of balsam, inasmuch as it has nearly 

 the same index of refraction as glycerine, and becomes 

 hard like balsam, doing away with the necessity of a 

 waterproof cement. The formula generally given in the 

 text-books for this solutionis not correct, and the author 

 has found that the following formula is more satisfac- 

 tory : — 



Picked gum arabic, 4 drachms. 

 Camphor water, 4 fl drachms. 

 Glycerine, 2 fl drachms. 



Dissolve and strain through muslin. 



Specimens to be mounted in this medium must first 

 be made transparent by soaking in strong glycerine, and 

 may then be mounted as though the solution were a 

 resinous mounting medium. Great care should be taken 

 to exclude air bubbles, as they cannot afterward be got- 

 ten rid of. This medium is especially adapted for delicate 

 animal membranes and soft vegetable tissues. 



Some specimens, especially vegetables, such as seeds, 

 pollen grains, sections of wood, etc., may often with ad- 

 vantage, be mounted dry, t, e., without much previous 

 preparation, and without any mounting medium, but they 

 must then be examined as opaque objects, and must be 

 viewed by reflected light. 



If an object is to be thus mounted, a disk is painted 

 with asphaltum varnish in the centre of the slide, which 

 is spun around upon the turn-table while applying the 

 I brush with varnish. A disk of dead black paper may 



