COLOURED VARNISHES DRY MOUNTING 445 



spirit lamp, plunged into the cement, which adheres in fair quantity, 

 and is instantly brought down upon the slide and the margin of the 

 cover. The fluid turpentine distributes itself evenly along the cover 

 and slide and hardens at once. We have no long experience of it, 

 but from some of its characteristics we are inclined to believe it will 

 prove a useful cement for this purpose. 



Marine glue, which is composed of shellac, caoutchouc, and 

 naphtha, is distinguished by its extraordinary tenacity, and by its 

 power of resisting solvents of almost every kind. Different qualities 

 of this substance are made for the several purposes to which it is 

 applied, and the one most suitable to the wants of the microscopist 

 is known in commerce as G K 4. The special value of this cement, 

 which can only be applied hot, is iixtottaching to glass slides the glass 

 or metal rings which thus form * cells ** for the reception of objects 

 to be mounted in fluid, no other cement being comparable to it 

 either for tenacity or for durability. The manner of so using it will 

 be presently described. 



Various coloured varnishes are used to give a finish to mounted 

 preparations, or to mark on the covering glasses of large preparations 

 the parts containing special kinds of noteworthy structure. A 

 very good black varnish of this kind is made by working up very 

 finely powdered lamp-black with gold-size. For red, sealing-wax 

 varnish may be used ; but it is very liable to chip and leave the glass 

 when hardened by time. The red varnish specially prepared for 

 microscopic purposes by Messrs. Thompson and Capper, of Liverpool, 

 spems likely to stand better. For \vhite, ' zinc cement ' answers 

 well, which is made of benzole, gum dammar, oxide of zinc, and 

 turpentine. But it is inexpensive, and either in Cole's or Ziegler's 

 formula may be obtained at the optician's. Blue or green pigments 

 may be worked up with this if cements of those colours be desired. 



For attaching labels to slides either of glass or wood, and for 

 fixing down small objects to be mounted 'dry' (such as foraminifera, 

 parts of insects, &c.), the Author has found nothing preferable to a 

 rather thick mucilage of gum arabic, to which enough glycerin has 

 been added to prevent it from drying hard, with a few drops of some 

 essential oil to prevent the development of mould. The following 

 formula has also been recommended : Dissolve 2 oz. of gum arabic 

 in 2 oz. of water, and then add J oz. of soaked gelatin (for the 

 solution of which the action of heat will be required), 30 drops of 

 glycerin, and a lump of camphor. The further advantage is gained 

 by the addition of a slightly increased proportion of glycerin to 

 either of the foregoing, that the gum can be very readily softened 

 by water, so that covers may be easily removed (to be cleansed if 

 necessary) and the arrangement of objects (where many are mounted 

 together) altered. 



Cells for Dry-mounting. Where the object to be mounted ' dry ' 

 (i.e. not immersed either in fluid or in any * medium ') is so thin as to 

 require that the cover should be but little raised above the slide, 

 a ' cement cell ' answers this purpose very well ; and if the ap- 

 plication of a gentle warmth be not injurious, the pressing down of 

 the cover on the softened cement will help both to fix it and to 



