78 



HARDWICKE'S SCIENCE-GOSSIP. 



r soiled by any amount of handling or exclusion 

 from the light ; and this idea was no sooner started 

 than the necessary steps for putting it iuto practice 

 were immediately taken, and which were attended 

 by complete success ; for the slides done at that 

 time are still as fresh and as perfect as at first. 



About that period, it having been found that gum- 

 dammar dissolved in pure benzole formed an ex- 

 cellent varnish, this solution, coloured with ver- 

 milion, was coming into use, which, relieved 

 with an inner circle of pure white, produced a 

 neat and pleasing appearance; but now a "happy 

 thought " occurred, which has resulted in making 

 the application of these coloured varnishes both 

 useful and ornamental. 



Introducing a variety of coloured rings upon the 

 white ground, the thought suggested itself that 

 these colours, with differently arranged combina- 

 tions, might be made to serve as distinguishing 

 characters for different classes of objects ; thus, for 

 instance, all vegetable substances might be noted 

 by a circle of green, while blue would serve to 



Fig. 34. Section of cell completed. 



A 



Fig. 35. Section of cell with too large a cover, showing the 

 weakness of its attachment. 



denote such as were of animal origin, and red or 

 yellow those of the mineral kingdom. Then, by 

 broad and narrow lines, and the order in which they 

 were placed, almost any amount of subdivision of 

 classification might be attained, and these would be 

 sufficiently conspicuous by artificial light to help 

 very materially in finding any particular slide that 

 might be required, and thus be the means of saving 

 both time aud trouble at a critical moment. 



Where strength is required, as in fixing down 

 celiYand putting on the covers, gum-dammar, how- 

 ever, must not be trusted to, as it is too brittle to 

 answer these purposes, and which brittleness is not 

 lessened by its solution in benzole ; yet, for other 

 purposes, it is invaluable, and being soluble without 

 heat and in any proportions, a solution may be 

 made of any consistence ; although about the 

 thickness of rich cream, to be diluted when being 

 used, if necessary, will be found the most con- 

 venient. Now, as security is one of the first ob- 

 j ects to be considered both in fastening down the 

 cell and in putting on the cover, any means which 

 can aid in attaining this object should not be neg- 

 lected ; and as immediately following the fixing of 

 the covers comes the most critical part of the whole 

 proceeding (for should any imperfection exist at this 

 stage, failure, in all probability, will be the result), 

 it is most important that this should be seen to 

 before going any further ; and whereas the slightest 



crevice or fissure will be sufficient to allow of 

 " running in," or admitting air, the chance of such 

 existing should be avoided by giving one coat of 

 thin asphalt over the whole, extending from the 

 glass slip on to the upper surface of the cover ; the 

 latter, however, should, when dry, be scraped off 

 clean, so that it sliall not project above but be on a 

 level with the glass disc. 



That there may be no risk of any subsequent 

 applicatioa of ..varnish softening or injuring the 

 asphalt, it is a commendable plan to apply a layer 

 of some kind of size or varnish not soluble in 

 benzole, as, for instance, either gum or shellac dis- 

 solved in spirit of wine, &c. 



Eor dry mountings, a solution of gum-arabic 

 answers perfectly; but this should have a small 

 proportion of sugar added to it to make it less 

 brittle when dry and not liable to crack, which 

 also increases its adhesiveness. It is also advisable 

 to add a small quantity of zinc-white in powder, as 

 this both adds to its body and renders it more con" 

 spicuous in application ; thus obviating the liability 



Fig. 36. Cell and slide seen edgeways, showing the shape 

 of the foundation. 



Fig. 37. Fig. 34 in section. 



of any part being missed. Zinc-white, or oxide of 

 zinc, may be purchased at almost any chemist's for a 

 few pence per ounce, and is far preferable to either 

 white lead, flake-white, or whiting ; and as the 

 dammar varnish is equally inexpensive, it is much 

 better for several reasons to use only a moderately 

 thin coating of gum, and to fill up the foundation 

 with dammar instead of any other gum compound. 

 To make the white dammar varnish, put a small 

 quantity of the zinc- white into a small wide-mouthed 

 phial, and pour over it three or four times its bulk 

 of benzole, when it will immediately form a smooth 

 fluid without any appearance of " lumpiness " ; after 

 which add a few pieces of dammar, and cork up till 

 dissolved. This solution, when well stirred up, and 

 a drop put upon a piece of glass, ought to dry with 

 a bright surface ; should it not do so, more dammar 

 must be added until it does, when it will be ready 

 for use. It may be observed, that in mixing any 

 kind of powder with a viscid solution, such as gum 

 or varnish, it is imperative that it be first moistened 

 with a thinner solvent, as with water for the gum, 

 or benzole for the others ; otherwise, if it be added 

 dry, it will inevitably become "lumpy," and scaroely 

 be able to be made smooth by any amount of stirring 

 afterwards. 



Having securely protected the cell from the 

 action of any solvent, we may now proceed to fill 

 up the angles, so as to get the most suitable shape 



