230 



HARDWICKE'S SCIENCE-GOSSIP. 



the two will be separated by this film of air until it 

 be got rid of by rubbing or heating. 



Place a drop of Canada balsam upon a cold slide, 

 and then hold it over a flame, when it will soon 

 become filled with minute air-bubbles, which are 

 generally attributed to the balsam boiling, but are 

 in reality ouly this air-film separated from the sur- 

 face. But make the slide hot in the first instance, 

 and then let the balsam fall upon it, when it will 

 instantly spread itself "as clear as crystal," without 

 the slightest , trace of an air-bubble making its 

 appearance unnecessarily, unless made hot. \ 



By the same rule, when a cold cell is filled with 

 fluid, this air-film lines the whole surface of the 

 interior ; but after a short time the liquid, by gra- 

 vitation, insinuates itself beneath it, and it then 

 becomes detached and accumulates as an air- 

 bubble. 



Now there are two ways of combating this evil. 

 Either, means may be taken to get rid of the air- 

 film first, either by wetting or warming, or it may 

 be left to the ordinary course, and the cover put on 

 only temporarily until the bubble has formed, when 

 it may be got rid of, and the mounting completed 

 without a like result being repeated. § Iu fixing 

 down the cover permanently upon a cell containing 

 fluid there are certain precautions indispensable to 

 success. In the first place, the cement must be of 

 a nature not to be softened by the fluid inside. In 

 the next place, its composition must be such that it 

 will harden throughout, and not mere'y on the 

 outer surface, while it remains soft and tacky 

 within. But in addition to these it is essential 

 that the surface of the glass should be perfectly 

 clean and free from grease, through being handled, as 

 well as devoid of the separating air-film. If these 

 points be strictly attended to, there need be then no 

 fear of subsequent imperfections. 



Eor almost every other description of fluid except 

 spirit there is no better material for securing the 

 covers with than good shellac dissolved in wood- 

 naphtha : this makes a cement which is very adhe- 

 sive, and remains tough for some considerable 

 period, and has but little tendency to crack or 

 separate from the glass ; but then, like Ackland's 

 asphalt,'"' it must be remembered that it is not a 

 finishing varnish, but is to be considered only as a 

 fastening -down agent for putting on covers, and 

 making thin cells, and keeping tbem tight, and to 

 be covered over with some external protective of 

 greater lasting toughness. 



Having filled the cell/ and got rid of the air- 



* When newly-made, this cement was found to be disposed 

 to become lumpy on being thinned with benzole, but by age 

 and keeping sufficiently thin it has quite lost that tendency, 

 and now works as freely as can be desired. At present, I 

 believe, this cement is only to be obtained at Home and 

 Thornthwaite's, in the Strand ; but as it is a most undoubted 

 boon to the mounter and can hardly be spoken too highly of, it 

 to be hoped that ere long it will be obtainable at every 

 micro dep6t in the kingdom. 



bubble, the next point will be to keep the cover 

 from shifting whilst cleaning off the superfluous 

 liquid. Take a piece of fine sewing-cotton, about 

 three or four times the length of a slide, wet it, and 

 then bind it over the middle of the cover and aroutd 

 under the slide, making four or five turns, and 

 securing the ends by twisting them together. 

 Now clean off the fluid roughly from the surface, 

 either by blotting-paper or fine rag, and then put a 

 minute portion of the lac solution iu the angle at 

 the edge of the cover at four equidistant points, and 

 a minute or two afterwards remove the thread and 

 leave the slide for a few hours to dry. After this 

 the surface may be more thoroughly cleansed, and 

 the ring of cement completed on the turn-table. It 

 is not sufficient that the cement be only suffered to 

 flow over the surface of the glass merely; but a 

 small quantity should be first well rubbed down, so 

 as to get rid of the air-film, and secure a perfect 

 contact with the glass ; and hence it is that a sable- 

 brush is better than a camel-hair pencil, on account 

 of its being stiffer and more elastic ; after which 

 more cement maybe added until the required thick- 

 ness has been attained. Eor obtaining a really 

 clean surface there is nothing equal to the plan of 

 dusting it over with fiuely-powdered whiting — or 

 " creta prseparata " of the shops is best, — and polish- 

 ing this off with a clean soft cloth. The lac varnish 

 will be greatly improved both in appearance and in 

 quality by having two or three crystals of magenta 

 added to each ounce of the solution, in which 

 it readily becomes dissolved ; but as it is not always 

 easy to form a neat narrow ring without the cement 

 spreading further than desired upon the glass slip, 

 this need not be heeded at the time, but should be 

 trimmed off on the turn-table afterwards. It should 

 be kept in mind that if the slide be expected to have 

 a neat and perfect finish when completed, care must 

 be taken with the first layers to have them smooth 

 and even, otherwise it will be almost impossible 

 afterwards to obtain a good shape and surface upon 

 a rough foundation. 



With glycerine and castor-oil it is thought to be 

 a difficult matter to secure the cells from leaking ; 

 but it may be set down to be entirely a matter of 

 care and perfect manipulation as against sloven- 

 liness and insufficient attention to cleanliness of 

 surface. As all fluids expand and contract by 

 difference of temperature, it is needful to have a 

 good body of cement over the cover's edge to resist 

 the strain, and, as an external coating, the engine - 

 maker's putty (white lead, red lead, and litharge, 

 with drying-oil) is probably the strongest and best- 

 holding material that can be used. This is some- 

 times made up with japanners' gold size, and in this 

 case there is no objection to the latter material, 

 because both red lead and litharge are powerful 

 " driers," and soon convert the whole into a hard 

 and solid mass : it is only when no driers are made 



