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HARD WICKE 'S SCIENCE - G OS SI P. 



the metal plate, I place it from five to six inches 

 below ; and instead of heating the slide from ten to 

 fifteen minutes, I heat it for about an hour. The 

 result is precisely the same by either process ; the 

 only advantage of the longer being, that it does not 

 require so close watching, as the heat is not so fierce, 

 the damar does not boil, and therefore no air-bubbles 

 form ; besides which, one can work at something 

 else whilst the slide is being heated. I have tried 

 Mr. Williams's mode of dropping the damar on the 

 object previous to placing the glass cover on it, with 

 good result ; a few bubbles accumulating occasionally, 

 which, however, the heat has expelled. The way to 

 ascertain when it has been heated long enough, is to 

 dip the point of a pin into any superfluous damar 

 that may have collected round the edge of the cover : 

 if it forms a HARD globule— not in the slightest 

 degree sticky — on the point, when cold, you can 

 remove the lamp and finish off. The second question 

 is, "Does not Japan varnish ever run into the damar 

 when there is no other varnish between ? " Ever 

 since I have used Japan, which has been for a con- 

 siderable time, only once I have found it run in ; and 

 that was because I had not heated the damar enough 

 for it to get hard. I find it very TOUGH and trust- 

 worthy, and prefer it to asphalt, being easier to work 

 with. Of course, if made very liquid with turps, it 

 would have a tendency to run in, as turps dissolves 

 damar ; but not otherwise ; at least, I don't think so. 

 If "J. A. Le M. H." will send me his address, I shall 

 be most happy to send him a slide so finished. — E. 

 B. L. Brayley, Bristol. 



Spontaneous Generation. — Professor Tyndall, 

 in a paper read at a recent meeting of the Royal 

 Society, showed that repeated heatings for a short 

 lime destroy the living germs from which infec- 

 tious growths proceed, much more effectually than 

 any continuous heating for a long time, even though 

 that time should be much longer than all the shorter 

 periods added together. His view is that living germs 

 exist in all stages of growth, in some of which they 

 are hard and insensible to heat, in others plastic and 

 instantaneously destroyed by heat, and he thinks that 

 by repeating the heating process very often, the heat 

 catches the different germs in all their stages, while if 

 one heating takes place, even though it last for many 

 hours, some of the germs may live through it, owing 

 to their not having reached the age of development in 

 which they are destroyed by heat. Another way of 

 destroying the vitality of these germs is to deprive 

 them completely of air by the use of the Sprengel 

 pump, after five or six hours' exposure to which they 

 will be rendered permanently barren. Dr. Bastian 

 may find that this discovery of Professor Tyndall's 

 accounts for some of the seeming successes which he 

 has achieved in producing life out of tubes previously 

 raised to a very high temperature, and sustained at 

 that temperature for many houi-s. 



Cement. — Some of your correspondents have 

 been asking for a good cement. I can strongly recom- 

 mend the white cement sold by Mr. White, of 

 Litcham, Norwich. If put on as a thin layer first, 

 and this allowed to dry, there is no fear of lamning 

 in, even with quite soft balsam or damar ; in fact, I 

 have been in the habit of putting the white ring round 

 at once. The white cement dries very rapidly, and 

 especially if the [turn-table is twisted quickly for a 

 few minutes. If there seems to be any danger of 

 running in, I put some shellac and castor oil cement 

 round first. — Fred. Ahn, jM.D. 



Dry Mounting. — I would like to draw the atten- 

 tion of those readers who are in want of a good 

 method for mounting objects diy, with asphalt cells, 

 to a method I found out some time ago, and which I 

 have since used with complete success. The methods 

 to be found in text-books, at present, are briefly as 

 follows : — I. Make a ring, dry it, warm over a lamp 

 until slightly soft, and having placed the object in 

 position, adjust the cover. 2. The former method is 

 sometimes varied by making two rings ; the second 

 after the first has dried. 3. Narrow rings of paper 

 are introduced between the ring and cover, and a few 

 other modifications of these processes. The whole of 

 the foregoing methods are liable to the objection that 

 the medium employed for making the cell, asphalt 

 and rubber, or whatever else it may be, runs in by ca- 

 pillary attraction, and either spoils the object or ren- 

 ders the slide unsightly. Of the above-mentioned 

 methods, I decidedly prefer the first one, but I could 

 not depend on it six times out of ten, and have many 

 a time spoiled both slide and temper. Most micro- 

 scopists seem to have battled against the material 

 "running in," a propensity which I have, to some 

 degree, taken advantage of. Take a slide, and with 

 the turn-table make two narrow concentric rings of 

 asphalt-and-rubber varnish, the inner one-half, and 

 the outer seven-eighths of an inch in diameter, and 

 fill up the space between the two w ith varnish, so as 

 to make a thin cell of varnish, with an interior a 

 half-inch in diameter. Dry the slide in an oven, 

 slightly warmed, and when quite dry, make a nar- 

 row ring of varnish on the extreme outer edge of 

 the cell, and having placed the object in position, 

 or, according to circumstances, before the first ring 

 was made, adjust a cover, pressing it down slightly. 

 The varnish is generally only flattened out, and only 

 occasionally spreads to the edge of the cell encircling 

 the object. The reason for its not "running in," 

 is simply because very little capillary attraction 

 is offered to the film of varnish by the dry cell 

 [ and cover, compared with the capillary attraction 

 offered to varnish by two plain surfaces of glass, as 

 i is the case when the old methods are employed. 

 1 When the cover has become fixed, the slide should 

 j be finished by making a ring on the cover, cor- 

 ! responding with the cell beneath. My experience 



