380 



• KNOWLEDGE 



[Nov. 



1884. 



some people associate the caddis-flies. The stone-flies and 

 willoiv-fly, together with the angler's " Yellow Sally," con- 

 stitute the family Perlidcp., a group of that section of the 

 order called Pseudo-neuroptera. The alder-fly belongs to 

 the Sialhla, a family of another section called Planipennia, 

 which contains also the beautiful lacewing-flies, or "goldeu- 

 eyes." But a more detailed notice of these must be reserved 

 for our next pappr. 



{To he cnnfinv.ed.) 



FRENCH BALLOON EXPERIMENTS. 



By Richard A. Peoctor. 



I HEARD with suspicion rather than surprise of the 

 apparent success of Captains Renard and Krebs in 

 directing a balloon against the wind. It is so manifest that 

 the balloon is unsuited for atrial navigation that I took it for 

 granted there could have been very little wind, if any. 

 Possibly, even, an upper current in a different direction 

 from the lower one favoured the aeronauts during that 

 first experiment. When there was a breeze worth men- 

 tioning, and its direction adverse, the experiment failed, 

 as it was bound to do. When we consider the nature of 

 the task which a balloon director has to achieve, we 

 readily see that the problem is a hopeless one. To sup- 

 port the weight of a man, a balloon must have more than 

 a thousand times a man's volume. To support an engine, 

 the balloon must have at least six thousand times the 

 volume of the engine. To support several men and an 

 engine equal in weight to several more, the balloon must 

 have theoretically a volume of forty or fifty thousand 

 cubic feet ; but really a much greater volume, because of 

 the weight of the balloon itself, the necessity of carrying 

 ballast, and so forth. Again, the hydrogen is not pure, 

 and it is not at atmospheric jn-essure, but at greater pres- 

 sure, as the distension of the balloon shows. A spherical 

 balloon would have to be nearly 60 feet in diameter to 

 contain 100,000 cubic feet, or thereabouts, of gas. Such a 

 balloon would present to the wind an eSective resisting 

 surface of about 2,700 square feet, and certainly no force 

 of propulsion which any engine carried by such a balluon 

 could produce for more than a few minutes, would suflSce 

 to resist the action of the wind on such a surface when 

 there was anything of a breeze. By giving to the balloon 

 a fish-like shape, as Captains Renard and Krebs did, the 

 propulsion of the balloon directly in the teeth of the wind 

 is of course made easier; but, as in the great majority of 

 cases there would be no advantage in going directly 

 against the wind (it is always as unlikely that that would 

 •be the best course, as a course directly with the wind), the 

 fish-like shape is disadvantageous rather than otherwise, for 

 it enormously increases the resisting surface when the 

 ■^wind is abeam. A great mistake, indeed, is made by those 

 who compare the movements of a balloon in the air with 

 the movements of a fish in the sea. They seem to suppose 

 that the fish shape will be a help against the action of the 

 wind ; but this is as absurd as it would be to suppose that 

 a fish placed in the midst of a current or river carrying it 

 bodily along would l>e helped by its shape to resist the 

 action of that current The fish can swim athwart the 

 current or river, but all the time it shares the motion of 

 the stream in which it is placed. Until it is shown to be 

 possible to urge a mon,strous balloon through the air at a 

 rate considerably exceeding that of a stiff breeze, nothing 

 is to be hoped for from balloon-supported propellers. Of 

 course, this is hopeless ; but a propulsive action, much less 

 effective, would suffice to keep afloat a flying machine sup- 



ported (as a condor or an albatross is supported) by a 

 widely - extended horizontal surface. — Newcastle Weekly 

 Chronicle. 



ELECTRO-PLATING. 



By W. Slingo. 



Xni.— SILVERING SOLUTIONS {continued). 



THE process of dissolving silver in nitric acid may be 

 dispensed with by purchasing the nitrate of silver 

 prepared in the crystalline form. It is a rather dear way 

 of going to work, but if a good price be paid at a good 

 shop, purity may be relied upon, and this is more than can 

 always be said for an amateur's work. Experience, how- 

 ever, would very speedily put matters right in this direc- 

 tion. Care should be taken that the nitric acid is of good 

 quality, as impurities are certain to lead to waste, if not 

 disaster. Hydrochloric acid, one of the more generally 

 present impurities of nitric acid, precipitates a portion of 

 the silver as the chloride, which portion is consequently 

 wasted. 



Having prepared the nitrate of silver solution, a solution 

 of cyanide of potassium is added gradually ; the latter 

 solution being made by dissolving about a quarter of a 

 pound of cyanide of potassium in a pint of water. The 

 effect of the cyanide of potassium upon the nitrate of 

 silver is to convert it into the cyanide of that metal, 

 which, being insoluble in water, falls to the bottom 

 of the vessel. The liquid then becomes nitrate of 

 potassium instead of silver. Care must be taken not to 

 add too much of the cyanide of potassium solution, in con- 

 sequence of its having the power of dissolving the cyanide 

 of silver. If too much should be accidentally added, it is 

 best to neutralise it by the addition of an extra quantity of 

 nitrate of silver solution. These operations are best con- 

 ducted in a glass vessel, bo as to enable the progress of the 

 precipitation of cyanide of silver to be carefully watched. 

 When the precipitation ceases, and the whole of the silver 

 has been allowed to settle at the bottom of the vessel, the 

 clear supernatant liquid is gently decanted, and the pre- 

 cipitate washed a few times. The washing process consists in 

 simply pouring a quantity of water on to the cyanide of silver, 

 and, after allowing a settlement, pouring it off again. It 

 is advisable to " test," or try, the solution as the cyanide is 

 added. Although the white cyanide of silver forms a some- 

 what dense precipitate, it nevertheless takes some time to 

 settle, the worker is consequently liable to mistake the 

 degree to which he has converted the silver solution. 



One tolerably sure indication that the necessary amount 

 of cyanide of potassium has been exceeded is that, where 

 the superfluous cyanide passes, it clarifies the solution by 

 redissolving the suspended particles of cyanide of silver. The 

 safest plan, however, is to allow the sediment to settle, and 

 then to remove a little of the clear supernatant liquid, and, 

 placing it in a test-tube or other small glass vessel, add a 

 drop or two of the cyanide of potassium solution ; the 

 .slightest cloudiness supervening is a sure indication that 

 some of the silver remains unprecipitated, and accordingly 

 more of the cyanide of potassium solution should be 

 added to the main bulk of the nitrate of .silver solution, 

 and after a short time a further test should be taken in 

 the manner above indicated. Should it happen, however, 

 that the addition of the cyanide to the teat-tube fails to 

 precipitate any silver, there may possibly be already pre- 

 sent an excess of the cyanide. This will be proved if, on the 

 addition of nitrate of silver, cloudiness ensues. Of course, 

 more nitrate must be added than would be necessary to 



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