48 



• KNOWLEDGE ♦ 



[Jan. 16, 1885. 



sink till it comes to the water, which is strong enough 

 to bear it up : So that in this liquid composed of two 

 liquor?, which do not mingle, two bodies of an unequal 

 weight will naturally assume two different places ; the one 

 will never ascend, the other will never descend. If we put 

 still other liquors which do not mingle, and throw other 

 bodies on them, it will be the sanie thiag. Fancy then 

 that the celestial matter, which fills this great vortex, has 

 several resting-places one by another, whose weight are 

 different, like that of oil, water, and other liquors; the 

 planets too are of a different weight, and consequently 

 every planet settles in that place which has a just strength 

 to sustain aud keep it equilibrate : so you see 'tis impossible 

 it should ever go beyond." 



"1 Very well apprehend," says she, "that these weights 

 keep their stations regularly. Would to God our world 

 were as well regulated, and every one among us knew their 

 proper place. I am now in no fear of being over-run by 

 Jupiter ; and since he lets us alone in our vortex with our 

 moon, I do not envy him the four which he has." 



"Did you envy him," I reply'd, "you would do him 

 wrong, for he has no more than what he has occasion for ; 

 at the distance he is from the suu, his moons receive and 

 send him but a very weak light.''""' 



" It is true, that as he turns upon himself in ten hours, 

 his nights, by consequence, are but five hours long ; so one 

 would think there is no great occa^on for four moons. But 

 there are other things to be considered : Here under the 

 Poles they have six months day, and six months night, 

 because the Poles are the two extremities of the earth, the 

 farthest remov'd from those places where the sun is over 

 'em in a perpendicular line. The moon seems to keep 

 almost the same course as the sun ; and if the inhabitants 

 of the Pule see the sun during oue-half of his course 

 of a year, and during the other half do not see him 

 at all ; they see the moon likewise during one-half 

 of her course of a month ; that is, she appears to 

 'tm fifteen days, but they do not see her during the 

 other half. Jupiter's year is as much as twelve of ours ; so 

 that there must be two opposite extremities in the planet, 

 where their night and their day are six years each. A 

 night six years long is a little disconsolate, and 'tis for 

 that reason, I suppore, they have four moons; that which 

 (in regard to Jupiter) is uppermost, finisheth its course 

 about him in seventeen days, the second in seven, the third 

 in three days and a half, and the fourth in forty-two hours : 

 and tho' they are so unfortunate as to have six years' night, 

 yet their course being exactly divided into halves, they never 

 pass above twenty-one hours, wherein they do not see at 

 least the last moon, which is a great comfort to so tedious 

 a darkness : so that be where you will, these four moons 

 are sometimes the prettiest sight imaginable ; sometimes 

 they rise all four together,! and then separate according to 

 the inequality of their course ; sometimes they are all in 

 their meridian, rang'd one above another ; sometimes you 

 see 'em at equal distances on the horizon ; sometimes when 

 two rise, the other two go down. Oh ! how I should love 

 to see this pleasant sport of eclipses ! for there is not a day 

 passes, but they eclipse the sun, or one another ; and they 

 are so accustomed to these eclipses in that planet, that they 

 are certainly more of diversion than of fear." 



(To be continued.) 



* It is worthy of notice that many writers who have dealt with 

 this point since Fontenelle's time, have failed to notice the point 

 on which he here touches. — E. P. 



t It is impossible th<at all the moons of Jupiter can rise, and be on 

 the meridian at the same time. For the movements of the three 

 inner ones are so related that when two are in conjunction the third 

 is in opposition to them. — R. P. 



ELECTRO-PLATIXG. 



By W. Slisgo. 



XVII. 



As indicated in the previous article, the recovery of the 

 silver from the stripping solution is, from an econo- 

 mical point of view, necessary. A copious accumulation of 

 crystals, observable on allowing the solution to cool down, 

 is a sure sign that the solution has done its work. 

 Warning is also given by more or less persistent slow- 

 working. Being satisfied that the solution is ready for 

 de-silvering, it shotdd be diluted with a quantity of cold 

 water. A ccnsiderable proportion of the silver may then 

 be precipitated in greyish crystals by the addition of a few 

 pieces of zinc. Chemically the zinc displaces the silver from 

 its solution. The whole of the silver is not easily separated 

 by this process, but the addition of a small quantity of 

 hydrochloric will complete the precipitation. The acid 

 should be added so long as the characteristic white precipi- 

 tate of chloride of silver is produced. The supernatant 

 liquid is then decanted, ai;d the precipitate washed several 

 times in fresh water, the zinc remaining undissolved being- 

 removed during the washing. The precipitate, being next 

 dried, is mixed with a little dry carbonate of potash, and 

 heated in a crucible until the whole of the silver is- 

 separated and collects in a globule or button of fine metal 



By another process, the whole of the silver may be con- 

 verted into the chloride by adding to the diluted stripping 

 solution, a solution of common salt. The chloride of silver 

 being precipitated, is washed well and dried, and then mixed 

 with carbonate of potash, after which it is heated in a 

 crucible, and the silver separated as before. This plan 

 answers well. 



Another plan, less commendable, is to place the article 

 to be stripped in the electrolytic bath, and, using it as the 

 anode or positive pole, to remove the silver by electrolysis. 

 This method will almost inevitably cause the plating solu- 

 tion to be spoiled ; but this trouble may be got over by 

 using a separate bath for the purpose, and employing the 

 end of a wire as the cathode upon which to deposit the 

 silver. The passage of a strong current speedily strips the 

 article of its coating, which may be collected in small 

 granules as they fall from the wire. 



Theoretically, a solution once made should last indefi-- 

 nitely, and with great care it may remain efficient for a 

 very lengthy period. There are, however, some difficulties 

 and troubles, to prevent which the greatest precautions are 

 impotent, and for this reason, if for no other, the solution 

 gradually loses its power or quality, the deposit becomes 

 bad in colour, and is liable to strip when subjected to the 

 polishing processes. 



Amongst the causes producing a deterioration of the- 

 solution may be mentioned the gradual absorption of car- 

 bonic anhydride (or carbonic acid) from the air, and the 

 consequent conversion of the free cyanide into carbonate of 

 potash. This is fur a time rendered harmless, or rather 

 the dissolving quality of the solution is maintained, by the 

 addition of free cyanide. 



The greatest care is insufficient to prevent this action. 

 There are, however, a number of troubles introducible by 

 the experimenter. Sometimes the solution is improperly 

 I)repared, sometimes too large a proportion of brightening 

 solution is added, and sometimes too much free cyanide 

 is added. These are all mistakes which are easily made ; 

 while, again, efforts to improve the solution are a fruitful 

 source of difficulty. Impurities, too, become troublesome, 

 and if not carefully watched and guarded against, they 

 will speedily render a solution unfit for use. 



Amongst the plans or methods available for the separa- 



