278 



KNOWLEDGE ♦ 



[April 3, 1885. 



mental discoveries) only one metal is known to be deposited 

 by it. For example, a solution of sulphate of copper would 

 not dissolve and deposit zinc at the same time that it is so 

 dealing with cojiper. On the other hand, a single metal 

 may be deposited from a complex liquid by employing a 

 curreut of suitable proportions. 



If we take a number of electrolytic baths, each con- 

 taining a ditfereut solution, and then divide the current 

 from a battery of large cells between them, we should find, 

 presuming the current to be powerful enough, that a dif- 

 ferent metal would be capable of being deposited in each 

 cell — say, for example, copper, silver, iron, nickel — but the 

 current that will decompose one solution so as to deposit 

 its metal, will not necessarily act in a similar manner upon 

 others. The curreut must be strong enough to overcome 

 the most obdurate ; and even then a difficulty will be expe- 

 rienced, as a larger proportion of the energy than is 

 required will be expended on the more easily aflected 

 solution. Presuming, then, that such work is possible, there 

 follows, as a natural consequence, the assumption that similar 

 results would follow were solutions of the various metals 

 collected in a single bath and the current suitably divided 

 between auodes of those various metals, uniting again at 

 the cathode. In this way the cathode would receive an 

 alloyed deposit of the ditterent metals. It is found 

 that difl'erent metals, or the same metal in dill'erent 

 solutions, necessitate the adoption of a varying number 

 of battery cells. Thus copper is precipitated from its 

 sulphate by a single cell, while other metals may require 

 two, and some three, or even more. Suppose, then, for 

 experimental purposes, we wished to precipitate a number 

 of metalj upon one and the same cathode .simultaneously, we 

 could do so by preparing a suitable solution of such metals 

 and suspending from a rod of insulating material, or 

 from separate insulated blocks, auodes of the different 

 metals, and connecting to each anode the positive pole 

 of its necessary battery, the whole of the negative poles 

 beiug connected, directly or indirectly, to the cathode. A 

 similar result might be accomplished by using a single 

 battery comprising a number of cells sufficient to perform 

 the most difficult precipitation, and then dividing the 

 oirrent between the various anodes, sufficient resistance 

 beiug introduced between the battery and each anode to 

 reduce the current between the cathode and the individual 

 anodes to the requisite strength. Suppose, for example, 

 thUt the electro-motive force required to be expended be- 

 tween anode A and the cathode to be three volts, and that 

 between anode B and the cathode only one volt, then if 

 resistance be introduced between the battery and B equal 

 to twice that between B and the cathode, the electro- 

 motive force expended between the two latter points will 

 be reduced to a third of what it is between A and the 

 cathode, or of what it would be between B and the cathode 

 were the resistance removed. 



The plan more generally adopted, however, is to use an 

 anode comi>osed of the alloy similar to that which it is 

 desired to deposit, and dissolving either a part of the alloy, 

 or certain proportions of the separate metals composing it, 

 in a suitable solution, and then using a current of sufficient 

 intensity aud electromotive force to produce a deposition of 

 the more obstinate metal, the others being deposited with it. 

 The danger is that the more easily-deposited metals will be 

 precipitated to a greater extent than is desired, while the 

 others share but little, if at all, in the deposition. That 

 this would be so becomes very apparent on the briefest 

 consideration. To take a more impre.ssiouable simile, we 

 should scarcely expect that in a forest of young and of 

 matured trees, a rush of wind capable of levelling the 

 older trees, would not discriminate so impartially as to 



destroy an equal number of large and small trees, but 

 would rather anticipate that it would expend itself 

 upon a larger proportion of the weaker trees. Simi- 

 larly, with a rush of elecrtioity, there is the ten- 

 dency, in the presence of two metals — say copper and 

 zinc — to decompose more of the copper than of the 

 zinc salts. This difficulty is a serious one. In the case of 

 the forest, if we wished the destruction of an equal number 

 of large and small trees, we should, in the absence of any 

 other means, present fewer small than large trees to the 

 wind. So in our bath, were no other course open, less 

 copper than zinc would be allowed to be present. 



The alloy most frequently deposited is that of zinc and 

 copper — viz., brass. A useful and easily-made solution 

 may be prepared in the following way : — Dissolve two 

 ounces, or thereabouts, of good brass in four ounces of 

 nitric acid, diluted with a small quantity of water. As 

 dense red fumes of a very pungent and somewhat 

 poisonous nature are evolved in this process, it is best 

 carried on in the open air. If the acid is insufficient to 

 dissolve the whole of the brass, a little more may be added, 

 but care shoidd be taken to avoid having an excess of acid 

 — that is to say, more acid than would suffice to dissolve 

 the metal ; for this reason it is, perhaps, preferable to reject 

 a small quantity of undissolved metal, rather than run 

 the risk of having a superabundance of acid. In order to 

 make sure that the whole of the acid has been neutralised 

 by the metal, a gentle heat may be applied to the glass 

 or earthenware vessel containing the solution. The 

 absence of any further action indicates that there is no 

 free acid present (presuming, of course, that there is at 

 least a small quantity of brass in the solution undissolved). 

 Next decant the solution into a large jug or other con- 

 venient receptacle, and add halfa-gallon of water. Then 

 add slowly a solution of pot issio carbonate, consisting of 

 half-a-pound of the carbonate to a pint and a half of 

 water. Care is necessary in adding the carbonate, or the 

 energy of the reaction may cause the loss of more or less 

 of the brass solution. A precipitate of a light greenish 

 colour is thus_ produced, and when it has settled the 

 liquid is decanted and the precipitate well washed and 

 subsequently dissolved in strong liquid ammonia, adding 

 the latter gradually, stirring at the time, until the whole of 

 the precipitate is again dissolved as a beautiful blue solu- 

 tion. A strong solution of cyanide of potassium is next 

 added until the solution again becomes clear, when a 

 small quantity of free cyanide must be added. Dilute 

 this with sutiicient water to make a gallon of solution, 

 when it will be ready for the bath. The anode should be 

 a sheet of brass of the same quality as that used in pre- 

 paring the solution. 



ORBIT OF THE SUN. 



Bv E. A. Proctor. 



ANOTHER subject about which many mistakes are 

 made is that of the sun's motion in interstellar space. 

 It is supposed by many that the sun has been shown to be 

 travelling in an orbit around the star Alcyone, the chief of 

 the Pleiades. Anything much farther from the truth can 

 hardly be imagined. Absolutely nothing has been proved, 

 or even rendered probable, about the sun's orbital motion 

 through space, except that the sun's present course is 

 directed towards the constellation Hercules. If it were 

 certain that the sun is travelling in a nearly circular course 

 around a centre, then of course it would follow that this 

 centre lies in sotne direction at right angles to that towards 



