THE POPULAR EDUCATOR. 



VOLTAIC ELECTRICITY. X. 



ELECTRIC DECOMPOSITION (continued) INFLUENCE OF WEAK 

 CURRENTS IN THE FORMATION OF MINERALS ELECTRO- 

 TYPING ELECTEO-PLATING ETCHING BY ELECTRICITY. 



BEYOND those whose lives almost depend upon the amount of 

 knowledge they are able to store in their minds, there is a 

 large class of amateur students, if we may call them so, who 

 adopt such a branch of science as electricity as a hobby or 

 pastime for their leisure hours. When the art of electrotyping 

 first became known there were many of these amateurs of 

 science who took it up, and helped towards bringing it to 

 perfection. With a battery cell and some sulphate of copper 

 they were able to reproduce medals, or fac-simile copies of 

 scarce coins, natural objects, etc. greatly to their own satis- 

 faction and thus electrotyping became for a time the rage. 

 But photography, with its beautiful results, and its especial 

 charm of portraiture, came upon the scene, and electrotyping 

 was henceforth committed to trade purposes only. The sun 

 proved to be a far quicker artist than the electric battery ; and 

 perhaps also the paintings of the former were more generally 

 attractive than the sculptures of the latter. Experimentally, 

 however, electrotyping is full of interest. 



As a continuous current of small intensity is of great moment 

 in many of these experiments, a 

 single cell of Daniell's battery is 

 frequently employed. A slight 

 modification of this, as represented 

 in the annexed figure (Fig. 60), 

 was recommended by the late Dr. 

 Golding Bird. B is the battery 

 cell, which, however, differs from 

 the ordinary construction, the 

 sulphate of copper being placed 

 in the inner vessel, c. This cell is 

 made of glass tube, closed below 

 by a disc of plaster, and inside it 

 is a piece of copper plate usually 

 coiled round so as to present a 

 large surface. The outer cell is 

 filled with a weak solution of salt, 

 and a plate, or sometimes a spiral 

 riband of zinc, is placed at the 

 bottom of it. By means of this 

 cell a weak but nearly uniform 

 current, lasting several weeks, 

 may be obtained. 



The decomposing cell, A, is 

 very similar to that described in 



our last lesson ; the tube, D, being filled as before with the 

 solution to be decomposed. In it is immersed a strip of plati- 

 num, E, which is connected with the negative pole of the 

 battery, and serves as the negative electrode, while the plate of 

 zinc in the outer cell is connected with the copper plate in c. 

 The metal is precipitated on the surface of E. 



By this apparatus, and a careful choice of the salts employed, 

 nearly all the metals may be deposited, and for the most part 

 they exhibit a highly crystalline structure ; this, however, 

 depends to a great extent upon the power of the current gene- 

 rated, it being found as a general rule that when the current is 

 weak the deposit is harder and more polished, while a powerful 

 current causes a more loose and spongy precipitate. 



A very interesting series of experiments was conducted some 

 years ago by Mr. Crosse, with a view of ascertaining the effects 

 of weak continuous currents on various minerals, and also to 

 discover how far the formation of many of the rarer minerals 

 could be explained in this way. This gentleman, whose experi- 

 ments on aerial electricity have already been referred to, was 

 one of the most original investigators of electrical phenomena, 

 and has shown how much may be learnt of the secrets of 

 Nature by patient inquiry into apparently simple phenomena. 



He employed a great number of very weak batteries, water 

 being in many of them the exciting liquid. The wires leading 

 from these were then connected with pieces of stone of various 

 kinds, placed in jars so as almost to touch one another, and 

 covered with spring water or solutions of different substances. 



After the lapse of some months they were carefully examined, 

 and in most cases crystals of different minerals were formed. 



Fig. 60. 



Many other remarkable effects were also discovered. In some 

 instances arragonite and other minerals were closely imitated. 



In a similar way, by placing pieces of brick or stone in me- 

 tallic solutions, crystals and streaks of the metals were formed 

 in their substance. The results of all the experiments seemed 

 to indicate that the electric currents which are known to exist 

 in the crust of the earth have had a great influence in the for- 

 mation of many minerals, and in the production of metallic 

 veins. 



Several works especially devoted to electricity will furnish the 

 student with more detailed accounts of these and similar experi- 

 ments, which will be found extremely interesting, and will serve 

 as a guide to him in repeating them and trying fresh ones. 



The chemical effects of the electric current were at first 

 looked upon merely as scientific curiosities : now, however, they 

 are found to be of great practical importance, as the arts of 

 electrotyping and electro-plating are merely applications of 

 them. Mr. Spencer was the first in this country to discover the 

 art of depositing copper on moulds, though a Russian, named 

 Jacobi, slightly anticipated him. He was, however, quite an 

 independent discoverer, and this is not the only instance in which 

 an important scientific discovery has been effected almost simul- 

 taneously by two independent workers. 



His apparatus was of the most simple kind ; a common glass 

 tumbler (Fig. 61), filled with a 

 solution of sulphate of copper, 

 serving for his outer cell, while a 

 lamp-glass, with its lower end 

 closed by a disc of plaster of 

 Paris, was suspended in it to 

 serve as the inner. This was 

 filled with a weak solution of 

 common salt, and had immersed 

 in it a plate of zinc with a wire 

 fastened to one end. To the other 

 end of this wire a medal was 

 attached, which was placed in the 

 copper solution immediately under 

 the porous disc. In a short time 

 the copper from the solution 

 began to deposit on the coin, and 

 after the lapse of a few hours it 

 was thick enough to be removed, 

 when it was found to present an 

 exact copy of the original, every 

 line and mark being faithfully 

 and accurately reproduced. 



On repeating the experiment 

 with the same coin, intending 



to procure a thicker copy, the deposit was found to adhere so 

 closely that it could not be removed. To guard against the 

 recurrence of this, the object to be copied was warmed and 

 rubbed over with wax, which was then wiped off as cleanly as 

 possible. Enough, however, remained to prevent the deposited 

 metal uniting with the coin, and when a sufficiently stout layer 

 of copper had been thrown down, it was taken from the solution, 

 and on being held over the flame of a spirit-lamp, the cast at 

 once separated from the original. Of course the cast was a 

 reverse, the raised parts in the one being sunk in the other. By 

 employing it, however, as a fresh mould, and depositing on that, 

 an exact reproduction of the original was obtained, and any 

 number of these could be procured from the mould. 



The next great step in this art was the discovery that non- 

 metallic substances might be employed for the construction of 

 the mould, if some conducting material were applied to their 

 surface. Considerable difficulty and expense would frequently 

 be incurred were a metal mould indispensable ; in fact, but little 

 practical application could be made of the art. Now, however, 

 the mould is made of some material easily worked, common white 

 wax mixed with a little spermaceti being most usually employed. 

 Gutta-percha and similar substances are, however, frequently 

 used, and answer well. The surface of these is thoroughly 

 rubbed over with finely-powdered black lead, and a wire is then 

 fastened to one side of the mould, by which it is connected with 

 the zinc plate. 



We must refer the student who wishes to become acquainted 

 with all the practical details of the process, fco one of tbo 

 many works specially treating of the subject. These will give 



