CHEMICAL EFFECTS.] 



TJNDULATORY FORCES. ELECTRICITY . 



195 



coloured by a little litmus, may be used. The chlorine, 

 Fig. 56. set free by the voltaic cur- 



rent, will bleach that side 

 of the liquid in which the 

 wire from the platina of 

 the battery is immersed, 

 whilst the other side will 

 retain its blue appearance. 

 Experiment 46. Employ 

 a solution of sulphate of 

 soda, also coloured with 

 litmus. The salt will be 

 decomposed ; the side in 

 which the wire from the 

 zinc is immersed, will re- 

 tain a blue colour ; whilst 

 that containing the other pole, will present a red ap- 

 pearance, from the liberated sulphuric acid acting on the 

 litmus. 



.Experiment 47. Dissolve some iodide of potassium in 

 water, and add a little arrowroot (nearly pure starch). 

 On inserting the two poles of the battery, the iodide will 

 be decomposed, and one side of the liquid will turn 

 to a deep-blue colour, owing to the liberation of free 

 iodine. 



The above are instances of chemical decomposition 

 effected by means of the current of electricity passing 

 through the liquid ; and we may here remark that 

 liquidity is essential for such changes. We shall, 

 perhaps, draw too largely on the chemical knowledge of 

 some of our readers, if we state, that a direct connection 

 s between the equivalents of the metal dissolved in 

 the battery, with the amount of the elements set free in 

 the gla<)s decomposing-vessel. What we mean is simply 

 this for every thirty-two grains of zinc dissolved in one 

 cell of a battery arrangement, one grain of hydrogen, 

 and eight grains of oxygen, are set free during the 

 decomposition of water. If five battery cells are used, 

 then five times as' much zinc is consumed during the 

 evolution of one grain of hydrogen and eight of oxygen ; 

 so that the quantity of gases evolved by the action of 

 a battery in decomposing water, is a measure of the 

 quantity of electricity which is set free. 



There are other modes of ascertaining the strength of 

 a voltaic current; and although such would be better 

 described under the head of Electro-Magnetism, we may 

 just state, that the galvanometer, by the deflection of a 

 magnetic needle, when a current of voltaic electricity 

 passes over it, is often employed as a test of the presence, 

 and also as a measure of the amount, of electricity 

 passing over its wires. One of these instruments ia 

 represented >n the following engraving. 



Fig. 47. 



Two wires may be observed on the left-hand of the 

 out : these convey the electricity from the battery. A 

 coil of foe covered wire is wound repeatedly over, but 



not touching, a magnetic needle ; and just in proportion 

 as the quantity of electricity in the current is great, so 

 the needle is deflected from zero, or 0. This deflection 

 rarely exceeds 70, even when a powerful battery is em- 

 ployed. But neither this instrument or the decom- 

 posing apparatus give any measurement of the intensity 

 of the current. This is best arrived at by interposing a 

 length of some bad conductor, such as pure water, between 

 one of the wires and the galvanometer, or the decom- 

 posing apparatus. The latter has been somewhat 

 ambiguously termed, by way of distinction, a voltameter. 



The secondary results of voltaic action in the decom- 

 position of metallic solutions, is so elaborately and prac- 

 tically dealt with in the next chapter, by Mr. Gore, of 

 Birmingham, that we shall not enter into any explana- 

 tion of the subject. Our readers will have the opportu- 

 nity of studying the theory and practice of electro- 

 typing, plating, and gilding, in all their details. We 

 have been compelled to make some alterations in the 

 original paper, because we have already explained many 

 points in our previous pages. All essential matters, 

 however, have been retained ; and we have made several 

 additions on processes which late discoveries and in- 

 ventions have introduced to public notice. 



In the early part of this century, Sir Humphry 

 Da^y created a great sensation in scientific circles, by 

 the publication of his researches in electro-chemistry. 

 He found that, on bringing the two poles of a voltaic 

 battery into contact with a piece of potash, that sub- 

 stance was decomposed, and a metal was liberated 

 which he called potassium. He extended his investi- 

 gations, and eventually obtained sodium, barium, and 

 other metals by a similar process. The student may 

 try the following experiments, which will illustrate this 

 ' interesting discovery. 



Experiment 48. Place a small piece of fused potass 

 on some platina foil, which must be connected with one 

 pole of the battery ; and bring the other pole, which 

 should be a piece of platina wire, into contact with the 

 potass. Chemical decomposition will soon take place ; 

 hydrogen will be given off, and potassium will be set 

 free. If a little mercury be used instead of the platina 

 foil, an amalgam of that metal with the potassium will 

 be produced ; and a little of this, if placed in cold water, 

 will show the presence of potassium by the production 

 of hydrogen gas, through the metal decomposing the 

 water, and seizing its oxygen to form potass. A similar 

 experiment may be performed with soda, when the metal 

 sodium will be produced. If the air be very dry, the 

 potass should be breathed on, so as to make it a con- 

 ductor by the moisture which it absorbs, as, in the dry 

 state, its power of conduction is very slight. 



By these experiments, it is shown, that potass, soda, 

 &c., are oxides of metals, similar in constitution to the 

 rust of iron, red lead, etc. At first these results were 

 chiefly of interest to philosophers only ; but during the 

 last few years, the cheap production of sodium has led 

 to that of aluminium, a metal obtained from common 

 clay, which has now come into extensive use for a great 

 variety of purposes. Our readers will not fail to ob- 

 serve how valuable are those philosophical researches 

 which at first seem to have no practical result, but 

 which, by patience and skill, may eventually turn out to 

 be of the utmost commercial and social benefit. 



Electricity, it is believed, has a great influence in pro- 

 ducing many of those valuable gems which adorn jewel- 

 lery ; and many attempts have been made to produce the 

 diamond, Jrc., by electro-chemical means. It has been as- 

 certained that a current of free electricity always exists in 

 mines fromwhich metals areobtained. This, however, may 

 be caused by the chemical changes which air and moisture 

 always produce on metals and their oxides. Mr. Crosse 

 and others have succeeded in producing minute crystals 

 of some minerals, by long-continued, intense voltaic 

 currents; and for this purpose small quantity is re- 

 quired. Indeed, a battery composed of small plates in 

 large numbers, charged with common water, is as 

 effective as any of the powerful arrangements to which 

 we have already alluded. Such an arrangement is 



