VOLTAIC ELECTKKTIT 



VOLTAIC ELECTRICITY. IX. 



DECOMPOSITION OF WATER VOLTAMETER THEORIES OF 

 DECOMPOSITION LEAD TREE MODES OF DECOMPOSING 

 8ALT8. 



THE decomposing action of the electric current WM flrat 



mg the effect it produced on water through 



\vhi< h it WM made to paH; and an this experiment ha* been 



IV lu.'utly repeated, and affords us one of the Ixmt illiutrations 

 of tlio uhomioal effect* of electricity, we will explain the mode 

 of j.i 



i-e of platinum is fixed to the end of each battery wire s a 

 glass tube, closed at one end, is then taken, and haring been filled 

 v, i i : i wutor, in inverted into a ressol likewise containing water, the 

 >) being kept fill, the air. The 



y wires oro now placed so that the platinum tips may be 

 insi.lo the tube, and may nearly 

 touch one another. In a 

 few seconds they will become 

 covered with small bubbles of 

 gas, which will unite and riao 

 in tho tube, displacing the 

 water which it contains. 



If a little acid be added to 

 tho water, a much greater 

 effect will \>c, produced, and 

 the tube will fill more rapidly. 

 iial explanation of thia 

 is, that the addition of tho 

 acid improves tho conducting 

 power of the liquid, and thus 

 diminishes the resistance op- 

 to tho current; some, 

 however, suppose that tho 

 acid becomes decomposed by 

 tho current in a way that will 

 be shortly explained, and thus 

 a larger quantity of gas is 

 liberated. Whichever expla- 

 nation is adopted, the fact 

 itself is very clear, as may 

 be seen by trying the experi- 

 ment, first with pure water, 

 and then with dilute acid. 

 The reason why platinum ia 

 employed for the ends of 

 the wires is that most other 

 metals would rapidly become 

 oxidised, owing to tho oxy- 

 gen gas liberated from the 

 water combining chemically 

 with them. 



If tho tube, when it has 

 become filled with the gas, 

 bo carefully removed, and 

 then held to a flame, the two 

 gases, oxygen and hydrogen, which it contains, will combine 

 with a loud report. Before attempting this, the glass should 

 be wrapped up in a cloth, so that in case it should bo 

 broken by the explosion, which is far from improbable, no 

 iiijury may be caused by the scattering of the fragments. A 

 soda-water bottle is frequently employed for this purpose, 

 and answers well, owing to its strength. These two gases 

 are the constituents of water, aa the student will have learnt 

 from the Lessons in Chemistry; the electric current, there- 

 fore, tears tho water apart into these elements, and when the 

 flame is applied they re-combine to form steam, which is at once 

 condensed. 



By the apparatus now described tho gases are obtained 

 mixed together. Frequently, however, it is desirable to collect 

 them separately. This may easily be accomplished by the 

 arrangement shown in Fig. 56. Tho glass vessel containing 

 the acidulated water has holes drilled in the bottom, through 

 which there pass two small strips of platinum foil, soldered to 

 wires which lead to tho binding screws seen at tho sides of the 

 stand of the apparatus. Two glass tubes are now employed, 

 one being placed over each electrode. As soon as the battery 

 wires ore connected with the instrument, gas will, as before, bo 



127 N.BL 



Fig. 56. 



ven off from each electrode ; it will, however,, soon b* 

 obMtvad that the tab* placed over the negative on* is filling 

 twice M rapidly M the other. 



When both are full, we may easily tact thmn by holding a 

 lighted match to the month of each, and we ahall find that UM 

 one over the negative polo is filled with a gas which exttngnih*ji 

 the match, bat become* itself ignited and boms si UM mouth 

 of the tube. This, then, is hydrogen. The content* of UM other 

 tab* are easily seen to be oxygen, for H pauses UM matrh to 

 burn much more brilliantly, and will even re-light it when ex- 

 tinguished, if a faint spark be left 



We see, then, that hydrogen is liberated at the negative pole 

 and oxygon at the positive, and farther, that the volume of 

 hydrogen is twice as great as that of the oxygen. As, however, 

 any given bulk of oxygen weighs sixteen times as much as a 

 similar bulk of hydrogen, the proportion by weight in which UM 



gases are given off is sixteen 

 parts of oxygen to on* of 

 hydrogen. Now tbsss are 

 their atomic weights, and 

 by further investigation we 

 shall discover that the quan- 

 tities of the different ele- 

 ments which are liberated 

 at the poles when any body 

 is decomposed, are always 

 in the proportion of their 

 atomic weights. This general 

 rule will be found of great 

 service. We shall further 

 observe that all the metals, 

 and hydrogen, which there 

 is every reason to suppose 

 is also a metal, are set free 

 at the negative pole, and 

 hence are cation*. Sometimes 

 they are distinguished also 

 as electro-positive elements, 

 while those liberated at the 

 positive polo are called electro- 

 negative, because at the mo- 

 ment of separation they are 

 supposed to be charged with 

 negative electricity, and are 

 therefore attracted to the 

 opposite pole. 



The oxygen liberated at 

 the positive pole possesses 

 strongly tho smell of ozone, 

 and appears to have its 

 chemical affinities greatly 

 heightened, as is tho case 

 with many elements when in 

 a nascent state. 



The quantity of the current 

 is of more importance than 

 the intensity, in these experiments ; and hence a much greater 

 amount of gas will be evolved when the cells are arranged for 

 quantity than when put together in the ordinary way. 



A voltameter was devised by Faraday, in which the strength 

 of the current is measured by the quantity of gas liberated by 

 it. This instrument is represented in Fig. 57, and one may 

 easily bo constructed by the student. A wide-mouthed bottle, 



A, is taken, and a sound bung is fitted tightly to its mouth, so 

 as to render it air-tight. Through this are passed two wires 

 with binding screws attached to their upper ends, while below 

 are fastened two small strips of platinum foil, about an inch 

 long and a quarter of an inch wide. These are placed parallel 

 to one another, at a little distance apart A bent tube, D, passes 

 through tho cork, and its other end dips into a vessel of water, 



B, and has a graduated glass tube, c, filled with water, inverted 

 over it. A is nearly filled with water mixed with one-eighth 

 part of sulphuric acid, and when the battery wires are connected 

 with the binding screws on the cork, oxygen and hydrogen are 

 given off from the surface of the platinum plates. These gases 

 pass off by the bent tube, and are collected in the receiver, c. 

 By means of the graduations we can at once ascertain the 

 number of cubic inches given off in a minute, and thus loam 



