THE ELECTRIC CURRENT

125.] Now Clausius supposes that it is on the constituent molecules in their intervals of freedom that the electromotive force acts, deflecting them slightly from the paths they would otherwise have followed, and causing the positive constituents to travel, on the whole, more in the positive than in the negative direction, and the negative constituents more in the negative direction than in the positive.

The electromotive force, therefore, does not produce the disruptions and reunions of the molecules, but finding these disruptions and reunions already going on, it influences the motion of the constituents during their intervals of freedom. The amount of this influence is proportional to the electromotive force when the temperature is given. The higher the tempera- ture, however, the greater the molecular agitation, and the more numerous are the free constituents. Hence the conductivity of electrolytes increases as the temperature rises.

This effect of temperature is the opposite of that which takes place in met- als, the resistance of which increases as the temperature rises. This difference of the effect of temperature is sometimes used as a test whether a conductor is of the metallic or the electrolytic kind. The best test, however, is the existence of polarization, for even when the quantity of the free ions is too small to be observed or measured, their presence may be indicated by the electromotive force which they excite. ∗ [Leçons sur la Dissociation, professées devant la Société Chimique. L. Hachette et Cie , 1866.]119 WATER NOT AN ELECTROLYTE. 126.] Kohlrausch∗ finds that if the electromotive force is one volt per cen- timetre in length of the electrolyte, then if the electrolyte differs but slightly from pure water at 18°C the velocity of hydrogen is about 0·0029 centimetres per second, and that the actual force on a gramme of hydrogen in the solution required to make it move at the rate of one centimetre per second through the solution is equal to the weight of 330,000,000 kilogrammes. The velocities of the components of unibasic acids and their salts were found by Kohlrausch to be in the following proportion:— Table I. HKNH4NaLi1 Ba 21 Sr 21 Ca 21 Mg 2 2734846301931282421 IBrClFNO3ClO3C2 H3 O255535029473622 127.] The specific molecular conductivity (l) of an electrolyte is the sum of the velocities of its components† , and the actual conductivity of any weak solution is found by multiplying the number l by the number of grammes of the substance in a litre and dividing by the molecular weight of the substance, that of hydrogen being 1. 128.] We have reason to believe that water is not an electrolyte, and that it is not a conductor of the electric current. It is exceedingly difficult to obtain water free from foreign matter. Kohlrausch‡ , however, has obtained water so pure that its resistance was enormous compared with ordinary distilled water. When exposed to the air for [4·3 hours its conductivity rose 70 per cent.], and [in 1060 hours it was increased nearly fortyfold. After long exposure to the air the conductivity was more than doubled in 4·5 hours by the action of tobacco smoke.] Water kept in glass vessels very soon dissolves enough of foreign matter to enable it to conduct freely. ∗ Göttingen Nachrichten, 5 Aug., 1874, 17 May, 1876, and 4 April, 1877. [Compare Cavendish Papers, pp. 446, 447.] ‡ [Poggendorff, Ergänzungsband, VIII (1876), pp. 7, 9, 11.] †120 WATER NOT AN ELECTROLYTE. Kohlrausch∗ has determined the resistance of water containing a very small percentage of different electrolytes, and he finds that the results agree very well with the hypothesis that the velocity with which each ion travels through the liquid is proportional to the electromotive force, the velocity correspond- ing to unit of electromotive force being different for different ions, but the same for the same ion, whatever the other ion may be with which it is com- bined. The velocities of different ions in centimetres per second, correspond- ing to an electromotive force of one volt, are given in Table II. Table II. H ·0029 K ·00051NH4 ·00049Na ·00032Li ·00020Ba ·00033Sr ·00030Ca ·00025 I ·00058Br ·00056Cl ·00053F ·00031NO3 ·00050ClO3 ·00038C2 H3 O2 ·00023 Mg ·00022 When the water contains a large percentage of foreign matter the velocities of the ions are no longer the same, as it is no longer through water, but through a liquid of quite different physical properties that they have to make their way. It appears from Table III† that though for small percentages of sulphuric acid in water the conducting power is proportional to the percentage of acid, yet as the proportion of acid increases the conducting power increases more slowly till a maximum conducting power is reached, after which the addition of acid diminishes the conducting power‡ . ∗ [Pogg. Ann. Vol. CLIV (1875), p. 215; Vol. CLIX (1876), p. 242; Phil. Mag. June 1875.] [See also p. 226.] ‡ [A similar result was found with nitric acid and some viscous saline solution.] †121

PROPERTIES OF DIELECTRICS. Table III.

Conductivity of Sulphuric Acid at 18°C referred to that of Mercury at 0°C as unity. Percentage of H2 SO4 1 2 ·5 5 10 15 20 25 30 35 40 45 50 55 108 KPercentage of H2 SO4108 KPercentage of H2 SO4108 K 429 1020 1952 3665 5084 6108 6710 6912 6776 6361 5766 5055 428060 65 70 75 78 80 81 82 83 84 85 863487 2722 2016 1421 1158 1032 985 947 924 915 916 92687 88 89 90 91 92 93 94 95 96 97 99·4944 965 986 1005 1022 1030 1024 1001 958 885 750 80

129.] The oxygen and hydrogen which are given off at the electrodes in so many experiments on water containing foreign ingredients are, therefore, not the ions of water separated by strict electrolysis, but secondary products of the electrolysis of the matter in solution. Thus, if the cation is a metal which decomposes water, it unites with an equivalent of oxygen and allows the two equivalents of hydrogen to escape in the form of gas. The anion may be a [compound radicle] which cannot exist in a separate state, [but which exists in the nascent condition, and] contains one equivalent [or more] of [some electronegative element which reacts upon water and liberates oxygen.]