made in this way was to find, as he expressed it, "what power of the velocity the resistance is proportional to *."

Cavendish means by "resistance" the whole force which resists the current, and by "velocity" the strength of the current through unit of area of the section of the conductor.

(In modern language the word resistance is used in a different sense, and is measured by the force which resists a current of unit strength.)

By four different series of experiments on the same solution in wide and in narrow tubes, Cavendish found that the resistance (in his sense) varied as the

1.08, 1.03, 0.976, and 1.00

power of the velocity.

This is the same as saying that the resistance (in the modern sense) varies as the

0·08, 0·03, -0.024

power of the strength of the current in the first three sets of experiments, and in the fourth set that it does not vary at all.

This result, obtained by Cavendish in January, 1781, is an anticipation of the law of electric resistance discovered independently by Ohm and published by him in 1827. It was not till long after the latter date that the importance of Ohm's law was fully appreciated, and that the measurement of electric resistance became a recognised branch of research. The exactness of the proportionality between the electromotive force and the current in the same conductor seems, however, to have been admitted, rather because nothing else could account for the consistency of the measurements of resistance obtained by different methods, than on the evidence of any direct experiments.

Some doubts, however, having been suggested with respect to the mathematical accuracy of Ohm's law, the subject was taken up by the British Association in 1874, and the experiments of Professor Chrystal, by which the exactness of the law, as it relates to metallic conductors, was tested by currents of every degree of

• Arts. 574, 575, 629, 686.

intensity, are contained in the Report of the British Association for 1876.

The laws of the strength of currents in multiple and divided circuits are accurately stated by Cavendish in Arts. 417, 597, 598.

Cavendish applied the same method of experiment to compare the resistance of the same liquid at different temperatures*, and he found that "salt in 69 [of water] conducts 1.97 times better in heat of 105 than in that of 581." He also found that "the proportion of the resistance of saturated solution and salt in 999 to each other seems not much altered by varying heat from 50 to 95."

Kohlrausch, who has made a most extensive series of experiments on the resistance of electrolytes, gives results from which it appears that the ratio of the resistances of salt in 69 at 105° F. and at 58° F. would be 159. He also finds that the temperature coefficient for solutions of salt alters very little with the strength. See Note 33.

Cavendish also tested the resistance of solutions of salt of strengths varying from saturation to one in 20000 of distilled water, and arrived at the result, which Kohlrausch has shown to be nearly accurate, that for weak solutions the product of the resistance into the percentage of salt is nearly constant.

Of all substances, that for which different observers have given the most different measures of resistance is pure water.

It has been found indeed that the presence of the minutest trace of impurity in water diminishes its resistance enormously. Thus Kohlrausch found that it was necessary to use water quite freshly distilled in platinum vessels, for if placed in a glass vessel it rapidly diminished in resistance by dissolving a minute quantity of the glass, and a few minutes exposure to the air of the laboratory, by impregnating the water with a trace of tobacco smoke, was found sufficient to spoil it for a determination of resistance. Kohlrausch indeed estimates that the electric conductivity which he observed in the purest water he could obtain might be accounted for by the presence of no more than one ten millionth part of hydrochloric acid, a quantity which no chemical analysis

* Art. 691.

could detect. Hence the hypothesis that water is a non-conductor of electricity, if not true, cannot be disproved.

Some of these remarkable properties of water were detected by Cavendish. He found that the resistance of pump water was 4 times less than that of rain water, and that of rain water was 2.4 times less than that of distilled water*.

In January 1777, he found that salt in 2999 conducted about 70 or 90 times better than some water distilled in the preceding summer but only about 25 or 50 times better than the distilled water used in the year 1776†, and that the conductivity of distilled water increased by standing two or three hours in a glass tube‡.

He also found that in order to make the conducting powers of his weakest solutions of salt agree with the hypothesis that they are as the quantity of salt in them, it would be necessary only to suppose that his distilled water contained one part of salt in 120000§.

It was found that distilled water impregnated with fixed air from oil of vitriol and marble conducted 2 times better than the same water deprived of its air by boiling ||, and that the presence of absorbed air in a weak solution of salt seemed to increase its conductivity T.

In order to find whether electricity is resisted in passing out of one medium into another in perfect contact with it, Cavendish prepared a tube containing 8 columns of saturated solution of sea salt enclosed between columns of mercury. He found that the shock was diminished in passing through a mixed column in which the length of salt water was 218 inches as much as in passing through a single column of the same size whose length was 22.94 inches**.

The difference would have been far greater if the comparison had been made with an ordinary galvanometer and continued currents which rapidly produce polarization, but with the small quantities of electricity which Cavendish used, the effect of polarization would hardly be sensible.

He also made a compound conductor consisting of 40 bits of tin soldered together. The shock through this appeared to be of the same strength as through a single piece of the same size. This experiment however is not of much value, as the resistance of the conductor was far too small compared with that of Cavendish's body to give good results*.

We now come to a very remarkable set of experiments which Cavendish made on a series of salts and acids in order to determine their relative electric resistance. They are recorded in Arts. 626, 627 and 694, and are dated Jan. 13 and 15, 1777.

The strength of the different solutions was such, as Cavendish tells us, "that the quantity of acid in each should be equivalent to that in a solution of salt in 29 of water."

The total weight of each solution was 3 pounds 10 ounces and 12 pennyweights, or 1116 pennyweights Troy. The quantity of each substance when reduced to pennyweights is in every case very nearly the equivalent weight of that substance in the system adopted at present, in which the equivalent weight of hydrogen is taken as unity+.

Now these experiments were made in 1777, and it is difficult to see from what source, other than determinations of his own, he could have derived these numbers. Wenzel's Lehre von den Verwandschaften was published in 1777. I have not been able to consult the work itself, but from the account of it given in Kopp's Geschichte der Chemie, the equivalent numbers seem to have been larger than those used by Cavendish. Richter's Anfangsgründe der Stöchyometrie was not published till 1792.

It is difficult to account for the agreement not only of the ratios but of the absolute numbers given by Cavendish with those of the modern system, in which the equivalent weight of hydrogen is taken as unity. I can only conjecture from several parts of his

* Art. 579. The resistance of a man's body, from one hand to the other, varies from about 1000 ohms when the hands are well wetted with salt water, to about 12000 when the hands are dry. When the outer skin is removed by a blister, the resistance is very much diminished. The resistance of the compound conductor was probably a fraction of an ohm. See Note 31.

See Note 34.

paper on Factitious Airs (Phil. Trans. 1766), that Cavendish was accustomed to compare the quantity of fixed air from different carbonates with that from 1000 grains of marble. Now the modern equivalent weight of marble is 100, so that if Cavendish took 100 pennyweights as the equivalent weight of marble, the equivalents of other substances would be as he has given them. This I think is more likely than that he should have selected inflammable air as his standard substance at a time when even his own experiments left it doubtful whether inflammable air was always of the same kind.

In his journal, Cavendish writes down these equivalent weights just as a modern chemist might do, without a hint that a list of these numbers was not at that time one of the things which every student of chemistry ought to know by heart. It is only by comparing the date of these researches with the dates of the principal discoveries in chemistry, that we become aware, that in the incidental mention of these numbers we have the sole record of one of those secret and solitary researches, the value of which to other men of science Cavendish does not seem to have taken into account, after he had satisfied his own mind as to the facts.

I take this opportunity of expressing my thanks to the many friends who have given me assistance in preparing this edition, and in particular to Mr C. Tomlinson, who gave me valuable information about the manuscripts; to Mrs Sime, who lent me a manuscript book of letters, &c., relating to Cavendish, collected by her brother, the late Dr George Wilson; to Mr W. Garnett, of St John's College, Cambridge, who copied out Arts. 236-294; and Mr W. N. Shaw, of Emmanuel College, who took the photographs from which the facsimile figures were executed; to Mr H. B. Wheatley, who furnished me with information connected with the history of the Royal Society; to Prof. Dewar, Mr P. T. Main, Mr G. F. Rodwell, and Dr E. J. Mills, who gave me information on chemical subjects; and Mr Dew Smith and Mr F. M. Balfour, of Trinity College, and Prof. Ernst von Fleischl, of Vienna, who gave me information about electrical fishes, and the physiological effect of electricity.