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 41 times less than that of rain water, and that of rain water was 24 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 24 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 21.8 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. Sce 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.

P. S. 14th June, 1879.

Just before sending this sheet to press I have received from Mr Robert H. Scott, F.R.S., a small packet marked "Cavendish Papers," which had been sent to the Meteorological Office by Sir Edward Sabine.

These papers relate entirely to magnetism, and do not fall within the scope of this volume, though they may supply important materials for the magnetic history of the earth, and are in all respects excellent specimens of Cavendish's scientific procedure.

I shall therefore only mention a few particulars in which these papers throw some additional light on Cavendish's life and work.

The descriptions of Cavendish by Cuvier, Young, Thomson and Wilson agree in representing him as living in London, and regularly attending the meetings of the Royal Society, but in other respects leading an isolated life, very much detached from the interests, whether social or scientific, of other men.

It has also been hinted that Lord Charles Cavendish, who, as we have already seen, was himself addicted to scientific pursuits, did not entirely approve of his son's devotion to science, or at least, for some reason or other, restricted him in the means of carrying on his work.

In these manuscripts, however, we have the details of a laborious series of observations undertaken to determine the errors of the variation compass and the dipping needle belonging to the Royal Society, and on Sept. 16, 1773, we find "Observations of needle in Garden by Father and Self," and a "Comparison of Society's compass in house and in soc[iety's] garden with Father's compass in room."

It appears, therefore, that Lord Charles Cavendish not only placed his instruments at his son's disposal, but made observations of the compass in concert with him, and that these observations were undertaken in order to make the instruments belonging to the Royal Society more available for accurate measurements. In the same Journal there are also "Measures taken for setting Dr Knight's magnets so that their poles shall be equidistant from var[iation] comp[ass] and dipp[ing] need[le] in 1775." The results of this enquiry are briefly stated by Cavendish in his paper on the Instruments belonging to the Royal Society in the "Philosophical

Transactions" for 1776. In the same volume there is an account of Dr Knight's great Magazines of magnets by Dr Fothergill.

A considerable portion of the MS. is taken up with "Directions for using the Dipping Needle," written out at greater or less length (probably according to the scientific capacity of the recipient) "for Captain Pickersgill," "for Captain Bayley," "for Dalrymple" [Hydrographer to the Honourable East India Company] &c.

There is also a treatise of 26 pages "On the different forms of construction of dipping needles."

Besides this, there is a series of observations of the magnetic variation and also of the dip, at various times, from 1773 to August 1809 (Cavendish died Feb. 24, 1810).

These observations were made for the most part only in the summer months, but during that time were carried on with the greatest regularity, and results for each year calculated from them.

We also find the record of "Trials of Nairne's needle in different parts of England in August, 1778."

It was tried" in Garden, Aug. 8. In Garden of Observatory at Oxford, Aug. 14. At Birmingham, in Bowling-green, Aug. 15. At Towcester, in Garden, Aug. 17. At St Ives, in Garden, Aug. 18. At Ely, in Garden, Aug. 18. At London, Aug. 19 and 22.” From these trials he finds that "Lines of equal dip should seem to run about 44° to south of west, and dip should increase about 42' by going 1° to N.W."

There is a long and valuable series of experiments on the magnetic properties of forged iron, blistered steel, and cast iron. "Some bars were got from Elwell 31 inch long, 21 broad, and about 5 thick. On May 29, 1776, one of each was made magnetical, the marked end being the south pole. In trying the experiment the bars were placed perpendicularly against a wall 25 inches distant from the center of the needle, 91° to west of usual magnetic north, either the top or bottom of the bar being always on a level with the needle. They were kept constantly with the marked end upwards till after the observations of June 30, after which they were kept with the marked end downwards."

Cavendish determines in every case the "fixed magnetism" and the "moveable magnetism" of the bar, and also its magnetism