The resistance of the discharging circuit of the tenth of a microfarad was 3600, so that r' = 3.6 × 10-4.

The values of C were inversely as the resistances, so that if the two shocks were, as I estimated them, nearly equal, the value of p would be 0.670.

This experiment was much more satisfactory and more easily managed than that with the induction-coil, and I thought it desirable to apply the same method to the comparison of the contractions of a muscle when its nerve was acted on by the discharge. I therefore availed myself of the kindness of Mr Dew-Smith, who prepared for me the sciatic nerve and gastrocnemius muscle of a frog, and attached the preparation to his myograph. The discharge was conducted through about 0.4 cm. of the nerve by means of Du Bois Reymond's unpolarizable electrodes, the resistance of the electrodes and nerve being 35000 Ohms. When the electrodes were in contact their resistance was 23000, leaving about 12000 as the resistance of the nerve itself.

I used two condensers, one 01 microfarad, and the other an aircondenser of 270 centimetres capacity in electrostatic measure, or about 3 x 10-1 microfarads.

The first was charged by one cell and the second by 25. The resistances were arranged so that the contractions produced in the muscle were much less than a third of a maximum contraction. The discharges were made alternately every 15 seconds, and when the resistances were 35000 and 140000 respectively, the alternate contractions as recorded on the myograph were as follows:

Here the time-modulus was 1.05 x 10-5 seconds for the small condenser and 1.4 × 10-2 for the large one, and the values of C were as 1 to 100, so that p = 640.

If we suppose that Cavendish took the shocks through pieces of metal held in his hands, the resistance of the circuit would depend on the state of his skin. He occasionally used a piece of apparatus, which he nowhere describes, but which he names in three places* a shock-melter.

From Art. 585 it would appear that it was filled with salt water, even when fresh water was the subject of the experiment, and from Art. 637 Cavendish seems to have considered it his last resource as a method of receiving shocks. I therefore think that it must have

* Arts. 585, 622, 637. See facsimile at p. 326.

been an apparatus by which his hands were well wetted with salt water, so that the resistance of his body would be between 1000 and 2000 Ohms.

The capacity of his battery of 49 jars was 321000 glob. inc., which comes to rather less than half a microfarad.

The discharges of this through 2000 Ohms would have a timemodulus of about one-thousandth of a second.

The following table gives the different results obtained by Cavendish and by myself, with the time-modulus of the discharges compared. The quantity p is such that the ratio of the initial strength of the two discharges is inversely as the p power of the ratio of the time-moduli when the shocks are equal in intensity, or

The number of jars among which a quantity of electricity must be divided in order to give a shock of a given intensity through a given

Experiments on the prepared nerve and muscle of a frog.

0.00001

0.014

0.640

This value of p does not differ much from 0-652, the only result which Cavendish has deduced in a numerical form from his experiments.

The most unaccountable of all the results arrived at by Cavendish is one which seems to have perplexed him so much that he has left the account of the experiments among which it occurs in a very imperfect state. He found (Arts. 639, 644) that the shock of a Leyden jar taken through a long thin copper wire produced a more intense sensation than when it was taken from the jar directly.

As in some of the experiments the wire was wound on a reel, and therefore the self-induction of the current might produce an oscillatory discharge, the physiological effects of which might be different from

those of the simple discharge; I charged two Leyden jars to the same potential, using Thomson's Portable Electrometer as a gauge electrometer, and took the discharge of one through the secondary wire of an induction-coil, the resistance of which was about 1000 Ohms, and that of the other through an ordinary resistance-coil of 1000 Ohms.

In every trial I found that the sensation was more intense when taken through the ordinary resistance-coil than when taken through the induction-coil, and it is manifest that in the latter case the current begins and ends much less abruptly, so that the result is quite in accordance with the modern theory, that the sensation depends on the rapidity with which the strength of the current changes. I am, therefore, quite unable to account for the opposite result obtained by Cavendish. At the same time it is quite impossible that Cavendish could be mistaken in this comparison of the intensity of his sensations, for he had more practice than any other observer in comparing them, and he repeated this experiment many times.

The only apparent objection to the experiment is that the resistance of the copper wires was only 430 in one case and only 1000 in the other, whereas the resistance of a man's body, from one hand to the other, varies from about 1000 when the hands are thoroughly wet, to about 12000 when they are dry, so that the resistance of the copper was small compared with the possible variations of the resistance of Cavendish's body.

The resistances of the tubes filled with solutions of salt, &c., were very much greater, being from 20000 to 900000.

NOTE 32, ARTS. 398, 576, 687.

Comparison of the Resistance of Iron Wire and Salt Water.

Cavendish never published the method by which he made this comparison, but the result given in Art. 398 seems to have been accepted by men of science on Cavendish's bare word, without any question as to how it was obtained.

It appears from Art. 576 that Cavendish made his body and the iron wire the branches of a divided circuit, and then tried how many inches of salt water must be put in the place of the iron wire, so that the shock might appear of the same strength.

By Matthiessen's experiments on the resistance of metals, the resistance of an iron wire of the dimensions given by Cavendish would be about 196 Ohms. As this is much less than that of a man's body from hand to hand, it would have made hardly any difference to the shock whether Cavendish took it through his body alone, or through his body and the iron wire in series.

By using the iron wire as a shunt and increasing the discharge so as to obtain a shock of easily remembered intensity, Cavendish was enabled to compare the wire with a column 5.1 inches long of saturated solution of salt.

By this experiment the resistance of saturated solution of salt is 355400 times that of iron.

By the statements in Art. 398, that the resistance of rain-water is 400,000,000 times that of iron wire, and 720 times that of a saturated solution of sea-salt, the resistance of saturated solution would be 555555 times that of iron wire.

It is true that this result given by Cavendish does not agree with the only experiment he has recorded, but we must remember that it is the only result which he published, and therefore he must have thought it the best he had.

By Kohlrausch's experiments on salt solutions combined with Matthiessen's on metals, the resistance of saturated solution of salt is 451390 times that of annealed iron, when both are at 18°C. The ratio of the resistances would agree with that given by Cavendish at a temperature of about 11°C.

The coincidence with the best modern measurements is remarkable.

NOTE 33, ART. 619.

Conductivity of Solutions of Salt.

According to the measurements of Kohlrausch* the electric conductivity k, of saturated solution of sodium chloride, the conductivity of mercury at 0° C. being taken as unity, is given by the equation

10% 1259 (1+0.0308t+ 0·000146).

When the temperature is near 18° C., we may use the equation 10% 2015+ 45.1 (18).

=

Saturated solution at 18° contains according to Kohlrausch 26.4

per cent. of salt.

Cavendish's saturated solution contained

of salt, which is equivalent to 26.45 per cent.

1

3.78

Kohlrausch finds that saturated solution of salt is one of the best standard substances for the comparison of the resistance of other electrolytes. Its conductivity seems to be sensibly the same, whether it is made with chemically pure salt or with the ordinary salt of commerce. The temperature coefficient is also smaller than that of many other electrolytes.

* Wiedemann's Annalen Bd. vi. (1879) p. 51.

For other solutions of sodium chloride he finds that at 18°

where p is the proportion, by weight, of the salt to the whole solution.

For the particular solutions examined by Cavendish we have

The substances mentioned by Cavendish are easily identified, with the exception of "calc. S. S. A." and "f. alk. D." The weights of the quantities furnish no indication, for they are so large as to show that a dilute solution was used. The letters A and D probably indicate the bottles in which the solutions were kept.

The expression f. alk. or fixed alkali occurs in several parts of Cavendish's writings, especially in the manuscripts lithographed by Mr Vernon Harcourt in the Report of the British Association for 1839. It certainly means pearl ashes or carbonate of potash. The full title seems to have been alkali fixum vegetabile, as distinguished from alkali fixum fossile, which is sodic carbonate, and other writers seem to have used the expression fixed alkali for either of these, but Cavendish always uses the expression as a synonym for pearl ashes, and distinguishes potassic hydrate by the name of "sope leys."

The conductivity as determined by Cavendish agrees much better with potassic carbonate than with potassic hydrate, the conductivity of which is much greater.

It seems likely that calc. S. S. was sodic carbonate, and the conductivity would agree very well with this explanation, only it is difficult to find among the names in use at the time any which could be written in this form. Mr Maine has suggested Calcined Salsola Soda. The burnt seaweed from the shores of the Mediterranean, from which soda was often extracted was, I believe, called salsola, but I doubt whether the word soda was then in use.

The weights of the other substances are, when reduced to pennyweights, not very far from the equivalent numbers now received, hydrogen being taken as the unit,