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evaporation of the fluid. With regard to other circumstances, such as the temperature of the air, the dryness or dampness of the weather, &c., I think it would be needless here to expatiate upon them."

In the second edition of “Cavallo's Treatise on Electricity,” page 42, we find that “electricity strongly communicated to insulated animal bodies, quickens their pulse and promotes their perspiration. If it is communicated to insulated fruits, fluids, and, in general, to every kind of bodies that are in a state of evaporation, it also increaseth that evaporation, and in a greater or less degree as those bodies are more or less subject to evaporate themselves, as the vessels that contain them are conductors or electrics, and as they have a greater or less surface exposed to the open air."

This is precisely the conclusion at which the Abbé Nollet arrived, with the exception of the extent of “surface exposed to the open air.” At page 101, Cavallo states that “ a considerable quantity of electricity exists in the atmosphere, and is certainly employed for some great purposes of nature.” “ Hitherto," again he says,

electricity has not been found concerned in any fermentation, evaporation, or coagulation, although the clouds, the rain, the hail, the snow, and the fogs, are almost always electrified."

Cavallo's Treatise was published in the year 1782, and although, as appears by the preceding paragraph, the author was not aware of the fact, yet Professor Volta, of Como, had previously discovered that electricity is produced by the evaporation of water, as will appear by the following extract from the “Philosophical Transactions of the Royal Society" for 1782. Volta says,

“I mentioned, that after various attempts I at last succeeded in obtaining undoubted signs of electricity from the simple evaporation of water, and from various chemical effervescences; but as this is a fact not less interesting than new, it seems proper to subjoin in this place a faithful account of the experiments made for that purpose. The first set of experiments were made at Paris, in company with M. Lavoisier and M. De la Place, two intelligent philosophers, and members of the Royal Academy of Sciences. After I had shewn them my experiments with my condenser, they, as well as myself, began to entertain hopes of succeeding in the experiments on the evaporation, &c. Accordingly, M. Lavoisier ordered a large condenser, with a marble plane, to be made. The first experiment I attempted with this instrument, in company with M. De la Place, proved unsuccessful; but the weather at that time was bad, the room was narrow and full of vapours, and the apparatus was not quite in good order. M. De la Place and M. Lavoisier repeated those experiments in the country, and then they were attended with success, which incited us to repeat and diversify the experiments, by which means the discovery was completed; having obtained unequivocal signs of electricity from the evaporation of water, from the simple combustion of coals, and from the effervescence of iron filings

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in diluted vitriolic acid. This observation was made on the 13th of April of the present year (1782), and the experiments were performed in the following manner :-In an open garden a long metal plate was insulated, which, by means of a large iron wire, was made to communicate with the metal plate of the condenser, laid on the piece of marble, which was kept continually warm by some lighted coals set underneath. This done, some chafing dishes, containing burning charcoal, were placed on the large insulated plate. The combustion of the coals was helped by a gentle wind. Some minutes after, the iron wire, by which the large insulated plate was connected with the metal plate of the condenser, was taken off ; then, the metal plate being removed from the marble by its insulating handle, and presented to Mr. Cavallo's electrometer, it made the balls of it diverge with negative electricity. The experiment was repeated by placing on a large insulated plate four vessels, containing iron-filings and water, instead of the chafing-dishes; then some vitriolic acid was poured into those four vessels, sufficient to cause a vigorous effervescence, and when the strongest ebullition was going to subside, the metal plate of the condenser was removed from over the marble, and being examined, not only electrified the electrometer with negative electricity, but gave a sensible spark. At this time, having tried to obtain electricity from the evaporation of water, the effects were equivocal or hardly sensible. The same thing happened a few days after, when, however, we obtained clear signs of electricity from those effervescences, which produce fixed and nitrous air. One day the electricity arising from the evaporation of water seemed to be positive; but subsequent experiments, and other circumstances, indicate that such a phenomenon must be attributed to a mistake.

“The experiment on the evaporation of water, which did not answer so well at Paris, succeeded much better in London, where I bethought me of throwing water on the lighted coals, which were kept in an insulated chafing-dish. In this manner the electricity of the evaporation never fails to electrify the chafing-dish negatively, and strongly enough for the electricity to be discovered by the simple electrometer: it will even afford a spark, if the condenser be used. Another time this experiment was repeated with success at Mr. Cavallo's, in the following manner :-A small crucible, containing three or four small coals lighted, was insulated; then a spoonful of water was thrown on the coals, and immediately after an electrometer, which communicated with the coals by means of a wire, diverged with negative electricity.

“The experiments hitherto made, though not numerous, yet concur to shew that the vapour of water, and in general the parts of all bodies that are separated by volatilization, carry away an additional quantity of the electric fluid, as well as elementary heat, and consequently that those bodies, from the contact of which the olatile particles have been separated, remain both cooled and elec

trified negatively; from which it may be deduced, that whenever bodies are resolved into a volatile elastic fluid, their capacity for holding the electic fluid is augmented, as well as their capacity for holding common fire, or calorific fluid.* This is a striking analogy by which the science of electricity throws some light on the theory of heat, and alternately derives light from it : I mean on the doctrine of latent or specific heat, the first notions of which were suggested by the admirable experiments of Dr. Black and M. Wilkie, and which has been afterwards much elucidated by Dr. Crawford, who followed the experiments of Dr. Irwin. By following this analogy, it seems that as the vapours, on their condensing, lose part of their latent heat, on account of their capacity being diminished, so also they part with some electric fluid. Hence originates the positive electricity, which is always more or less predominant in the atmosphere when the sky is clear, viz., at the height at which the vapours begin to be condensed. Accordingly, the atmospherical electricity is stronger in fogs, in which case the vapours are more condensed, so as to be almost reduced into drops, and is still stronger when thick fogs become clouds.

“ Hitherto we have accounted for the positive electricity, but it is easy to account for clouds negatively electrified ; for when a cloud, positively electrified, has been once formed, its sphere of action is extended a great way round, so that if another cloud comes within that sphere, its electric fluid, agreeably to the well-known laws of electric atmospheres, must retire to the parts of it which are most remote from the first cloud ; and from thence the electric fluid may be communicated to other clouds or vapours, or terrestrial prominences. Thus, a cloud may be electrified negatively, which cloud, after the same manner, may occasion a positive electricity in another cloud, &c. This explains not only the negative electricity, which is often obtained from the atmosphere in cloudy weather, and the frequent changes from positive to negative electricity, and contrarywise, in stormy weather; but also the waving motion often observed in clouds, and the hanging down of them so as nearly to touch the earth. After the forementioned discoveries, we need no longer wonder at the appearance of lightnings in the eruptions of volcanoes, as was particularly observed in the late dreadful eruption of Mount Vesuvius. The few experiments I have made, shew that the quantity of smoke, but much more the rapidity by which it is produced, tends to increase the electricity which arises from combustion, &c. How great, then, must be the quantity of electricity that is produced in such eruptions ?”

It will be observed, by comparing the opinions of Dr. Franklin and M. Volta, respecting the electricity of the clouds, that they are very different to each other. Franklin considers the general electric

This is precisely the view taken by Dr. Franklin, at least 33 years previously.-EDIT.

Now,

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condition of the clouds to be negative, and that positively electric clouds are comparatively but of rare occurrence. Volta, on the other hand, considers the general electric condition of clouds to be positive, and accounts for negatively electric clouds as a secondary occurrence, occasioned by the repulsive influence of a more formidable electro-positive cloud. Hence Volta's opinion is, that all clouds, at their formation, are positively electric. although the latter view is exactly the same our own, it must be confessed that M. Volta has by no means shewn how it happens that vapour becomes positively electrical. That philosopher, according to the views of Dr. Franklin, considers that water, when converted into vapour, assumes a greater capacity for the electric fluid than before; and, therefore, would become negatively electrical the moment that it assumes the shape of vapour, conformable to Franklin's can and chain apparatus. That this is the fact we are quite certain, even though the

vessel, with its remaining water, become negatively electric also. To clear up this point, we have only to consider that, as soon as the first stratum of steam or vapour is formed on the surface of the water, it has a greater capacity for the electric fluid than before, and consequently, for a short time, assumes a negative electric condition : but this new electric condition of the vapour allows of its being invaded by a portion of that electric-fluid which was left in the water, and also by a portion of that of the vicinal air ; by which means it becomes supplied to a certain extent, and thus becomes less negative than at first, and in a short time afterwards it will have attained the whole quantity that is due to it as a vapour, but not more: therefore, it would be impossible for the vapour to assume a positive electric state by the simple process of evaporation, though it is quite certain that it may assume a negative electric state for a considerable time when its formation is very rapidly carried on. Why, then, did Volta's experiments shew that the cloud of vapour was positively electrical ? Volta's experiments did not shew that the vapour was positively electrical. The indications of the electrometer, in those highly interesting experiments, were occasioned by the vapour condensing, and again becoming water, on the metallic appendages of his electroscope. The vapour, being condensed, had its capacity for the electric fluid lessened, and now yielded to the instrument a portion of the electric fluid which it had absorbed whilst in the state of vapour : hence the reason of the electroscope indicating positive electric action. It is easy to see how the dish and water would be left in a negative electric condition, which would also be the case with the vicinal air. It is thus, also, that aqueous vapours from the earth, by assuming a greater capacity for the electric fluid than they previously had whilst in the state of water, absorb, and carry up, an immense quantity of it; and that vapour must either be electronegative or electro-neutral, until condensation again takes place, and can never become electro-positive until the degree of condensation

is sufficiently great to lower the electric capacity beyond that required to maintain an electric equilibrium with the surrounding air.

The curiosity of philosophers, concerning Volta's experiments, having soon become subsided, we hear very little more of the production of electricity by evaporation till the celebrated experiments of Messrs. Armstrong and Pattison, in the neighbourhood of Newcastle-upon-Tyne, the particulars of which have been made known to the readers of these Annals of Electricity, &c., vol. v., pp. 452, 453, 456. Vol. vi., pp. 37, 42, 305, 311.

The results of the scientific part of these famous experiments are perfectly reconcilable to the theoretical views we have already taken respecting the electrical condition of vapours in M. Volta's original experiments, viz., that vapour or steam, at its first production, is in a negative electrical condition, and that it is only by recondensation that it shews positive electric action.

Dr. Charles Schafhaentl made a series of experiments, from which he also concludes that the developement of the positive electricity is occasioned by “the sudden condensation and separation of water from the steam."* But he also supposes that the steam itself, in order to produce electricity, should be produced in a peculiar manner, “by the sudden boiling of the water, and the conversion of a portion of it into a fine spray, &c.” Such an inference might also be drawn from Volta's experiments, in which he obtained the greatest electric action by throwing a spoonful of water on burning coals. There can be no doubt of the action being greater under these circumstances ; but we are far from considering that this condition is essential to the absolute production of electric action by evaporation. Indeed, our own experiments shew the contrary.-Edit.

• Dr. Schafhaentl's experiments and reasoning thereon will shortly be placed before the readers of these Annals.

An Account of the Magnetic Observations made at the Observatory

of Harvard University, Cambridge, by Joseph LOVERING, Hollis Professor of Mathematics and Natural Philosophy, and W. Cranch Bond, Astronomical Observer to the College.

(Memoirs of the American Academy.)

(Concluded from page 112.)

In 1837, Gauss published his “ Allgemeine Theorie des Erdmagnetismus.” This was the first attempt to subject the problem of the earth’s magnetism to strict mathematical analysis. The solution was embarrassed and complicated, being of the nature of those which had already been performed in determining the figure of the earth and the tides. It required the use of Laplace's celebrated co-efficients, a powerful instrument, but difficult of management.

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