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Historical Sketch of the Production of Electricity by Evaporation,
and of Evaporation by Electricity. The rage for steam electricity having now become greatly abated, we will endeavour to collect such theoretical opinions concerning the cause as we can find in the journals of the day; and also those which were entertained many years ago.
We are not certain that Dr. Franklin was the first philosopher who entertained the idea of the electricity in clouds being taken up from the earth in its aqueous vapours; but such was his opinion, however, as early as the year 1749, as is clearly stated in his fifth letter, which is highly probable to be the first time that such an idea received publicity. From the results of many experiments on atmospherical electricity, Franklin was led to suppose that the generality of thunder clouds are negatively electrical with respect to the earth. Hence he says, in his twelfth letter, “For the most part, in thunder strokes, it is the earth that strikes into the clouds, and not the clouds that strike into the earth.” And in order to give an explanation for this negative electric condition of clouds, he proceeds thus :
“I conceive that this globe of earth and water, with its plants, animals, and buildings, have diffused throughout their substance a quantity of the electric fluid, just as much as they can contain, which I call the natural quantity. That this natural quantity is not the same in all kinds of common matter under the same dimensions, nor in the same kind of common matter in all circumstances ; but a solid foot, for instance, of one kind of common matter, may contain more of the electric fluid than a solid foot of some other kind of common matter; and a pound weight of the same kind of common matter may, when in a rarer state, contain more of the electric fluid than when in a dense state. For the electric fluid
Ann. of Elec.-Vol. VIII.-No. 45.-March, 1842.
being attracted by any portion of common matter, the parts of that fluid (which have among themselves a mutual repulsion) are brought so near to each other by the attraction of the common matter that absorbs them, as that their repulsion is equal to the condensing power of attraction in common matter; and then such portion of common matter will absorb no more.
“Bodies of different kinds having thus attracted and absorbed what I call their natural quantity, i. e., just as much of the electric fluid as is suited to their circumstances of density, rarity, and power of attracting, do not, then, shew any signs of electricity among each other; and if more electric fluid be added to one of these bodies, it does not enter, but spreads on the surface, forming an atmosphere ; and then such body shews signs of electricity.
“I have, in a former paper, compared common matter to a sponge, and the electric fluid to water; I beg leave once more to make use of the same comparison, to illustrate further my meaning in this particular. When a sponge is somewhat condensed by being squeezed between the fingers, it will not receive and retain so much water as when in its more loose and open state. If more squeezed and condensed, some of the water will come out of its inner parts and flow on the surface. If the pressure of the fingers be entirely removed, the sponge will not only resume what was lately forced out, but attract an additional quantity. As the sponge, in its rarer state, will naturally attract and absorb more water, and in its denser state will naturally attract and absorb less water, we may call the quantity it attracts and absorbs, in either state, its natural quantity, the state being considered.
“Now, what the sponge is to water, the same is water to the electric fluid. When a portion of water is in its common dense state, it can hold no more electric fluid than it has; if any be added, it spreads on the surface. When the same portion of water is rarified into vapour, and forms a cloud, it is then capable of receiving and absorbing a much greater quantity: there is room for each particle to have an electric atmosphere. Thus water, in its rarified state, or in the form of a cloud, will be in a negative state of electricity ; it will have less than its natural quantity—that is, less than it is naturally capable of attracting and absorbing in that state. Such a cloud, then, coming so near the earth as to be within the striking distance, will receive from the earth a flash of the electric fluid ; which flash, to supply a great extent of cloud, must sometimes contain a great quantity of that fluid.”
Again, our philosopher states :—“Thus thunder-clouds are generally in a negative state of electricity compared with the earth, agreeably to most of our experiments ; yet, as by one experiment we found a cloud electrized positively, I conjecture that, in that case, such cloud, after having received what was, in its rare state, only its natural quantity, became compressed by the driving winds, or some other means, so that part of what it had absorbed was forced out,
and formed an electric atmosphere around it in its dense state. Hence it was capable of communicating positive electricity to any rod."
The principal object for bringing forward these quotations, is to shew that, in the opinion of this great philosopher, the aqueous vapour rising from the surface of the earth is negatively electrical with respect to the natural electrical condition of the water from which it ascended. There are, however, other topics of high theoretic importance contained in these few paragraphs. One of the great beauties in Franklin's theory is, that, under natural circumstances, each peculiar kind of matter is possessed of a specific quantity of the electric fluid, an idea parallel to that of Dr. Black, respecting the specific heat of bodies; and may eventually, perhaps, become of parallel practical importance. In support of this part of his theory, Dr. Franklin invented a very interesting experiment with a can and chain, now well known amongst electricians. A brass chain, of some few yards long, was coiled in a silver can, and a certain degree of electric action, measured by an electrometer, was given to this apparatus by a spark from a machine. One end of the chain was now lifted up, by a silk thread previously attached to it, until most of the chain was out of the can. With this increased surface the electric tension became much lessened ; but when the chain was again let down into the can, the electric tension increased almost to its original degree.
“Thus,” says Franklin, we see that increase of surface makes a body capable of receiving a greater electric atmosphere. But this experiment does not, I own, fully demonstrate my new hypothesis ; for the brass and silver still continue in their solid state, and are not rarified into vapour, as the water is in the clouds. Perhaps some future experiment on vapourized water may set this matter in a clearer light. One seemingly material objection arises to the new hypothesis, and it is this. If water in its rarified state, as a cloud, requires, and will absorb, more of the electric fluid than when in its dense state as water, why does it not acquire from the earth all it wants at the instant of its leaving the surface, while it is yet near, and but just rising in vapour ?"
In this doubtful position, we believe, Dr. Frankin left the subject, and it was not till many years after that Volta set the matter somewhat at rest, by producing electric action on the conversion of water into steam. In Franklin's 59th letter, dated Paris, 1767, we find that he has changed his opinion respecting the electricity of the clouds ; for he there says—“The clouds have often more of this fluid in proportion than the earth ; in which case, as soon as they come near enough (that is, within striking distance), or meet with a conductor, the fluid quits them and strikes the earth.”
About the year 1752, the Abbé Nollet pursued a series of experiments on the evaporation of water and other liquids by electric agency. The following were the results :
“1. Electricity augments the natural evaporation of fluids ; since, excepting mercury, which is too heavy, and the oil of olives, which is too viscous, all the others which were tried suffered a diminution which could not be ascribed to any other cause than electricity.
“ 2. Electricity augments the evaporation of those fluids the most which are most subject to evaporate of themselves, for the volatile spirit of sal ammoniac suffered a greater loss than spirit of wine or turpentine ; these two more than common water; and water more than vinegar or the solution of nitre.
"3 Electricity has a greater effect upon fluids when the vessels which contain them are non-electrics ; the effects always seeming to be a little greater when the vessels were of metal than when they were of glass.
4. This increased evaporation was more considerable when the vessel which contained the liquor was more open, but the effects did not increase in proportion to their apertures; for when these liquors were electrified in vessels, whose aperture was four inches in diameter, though they presented to the air a surface sixteen times larger than when they were contained in vessels whose aperture was one inch in diameter, they were, nevertheless, far from suffering a diminution proportioned to that difference.
“5. Electricity does not make any liquors evaporate through the pores either of metal or of glass ; since after experiments, which were continued ten hours, there was found no diminution of their weight, when the vessels in which they were contained were well stopped.”
On these experiments of the Abbé Nollet, Beccaria remarks, “The Abbé Nollet has found, by accurate experiment, that the electricity without sparks promotes the evaporation of liquids proportionally, not so much to their surface as to their natural propensity to evaporate; the real fact is this, the evaporation is not, and cannot be proportioned to the absolute surface (ceteris partibus) of liquids, but must be, and is, proportioned to the liberty of the surfaces themselves. By the words liberty of the surfaces, I mean their not being counteracted by an electricity homologous to that by which they are themselves animated.” The illustrious Beccaria then proceeds to the following description of experiments which he made on the same inquiry.
“I procured small cans of tin, two inches high, cylindrical, and eight lines wide. I filled one with water, and placed it at the bottom of the electric well : * another of the cans I left in the open air, and put into it only half an inch of water, and placed it six feet distant from the prime conductor. I placed next to it a third can, entirely filled with water; afterwards I insulated both the cans and the electric well, and made them communicate with the conductor
• For a description of the electric well, see “ Annals of Electricity, &c.," vol. vi. p. 415, plate r.
by iron wires that touched their bottoms. Now, neither the can placed at the bottom of the well, nor that in which the surface of the water was 18 lines lower than the brim of it, had, in three hours time, lost anything of their weight, but the can which was quite full had lost a grain, and even something more, of its weight.
"1. I suspended to the conductor one of the same cans, full of water, at the distance of three inches from the inferior surface of it, and to the bottom of this can I fastened a plate of tin, one foot wide. 2. I fixed another can on the conductor itself, which was also full. 3. I insulated two other cans, at six feet distance from the conductor; the one was on the one side of the conductor, and the other on the other side. Over the one I suspended, at five inches distance from its surface, a circular plate of tin, one foot diameter. The electricity having been continually excited for three hours together, I found that the can suspended to the conductor had lost nothing of its weight; that which was placed upon it had lost about half a grain ; and that which had nothing suspended over it had lost about a grain ; and the last, above which a plate of tin was suspended, had lost a grain and a half of its weight.
“Hence we see, that the evaporation was nothing where the surface of the water was counteracted on all parts by an homologous electricity. In the can placed on the conductor, only a little evaporation had taken place, because the atmosphere of the conductor bent itself over the surface of the water in that can, and lessened the evaporation. In the can placed at a great distance from the conductor, the evaporation was great, but it was still greater in the can over which the plate of tin was suspended, because a contrary electricity was actuated in that plate by the atmosphere of the can. In consequence of this, a continual dissipation of vapour was effected by invisible effluvia of the electric fire, which, through the plate were enabled to be diffused away. From the same fact we may also derive a new confirmation of the principle above mentioned, that the electric fire carries and disposes conducting (defferent) bodies in its
“Thence we see, that the evaporation of liquids is not always proportioned to the ampleness of their surfaces ; because, as the electric atmospheres of the liquids are much less counteracted by the reaction of the atmosphere of the adjacent parts, close to the sides of the vessels in which they are contained, than in other places, their evaporation must, of course, be proportionally greater.
“A number of other circumstances may also alter the quantity of electrical evaporation. 1. A square vessel will, as I have experienced, produce a greater evaporation than a cylindrical one, because the electric fire more easily dissipates through the angles of such vessels. 2. The liquor in a given vessel will evaporate the faster, as the height of its edge above the surface of the water will be in a smaller ratio to the width of it; this is because the electricity of the edge will then counteract less the electricity which promotes the