LETTER XXIV. - Euler
The true Principle of Nature, on which are founded all the Phenomena of Electricity.
The summary I have exhibited of the principal phenomena of electricity, has no doubt excited a curiosity to know what occult powers of nature are capable of producing effects so surprizing.
The greatest part of natural philosophers acknowledge their ignorance in this respect. They appear to be so dazzled by the endless variety of phenomena, which every day present themselves, and by the singularly marvellous circumstances which accompany these phenomena, that they are discouraged from attempting an investigation of the true cause of them.
They readily admit the existence of a subtile matter, which is the primary agent in the production of the phenomena, and which they denominate the electric fluid ; but they are so embarrassed about determining its nature and properties, that this important branch of physics is rendered only more perplexed by their researches.
There is no room to doubt, that we must look for the source of all the phenomena of electricity only in a certain fluid and subtile matter ; but we have no need to go to the regions of imagination in quest of it. That subtile matter denominated ether, whose reality I have already endeavoured to demontrate, is sufficient very naturally to explain all the surprizing effects which electricity presents. I hope I shall be able to set this in so clear a light, that you shall be able to account for every electrical phenomenon, however strange an -appearance it may assume.
The great requisite is to have a thorough knowledge of the nature of ether.
The air which we breathe rises only to a certain height above the surface of the earth ; the higher you ascend, the more subtile it becomes, and at last it entirely ceases. We must not affirm, that beyond the region of the air there is a perfect vacuum, which occupies the immense space in which the heavenly bodies revolve.
The rays of light which are diffused in all directions from these heavenly bodies, sufficiently demonstrate that those vast spaces are filled with a subtile matter.
If the rays of light are emanations forcibly projected from luminous bodies, as some philosophers have maintained, it must follow, that the whole space of the heavens is filled with these rays, nay that they move through it with incredible rapidity. You have only to recollect the prodigious velocity with which the rays of the sun are transmitted to us. On this hypothesis, not only would there be no vacuum, but all space would be filled with a subtile matter, and that in a state of constant and most dreadful agitation.
But I think I have clearly proved, that rays of light are no more emanations projected from luminous bodies, than sound is from sonorous bodies. It is much more certain, that rays of light are nothing else but a tremulous motion or agitation of a subtile matter, just as sound consists of a similar agitation excited in the air. And as sound is produced and transmitted by the air, light is produced and transmitted by that matter, incomparably more subtile, denominated ether, which consequently fills the immense space between the heavenly bodies.
Ether then is a medium proper for the transmission of rays of light, and this same quality puts us in a condition to extend our knowledge of its nature and properties. We have only to reflect on the properties of air, which render it adapted to the reception and transmission of sound. The principal cause is its elasticity or spring. You know that air has a power of expanding itself in all directions, and that it does expand, the instant that obstacles are removed. The air is never at rest, but when its elasticity is every where the same ; whenever it is greater in one place than another, the air immediately expands. We likewise discover by experiment, that the more the air is compressed, the more its elasticity increases : hence the force of air-guns, in whichthe air, being very strongly compressed, is capableof discharging the ball with astonishing velocity. The contrary takes place when, the air is rarefied : its elasticity becomes less in proportion as it is more rarified, or diffused over a larger space.
On the elasticity of the air, then, relative to its density, depends the velocity of sound, which makes a progress of about 1ooo feet in a second. If the elasticity of the air were increased, its density remaining the same, the velocity of sound would increase : and the same thing would take place if the air were more rare, or less dense than it is, its elasticity being the same. In general, the more that any medium, similar to air, is elastic, and at the same time less dense, the more rapidly will the agitations excited in it be transmitted. And as light is transmitted so many thousand times more rapidly than sound, it must clearly follow, that the ether, that medium whose agitations constitute light, is many thousand times more elastic than air, and, at the same time, many thousand times more rare or more subtile, both of these qualities contributing toaccelerate the propagation of light.
Such is the reason which leads to conclude, that ether is many thousand times more elastic and more subtile than air ; its nature being in other respects similar to that of air, in as much as it is likewise a fluid matter, and susceptible of compression and of rarefaction. It is this quality which will conduct us to the explanation of all the phenomena of electricity.