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reach of attraction, and being driven away with ex. ceeding great velocity by the force of reflection.*
Many of the chymical views which Sir Isaac thus published in the form of queries were in his own lifetime illustrated and confirmed by Dr. Stephen Hales, in his book on Vegetable Statics,-a work of great originality, which contains the germ of some of the finest discoveries in modern chymistry.
Although there is no reason to suppose that Sir Isaac Newton was a believer in the doctrines of alchymy, yet we are informed by the Reverend Mr. Law that he had been a diligent student of Jacob Behmen's writings, and that there were found among his papers copious abstracts from them in his own handwriting.f He states also that Sir Isaac, together with one Dr. Newton, his relation, had, in the earlier part of his life, set up furnaces, and were for several months at work in quest of the philosopher's tincture. These statements may receive some confir. mation from the fact, that there exist among the Portsmouth papers many sheets, in Sir Isaac's own writing, of Flammel's Explication of Hieroglyphic Figures, and in another hand, many sheets of Wile liam Yworth's Processus Mysterii Magni Philosophicus, and also from the manner in which Sir Isaac requests Mr. Aston to inquire after one Borry in Holland, who always went clothed in green, and who was said to possess valuable secrets; but Mr. Law has weakened the force of his own testimony, when he asserts that Newton borrowed the doctrine of attraction from Behmen's first three propositions of eternal nature.
* Mr. Herschel, in his Treatise on Light, 553, has maintained that Newton's Doctrine of Reflection is accordant with the idea that the attractive force extends beyond the repulsive or reflecting force. In the query above referred to, Sir Isaac, in the most distinct manner, places the sphere of the reflecting force without that of the attractive one.
† In a tract annexed to his Appeal to all that doubt or disbelieve the truths of the Gospel. See Gent. Mag. 1782, vol. lii. p. 227, 239.
It is stated in a letter of Mr. Law's, quoted in this magazine, that Charles I. was a diligent reader and admirer of Jacob Behmen; that he sent a well-qualified person from England to Goerlitz, in Upper Lusatia, to acquire the German language, and to collect every anecdote he could meet with there relative to this great alchymist.
On the 7th December, 1675, Sir Isaac Newton communicated to the Royal Society a paper entitled An hypothesis explaining properties of light, in which he, for the first time, introduces his opinions respecting ether, and employs them to explain the nature of light, and the cause of gravity. “He was induced,” he says, “to do this, because he had observed the heads of some great virtuosos to run much upon hypotheses, and he therefore gave one which he was inclined to consider as the most probable, if he were obliged to adopt one."*
This hypothesis seems to have been afterward a subject of discussion between him and Mr. Boyle, to whom he promised to communicate his opinion more fully in writing. He accordingly addressed to him a long letter, dated February 28th, 1678-9, in which he explains his views respecting ether, and employs them to account for the refraction of light,—the cohesion of two polished pieces of metal in an exhausted receiver,--the adhesion of quicksilver to glass tubes,
the cohesion of the parts of all bodies,—the cause of filtration,—the phenomena of capillary attraction, —the action of menstrua on bodies,—the transmutation of gross compact substances into aerial ones, —and the cause of gravity. From the language used in this paper, we should be led to suppose that Sir Isaac had entirely forgotten that he had formerly treated the general subject of ether, and applied it to the explanation of gravity. “I shall set down,” says he, “one conjecture more which came into my mind now as I was writing this letter; it is about the cause of gravity,” which he goes on to explain ;t and
* In a letter to Dr. Halley, dated June 20th, 1686, Sir Isaac refers to this paper, and observes, that it is only to be looked upon as one of his guesses that he did not rely upon.
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he concludes by saying, that "he has so little fancy to things of this nature, that, had not your encouragement moved me to it, I should never, I think, thus far have set pen to paper about them."
These opinions, however, about the existence of ether, Newton seems to have subsequently renounced; for in the manuscript in the possession of Dr. J. C. Gregory, which we have already mentioned, and which was written previous to 1702, he states, that ether is neither obvious to our senses, nor supported by any arguments, but is a gratuitous assumption, which, if we are to trust to reason and to our senses, must be banished from the nature of things; and he goes on to establish, by various arguments, the validity of this opinion. This renunciation of his former hypothesis probably arose from his having examined more carefully some of the phenomena which he endeavoured to explain by it. Those of capillary attraction, for example, he had ascribed to the ether “standing rarer in the very sensible cavities of the capillary tubes than without them,” whereas he afterward discovered their true cause, and ascribed them to the reciprocal attraction of the tube and the fluid. But, however this may be, there can be no doubt that he resumed his early opinions before the publication of his Optics, which may be considered as containing his views upon this subject.
The queries which contain these opinions are the 18th-24th, all of which appeared for the first time in the second English edition of the Optics. If a body is either heated or loses its heat when placed in vacuo, he ascribes the conveyance of the heat in both cases “ to the vibration of a much subtiler medium than air;" and he considers this medium as the same with that by which light is refracted and reflected, and by whose vibrations light communicates heat to bodies, and is put into fits of easy reflection and transmission.
This ethereal medium, according to our author, is exceedingly more rare and more elastic than air. It pervades all bodies, and is expanded through all the heavens. It is much rarer within the dense bodies of the sun, stars, planets, and comets, than in the celestial spaces between them, and also more rare within glass, water, &c. than in the free and open spaces void of air and other grosser bodies. In passing out of glass, water, &c. and other dense bodies into empty space, it grows denser and denser by degrees, and this gradual condensation extends to some distance from the bodies. Owing to its great elasticity, and, consequently, its efforts to spread in all directions, it presses against itself, and, consequently, against the solid particles of bodies, so as to make them continually approach to one another, the body being impelled from the denser parts of the medium towards the rarer with all that power which we call gravity.
In employing this medium to explain the nature of light, Newton does not suppose, with Descartes, Hooke, Huygens, and others, that light is nothing more than the impression of those undulations on the retina. He regards light as a peculiar substance, composed of heterogeneous particles thrown off with great velocity, and in all directions, from luminous bodies; and he supposes that these particles while passing through the ether, excite in it vibrations or pulses which accelerate or retard the particles of light, and thus throw them into their alternate fits of easy reflection and transmission.
Hence, if a ray of light falls upon a transparent body, in which the ether consists of strata of variable density, the particles of light acted upon by the vibrations which they create will be urged with an accelerated velocity in entering the body, while their velocity will be retarded in quitting it. In this manner he conceives the phenomena of refraction to be produced, and he shows how in such a case the
refraction would be regulated by the law of the sines.
In order that the ethereal medium may produce the fits of easy reflection and transmission, he conceives that its vibrations must be swifter than light. He computes its elasticity to be 490,000,000,000 times greater than that of air, in proportion to its density, and about 600,000,000 times more rare than water, from which he infers that the resistance which it would oppose to the motions of the planets would not be sensible in 10,000 years. He considers that the functions of vision and hearing may be performed chiefly by the vibrations of this medium, executed in the bottom of the eye, or in the auditory nerve by the rays of light, and propagated through the'solid, pellucid, and uniform capillamenta of the optic or auditory nerves into the place of sensation; and he is of opinion that animal motion may be performed by the vibrations of the same medium, excited in the brain by the power of the will, and propagated from thence by the solid, pellucid, and uniform capillamenta of the nerves into the muscles for contracting and dilating them.
In the registers of the Royal Society there exist several letters* on the excitation of electricity in glass, which were occasioned by an experiment of this kind having been mentioned in Sir Isaac's hypothesis of light. The society had ordered the experiment to be tried at their meeting of the 16th December, 1675; but, in order to secure its success, Mr. Oldenburg wrote to Sir Isaac for a more particular account of it. Sir Isaac being t'hus “put upon recollecting himself a little farther about it," remembers that he made the experiment with a glass fixed at the distance of the d of an inch from one end of a brass hoop, and only the sth of an inch from the other. Small pieces of thin paper were