a method of accounting for thofe effects, by vibrating inflexible folids, muft appear uncouth to thofe, who have been accuftom'd to believe, that the pores of denfe bodies are replete with a fubtile elaftic fluid, in the manner Sir Ifaac Newton affirmed, and as verified by electrical experiments *. 102. Could the matter be put to the test of a proper trial, the velocity of founds, by means of the medium contained in thofe bodies, I am fully perfuaded would appear equally instantaneous as the velocity of this fluid is found to be in electrical experiments. 103. Quære. Whether it be not unnatural to fuppofe fuch an extraordinary vibration of the parts of folid bodies, when at the fame time, neither reafon or any one sense informs us of any such motion in their parts. 104. N. B. A fmooth furface feems necef fary to the body that propagates the found $ but the chief requifite is, moft probably, the * To fhew I am not fingular in fuppofing that Sounds are propagated through denfe bodies by means of an elaftic fluid exifting in their pores, I fhall cite a quotation of Mr Jones's taken from Seneca, 'That a Medium muft be conceal'd within the most folid bodies; because found would not otherwife be tranfmitted through them as it moft certainly is. This is one of Du Hamel's arguments for a fubtile Ether. See Mr. Jones's First Principles of natural Philofophy. p. 213. H folidity folidity of the body, whofe pores are so minute, as to admit of the more pure and more elastic air only. Mr. Clare has a great opinion of the former, as appears by the following narrative. 1105. It might feem incredible,' faith he, that the voice of a Man might be distinctly heard at the diftance of ten or twelve miles: But a Gentleman of great veracity, who liv'd fome years at Gibraltar affirms, that he has at Old-Gibraltar heard the Watch-word of the night, viz. All's Well, given by the cen tinels to the patrole, as they paffed all along the Ramparts of New-Gibraltar, in a still serene night, when the water was perfectly fmooth, as plainly and diftinctly, as the Officers who walked the round, or himself ( had • he been upon the Rampart) could have done. . This is a fufficient proof of the fervice it is to places of hearing, that the furfaces of things fhould be in them as fmooth as poffible.' Mr. Clare's Fluids. p. 303. 6 106. Since the foregoing experiments fo clearly prove the existence of a fubtile fluid in the pores of grofs bodies, and in which there appears, not only a remarkable elasticity, but alfo a continuity of its parts: Can it then be any longer doubted, whether the motion of it in the experiments be propagated through the internal parts of the feveral bodies, or whether over and along their furfaces, as has been fuggefted? But if that can still be doubted; let may a wire of any imaginary length be fupported with filken ftrings through ever fo many rooms, and back again, one end of which to communicate with the prime conductor; by this means, when the wheel is in motion, the whole length of the wire will be electrifed, i. e. the active fluid will be accumulated on every part, as be easily proved by drawing fparks from it, and by feeing how brifkly light bodies are agitated, when placed near to any part of it on a plate: But if any part of the wire be made to communicate with the floor either directly or indirectly, all the Phænomena feem then at an end. To illuftrate this: Let a Perfon take hold of the infulated wire, even at the remoteft part from the Machine, and all the afore-mentioned accumulation vanishes. ; the light bodies all lie ftill on the plate, nor can any spark be drawn from any part of the Apparatus: A Perfon therefore unacquainted with thofe experiments would think all was at an end; whereas the fluid is all the time swiftly paffing into the earth through the internal fubftance of the wire, and through the Person who holds it at the end. To prove this: Let the fupported wire be clipp'd afunder at parts where it is fo fupported, that the two ends may not drop from each other, and the Fire will plainly appear between every fuch feparation, even though the experiment were made at noon day: Then let the Perfon release the end of the wire, or elfe ftep on the Refin-cake (either H 2 (either of which cuts off the communication with the earth) and then the accumulation again takes place; the light bodies alfo on the plate will be in motion as before, but no Fire appears at the feparations; for that Phænomenon never happens, unless the part of the wire beyond the feparations communicates with the floor. 107. Another effectual method to prove that it moves through the internal parts of nonelectrical bodies, may be the following: 3 108. Let Four Corks be fitted to a glass Tube as A B, having a wire through each of them and two of thofe Corks as CD, with their wires g h, and k 1, be thruft in towards the middle of the Tube, fo far, that the two Loops of the wires h and k, which project beyond the ends of the Corks, may come near each other in the middle of the Tube. After which, let the two ends of the Tube E F, be filled with water, and then stopped with the other two Corks G and H. Pl. II. Fig, 1. ་ 109. Let the glass thus prepared be made a part of the foremention'd Line of insulated wires, by taking out a part of the wire, and putting the tube in its ftead: This line thus compounded of the prime conductor, the di vided wires and the water-tube, will ftill anfwer the fame ends as before in every refpect. A Person standing on the Refin and communicating with the extremity or other parts of the wire is electrifed in the fame manner as at other times: The fire appears at every feparation of the wire as before, if a fpark be drawn from the electrifed Perfon, or the wire with which he communicates. 110. And in a word, the propenfity of this elaftic fluid to maintain an equilibrium among all its parts is so amazingly great, that if a spark be drawn off, tho' from the most distant part, a correfponding spark appears, not only at each feparation of the wires, but also between the two wires in the middle of the water-tube. III. Having thus far confider'd the great fubtilty and elafticity of the electrical Medium, I fhall in the following Chapter explain more particularly the natural effect of an elaftic fluid, viz. that of its expanding. INTUMESCENCE, &c. of an elaftic Fluid. *NTUMESCENCE is that Pro 味 I perty of an elastic Fluid, of expanding, 113. tending itfelf to larger Dimenfions. I This will be the better understood, if with Mr. Boyle, we conceive the primary Particles, |