8<5 Experiments and Inquiries refpeEilng Bound and Light. 



therefore probable that the hydrogen gas ufed in Profeflbr Chladni's late experiments, was 

 not quite pure. It muft be obferved, that in an accurate experiment of this nature, the 

 preflure caufing the blaft ought to be carefully afcertained. There can be no doubt but 

 that, in the obfervations of the French Academicians on the velocity of found, which 

 appear to have been conduced with all poflible attention, the dampnefs and coldnefs of the 

 night air muft have confiderably increafed its denfity : hence, the velocity was found to be 

 only 1 109 feet in a fecond ; while Derhanj's experiments, which have an equal appearance 

 of accuracy, make it amount to 1142. Perhaps the average may, as has been already 

 mentioned, be fafely eftimated at U30. It may here be remarked, that the well known 

 elevation of the pitch of wind inftruments, in the courfe of playing, fometimcs amounting 

 to half a note, is not, as is commonly fuppofed, owing to any expanfion of the inftrument, 

 for this (hould produce a contrary effeft, but to the increafed warmth of the air in the tube. 

 Dr. Smith has made a fimilar obfervation, on the pitch of an organ in fummer and winter, 

 which he found to differ more than twice as much as the Englifh and French experiments 

 on the velocity of found. Bianconi found the velocity of found, at Bologna, to differ 

 at different times, in the ratio of 152 to 157. 1 



X. Of the Analogy hetnueen Light and Sound. 



Ever fince the publication of Sir Ifaac Newton's incomparable writings, his doftrines of 

 the emanation of particles of light from lucid fubftances, and of the formal pre-exiftence 

 of coloured rays in white light, have been almoft univerfally admitted in this country, 

 and but little oppofed in others. Leonard Euler indeed, in feveral of his works, has 

 advanced fome powerful objeftions againft them, but not fufficiently powerful to juftify 

 the dogmatical reprobation with which he treats them ; and he has left that fyftem of an 

 ethereal vibration, which after Huygens and fome others he adopted, equally liable to be 

 attacked on many weak fides. Without pretending to decide pofitively on the controverfy, 

 it is conceived that fome confiderations may be brought forwards, which may tend to 

 diminifli the weight of objeftions to a theory fimilar to the Huygenian. There are alfo 

 one or two difficulties in the Newtonian fyftem, which have been little obferved. The 

 iirft is, the uniform velocity with which light is fuppofed to be proje£ted from all luminous 

 bodies, in confequence of heat, or otherwife. How happens it that, whether the pro- 

 jefting force is the flighteft tranfmiffion of ele£kricity, the fri£li6n of two pebbles, the 

 Joweft degree of vifible ignition, the white heat of a wind furnace, or the intenfe heat of 

 the fun itfelf, thefe wonderful corpufcles are always propelled with one uniform velocity ? 

 For, if they differed in velocity, that difference ought to produce a different refradion. 

 But a ftill more infupera:ble difficulty feems to occur, in the partial refle£tion from every 

 refrafting furface. Why, of the fame kind of rays, in every circumftance precifcly 

 fimilar, fome fliould be always reflefted, and others tranfmitted, appears in this fyftem to 

 be wholly inexplicable. That a medium rcfembling, in many properties, that which has 

 beta denominated ether, does really exift, is undeniably proved by the phxnomena of 

 4 cleftricityi 



