RESPECTING SOUND AND LIGHT. 



541 



IX. Of the Vibrations of different elastic 

 Fluids. 



All the methods of finding the velocity of 

 sound agree, in determining it to be, in fluids 

 of a given elasticity, reciprocally in the sub- 

 duplicate ratio of the density : hence, in 

 pure hydrogen gas it should be ^13 = 3.6 

 times as great as in common air ; and the 

 pitch of a pipe should be a minor fourteenth 

 higher in this fluid than in the common air. 

 It is therefore probable, that the hydrogen 

 gas, used in Professor Chladni's late experi- 

 ments, was not quite pure. It must be ob- 

 served, that in an accurate experiment of 

 this nature, the pressure causing the blast 

 ought to be carefully ascertained. There 

 can be no doubt but that, in the observa- 

 tions of the French Academicians on the ve- 

 locity of sound, which appear to have been 

 conducted with all possible attention, the 

 dampness and coldness of the night air must . 

 have considerably increased its density : 

 hence, the velocity was found to be only 

 1 109 feet in a second ; while Derham's ex- 

 periments, which have an equal appearance 

 of accuracy, make it amount to 1142. Per- 

 haps the average may, as has been already 

 mentioned, be safely estimated at 1 130. It 

 may here be remarked, that the well known 

 elevation of the pitch of wind instruments, in 

 the course of playing, sometimes amounting 

 to half a note, is not, as is commonly sup- 

 posed, owing to any expansion of the instru- 

 ment, for this should produce a contrary ef- 

 fect, but to the increased warmth of tlie air 

 in the tube. Dr. Smith has made a similar 

 observation, on the pitch of an organ in sum- 

 mer and winter, which he found to differ 

 more than twice as much as the English and 

 JVench experiments on the velocity of 



sound. Bianconi found the velocity of sound, 

 at Bologna, to differ at different times, in 

 the ratio of 152 to 157. 



X. Of the Analogy between Light and Sound. 



Ever since the publication of Sir Isaac 

 Newton's incomparable writings, his doctrine 

 of the emanation of particles of light, from 

 lucid substances, has been almost universally 

 admitted in this country, and but little op- 

 posed in others. Leonard Euler indeed, in 

 several of his works, has advanced some 

 powerful objections against it, but not suffi- 

 ciently powerful to justify the dogmatical re- 

 probation with which he treats it ; and he 

 has left that system of an ethereal vibration, 

 which after Hu^'gens and some others he 

 adopted, equally liable to be attacked on 

 many weak sides. Without pretending to de- 

 cide positively on the controversy, it is con- 

 ceived that some considerations may be 

 brought forwards, which may tend to dimi- 

 nish the weight of objections to a theory 

 similar to the Huygenian. There are also 

 one or two difficulties in the Newtonian sys- 

 tem, which have been little observed. The 

 first is the uniform velocity with which light 

 is supposed to be projected from all luminous 

 bodies, in consequence of heat, or other- 

 wise. How happens it that, whether the 

 projecting force is the slightest transmission 

 of electricity, the friction of two pebbles, 

 the lowest degree of visible ignition, the 

 white heat of a wind furnace, or the intense 

 heat of the sun itself, these wonderful cor- 

 puscles are always propelled with one uni- 

 form velocity ? For, if they differed in velo- 

 city, that difference ought to produce a dif- 

 ferent refraction. But a still more insupera- 

 ble difhcuity seems to occur, in the partial 

 reflection from eveiy refracting surface.- 



