PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 603 



oxjg'cn is mechanically, but thoroug-hly mixed with tlie hydro-carbon gas 

 before it arrives at the place of burning*. This is eflectcd by the Bunsen 

 burner, in which the air is admitted at the bottom and mixed with the gas 

 on its upward passage within the burner. 



The result of this simultaneous burning of both carbon and hydrogen 

 is an increased amount of heat and an almost entire absence of light. It 

 seems, therefore, to be essential to the production of light that the com- 

 bustion of the carbon should take place after that of the hydrogen. 



Incandescence. 



Steel filings dropped into a current of heated gases give forth brilliant 

 scintillations. Hare, soon after his invention of hydro-oxygen blow-pipe, 

 found that a pencil of lime lu^ld before it, in the burning gases, emitted a 

 light of intense brilliancy. Such a light, when its rays were thrown into 

 parallel lines by means of a parabolic mirror, has been seen in diffused 

 daylight at a distance of more than one hundred miles. But to assert that 

 light is generated because carbon or any other solid is incandescent, is not 

 to explain the phenomenon. 



Light is proved, beyond a doubt, to be the result of waves moving trans- 

 versely to the line of propagation; the solid from which it proceeds must, 

 therefore, have the power of prcxlucing such waves in the teth. The inter- 

 esting question to be settled is whetlier the solid itself, or the seth within 

 it, can be set into high vibratory action by means of waves of heat having 

 a lower rate of velocity. Eeasoniug from analogy, we must decide in the 

 aflBrmative. 



Wave Induction. 



Air waves have the power of exciting vibrations in solids which are 

 more rapid than the waves producing them. This fact was brought forci- 

 bly te my notice many years ago, when I found the low tone in which I 

 was conversing in a certain room was constantly followed, not by an echo, 

 but by a musical note of very high pitch. After a search, the sound was 

 found to proceed from a sheet of steel, six or eight feet long by as many 

 inches wide, standing on its end and resting against the wall. 



This sympathetic action can be accounted for by the laws of harmonics. 

 The proper tone of a bell is always accompanied by harmonic sounds readily 

 perceptible to a fine ear. It is asserted by some musicians that every 

 sound made by a musical instrument is thus accompanied. 



The vibratory action arising from periodic pulses sometimes appears to 

 be greater than the cause. This arises from the fact that a new impulse 

 is given just before the force of the previous impulse is expended. The 

 same remark may be applied to oscillations. In the gymnasium, tiie self- 

 swingers exert themselves only at the extremities of the arc. The danger 

 of regular pulses where weight is sustained is well known. Soldiers in 

 crossing a wooden bridge are required to break ranks and step out of 

 tune. I have ofteji seen the lonjr span of a timber bridge, which was firm 

 under the tread of a herd of cattle, thrown into quick vibration by the 

 rapid passage of a dog across it. 



The condition required iu this case is, that the tread of the dog shall 



