1881.] Radiant Energy into Sonorous Vibrations. b\9f 



the diathermancy of the exposed side of the cavity, on its dimensions 

 and surfaces, and on the absorbent character of the contained gas. 



The remarkable property which deposited carbon possesses of 

 reducing radiant energy to thermometric heat is strikingly shown by 

 these experiments, and it suggests an important field for inquiry for 

 those who are working in the region of radiant heat. 



It is only necessary to add that, in carrying out these experiments, 

 I have had the benefit of the presence and advice of Professor Hughes, 

 and the inestimable advantage of the great mechanical skill, philoso- 

 phical character, and experimental ability of Mr. Stroh, without whom, 

 in fact, the inquiry could not have been accomplished. 



Additional Note. Received March 14, 1881. 



Professor Stokes has suggested that the action is due not to the ex- 

 pansion of the contained vapour through its absorption of thermome- 

 tric heat generated on the lamp-black surface, but to the contact of the 

 air molecules with this surface. It is clear that when the carbon is 

 warmed, the rapidly moving air molecules which strike it and bound 

 off have their retreating motion increased in velocity. This increased 

 velocity means increased pressure, which in its turn produces increased 

 volume, and this when intermittent, produces sonorous vibrations. This 

 explanation is quite in accordance with the observed facts, for the 

 difference of the intensity of the sounds emitted by dry air and air 

 charged with absorbent vapours is very much less, when a lamp-black 

 surface is used, than was anticipated. Dry air gives excellent results 

 with lamp-black, but is silent without it. Indeed it leads one to 

 conceive that as charging the air with heavy smoke produces the 

 same effect as coating the containing surface with lamp-black, each 

 particle of smoke becomes a warmed surface from which the colliding 

 air molecules, whose dimensions are infinitely smaller, retreat with 

 increased velocity. Moreover, if we conceive the . smoke particles 

 replaced by the compound molecules of absorbent vapours such as sul- 

 phuric ether, olefiant gas, or ammonia, and if we assume the dimen- 

 sions of these molecules greater than those of the air, we have an 

 explanation not only of the absorbent power of these gases, but of the 

 reasons for their behaviour in converting radiant energy into sonorous 

 vibrations. It is clear that if the compound molecule act as a smoke 

 particle, the radiant energy becomes degraded into thermometric heat, 

 for the motion of the ether is transferred to the motion of the mole- 

 cule. The minute air molecules move unaffected in the undulating 

 ether, but the larger compound molecules of the absorbent gas take up 

 the waves of the ether, and assume that form of motion which is 

 known as thermometric heat. Hence, the greater the number of 

 molecules a?id the larger their dimensions, the greater the absorption 



2 o 2 



