Radiant Heat, and its Conversion thereby into Sound. 489 



naked face of the pile was exposed to the common air of the 

 laboratory, it, of course, contracted a film corresponding to 

 the humidity of the air. The sweeping away of this film by 

 .dry air produced a very small deflection indicating cold. 

 When, on the contrary, humid air was urged against the salt, 

 the deflection indicating heat was prompt and large. We 

 ■ have now to test the action of the liquid film thus formed 

 , upon radiant heat. The circular plate of rock-salt before 

 referred to was mounted, with the incandescent spiral on one 

 side and the pile on the other. The slit -was so arranged 

 that dry and humid air could be sent in succession oyer the 

 surface of the plate. As before, the radiation from the spiral 

 i was neutralized by the compensating cube, the needle pointing 

 to zero when the two sources were in equilibrium. On sweep- 

 ing dry air oyer the plate of salt, so as to remove the film 

 contracted from the laboratory air, there was no motion of 

 the needle. On pouring humid air from the slit oyer the salt, 

 there was no motion of the needle. To moisten the air it was 

 urged from bottom to top through a tall jar filled with wet 

 bibulous paper. The same apparatus had furnished humid air 

 which produced a swing of 80° when urged against the thin 

 plate of rock-salt resting on the face of the pile. As a barrier 

 to radiant heat it was nevertheless powerless. The conden- 

 sation may even be considerably enhanced without producing 

 any sensible effect. Through a glass tube I urged my breath 

 against the plate of salt so as to produce the colours of thin 

 plates, without any sensible effect upon the galvanometer. 



Having thus clearly shown what films, even in an exag- 

 gerated form, cannot accomplish, I hope, in testing the 

 action of humid air, that the use of the silvered tube, closed 

 with polished plates of rock-salt, on which no trace of visible 

 moisture is deposited, will be conceded to me. This tube 

 involves the use of a source of heat of small dimensions. The 

 lime-light, though fulfilling this condition, is not suitable, 

 because of the high refrangibility of its heat. The incan- 

 descent platinum spiral would be better; still the radiation 

 from this source is but feebly absorbed by the aqueous vapour 

 taken up by air at ordinary temperatures. The oxyhydrogen 

 flame fulfils the required conditions best*. It has the advan- 

 tage of high temperature and low refrangibility; while the fact 



* I was careful to assure myself that, unless it amounted to the visible 

 wetting of the plates of salt, there was no sensible stoppage of the rays 

 from the oxyhydrogen flame. This quite agrees with the result obtained 

 by Magnus himself, in the experiment -with the concave mirror already 

 referred to. Unless he visibly wetted the mirror, he failed to impair the 

 energy of the reflected beam. 



