Radiant Heat, and its Conversion thereby into Sound. 443 



vapour of our atmosphere was again taken up, and proved to 

 be, not 18 times, as I had at first supposed, but, on fairly 

 humid days, at least 60 times that of the air in which it was 

 diffused. When carefully dried air was caused to pass over 

 moistened glass, and then carried into the experimental tube, 

 the absorption was still greater. 



A power has been claimed for mist or haze which has been 

 denied to aqueous vapour ; but in these experiments concen- 

 trated luminous beams, which would have infallibly brought 

 into view the least trace of suspended matter, revealed no 

 mist or dimness of any kind. It is, moreover, demonstrable 

 that an amount of turbidity, rendered strikingly evident by a 

 luminous beam, exerts only a fractional part of the action of 

 the pure aqueous vapour. When well-dried air Avas led, not 

 through water or over wet glass, but over bibulous paper, 

 taken apparently dry from the drawers of the laboratory, the 

 amount of vapour carried forward, from the pores of the paper 

 produced 72 times the absorption of the air which carried it. 

 After five repetitions of the experiment, wherein the same air 

 was carried over the same paper, a quantity of vapour was 

 still sent forward capable of exerting 47. times the absorption 

 of the air in which it was diffused. 



Here the possible action of odours upon radiant heat 

 naturally suggested itself. Many perfumes were accordingly 

 subjected to examination, the odorous substance being in each 

 case carried into the experimental tube by a current of dry 

 air. Thus tested, pachouli exerted 30 times, cassio 109 times, 

 while aniseed exerted 372 times the absorption exerted by the 

 air in which it was diffused. 



A novel method of exhibiting the absorption and radiation 

 of gaseous bodies, the germ of which had been previously 

 discovered *, was illustrated and developed by the new appa- 

 ratus. Suppose the experimental tube exhausted, and the 

 needle, under the joint action of the two sources, to be at 0°. 

 On admitting a strong vapour the usual deflection would 

 occur. Suppose it to be 50 galvanometric degrees. Let dry 

 air be now introduced until the experimental tube is filled. 

 Although fresh matter is thus thrown athwart the rays of 

 heat, the needle behaves as if the matter within the experi- 

 mental tube had wholly disappeared. It sinks to zero, and 

 not only so, but passes, say to 50°, on the other side. 



After the first moments of perplexity succeeding the obser- 

 vation of this effect, its cause became clear. On entering the 

 experimental tube the air, having its vis viva destroyed, was 



* Philosophical Transactions, vol. cli. p. 32. 



