Radiant Heat, and its Concersion thereby into Sound. 457 

 A still further augmentation sent it up to 



75°. 



This last deflection was equivalent to more than 400 of the 

 degrees in the neighbourhood of zero. 



The radiating column was here considerably above the 

 flame. To examine the condition of the column, a concen- 

 trated luminous beam was directed upon it. There was no 

 precipitation. On the contrary, the suspended matter in the 

 air of the chimney was much less than that of the surrounding 

 air. Instead of a white mist, we had the blackness due to the 

 destruction of the floating matter by the hydrogen-flame. 



On quenching the flame, the needle returned accurately to 

 zero. 



In his objections Magnus, for the most part, dealt with true 

 causes; but he erred as to their scope of action. I never 

 denied the existence of the dangers which he emphasized. 

 The hygroscopic character of rock-salt, for example, to which 

 he recurred so often, cannot be questioned. It has a strong 

 attraction for moisture, especially when cold. On this point 

 my experience has been large; and I applied it in the execution 

 of my experiments. These, as I have so frequently stated, 

 were conducted with plates of salt as dry as polished glass or 

 rock-crystal. Thus, while conceding the fact that rock-salt is 

 hygroscopic, I demur to its application. 



A similar remark applies to the last solution offered by 

 Magnus of the differences between us. In 1867 he showed 

 that vapours were condensed by surface-attraction to a greater 

 extent than had previously been supposed. Blowing vapour- 

 laden air into a metal tube, he found that heat was generated. 

 He inferred, and rightly inferred, that this heat was produced 

 by the condensation which occurred on the interior surface. 

 This condensation he found to depend on the condition of the 

 surface, being greater when it was tarnished or coated than 

 when it was polished. He saturated air with moisture at a 

 temperature of 16° C, and then raised both it and his pile to 

 a temperature of 38°. When such air was blown against the 

 dry face of the pile, heat was generated. Condensation there- 

 fore occurred upon a surface 22° C. higher than the dew-point 

 of the vapour. Against this I have nothing to urge. But 

 the fact by no means justifies the inference drawn from it, 

 which was, that the \apours in my experimental tube were 

 converted by " vapour-hesion " into liquid layers of high 

 opacity to radiant heat; these layers, acting upon the calo- 

 rific rays impinging on the interior surface of my tube, pro- 

 duced the absorption which I had erroneously ascribed to the 



