Influence of Condensation on Diathermancy 249 



bodies — wax, paraffin, glass, quartz, mica, gypsum, and the most 

 dissimilar salts, also the metals, whether rough or polished, or 

 even covered with varnish — condense on their surface aqueous 

 vapour from the surrounding air which has the same tempera- 

 ture as themselves, and in consequence of this condensation 

 undergo elevation of temperature ; and that when the surround- 

 ing air is changed for air containing less moisture, then a part 

 of the previously condensed moisture should evaporate and cool 

 the surface of the body. 



Results perfectly similar to those obtained with vapour of 

 water were obtained by using vapour of alcohol, or of ether, or 

 other vapours. 



Generalizing, the most various vapours condense on the surface 

 of solid bodies in such quantity as to cause appreciable elevation 

 of temperature. 



From this it follows that at all times there is at the surface of 

 solid bodies a layer of condensed vapour, which is larger or smaller 

 according to the hygrometric state of the atmosphere. Under 

 some conditions this will, without doubt, exercise a by no means 

 unimportant influence. 



XLIV. On the Influence of Condensation in Experiments upon 

 Diathermancy. By G. Magnus*. 



RESPECTING the influence which condensation exercises 

 upon the thermo-pile, and from which this investigation 

 sets out, Dr. Tyndall says that it has been known to him for 

 yearsf. It is to be regretted that in his numerous publications 

 upon the transmission of heat, Dr. Tyndall has nowhere made 

 mention of this important circumstance, and that he has nowhere 

 given the precautions he has taken in order to avoid this disturb- 

 ing influence when blowing or sucking air through tubes open 

 at both ends. Dr. Tyndall J has had his experiments repeated 

 by Dr. Frankland, in order, as he says, to prove that he " had 

 not mistaken cold for hot, and hot for cold." Such a confir- 

 mation was, in my opinion, unnecessary. I have not implied an 

 error of that kind, but have only said that, on repeating the 

 experiments of Dr. Tyndall, it has not even once happened to me 

 to obtain the same result as he did. From that I infer that in 

 these experiments, and in the repetitions by Dr. Frankland, 

 some circumstance has come into play which up to the present 

 has remained unascertained, or at any rate unmentioned. Dr. 



* Translated by Prof. Wanklyn from Poggendorff's Annalen, vol. cxxi. 

 part 1 (1864, No. 1). 

 I f Phil. Mag. S. 4. vol. xxvi. p. 45, July 1863. 



j Ibid. p. 46 



