Prof. Tyndall' s Note on the preceding Paper. 425 



myself appears to consist in the hitherto unknown alteration of 

 reflexion produced by the aqueous vapour condensed on the sides 

 of the tube, and which are not visible to the eye. It is then 

 intelligible that the strong cooling of the pile observed by Pro- 

 fessor Tyndall, and also by Professor Wild, on forcing in moist 

 air was ascribed by them to a corresponding absorption by 

 aqueous vapours. But I am convinced that if Professor Tyndall 

 and M. Wild now repeat the experiments with tubes strongly 

 blackened on the inside or lined with velvet, they will share 

 the view that aqueous vapours have no such great absorptive 

 power. 



Professor Wild will then no longer adhere to the opinion that 

 the science of meteorology may without hesitation utilize the 

 great absorption of heat by aqueous vapour as a mode of expla- 

 nation. 



Note on the preceding Paper. 

 By Professor Tyndall, F.R.S. 



Dear Francis, 



It gives me pleasure to learn from you that a translation of 

 Professor Magnus's last paper on the Action of Aqueous Va- 

 pour upon Radiant Heat will be published in the forthcoming 

 Number of the Philosophical Magazine. In 1862, while walk- 

 ing with Professor Magnus through the streets of London, he 

 drew my attention to the untransparent state of the atmosphere, 

 and expressed the opinion that London smoke had much to do 

 with the results that I had ascribed to aqueous vapour. That 

 remark caused me to make a journey to the middle of the Isle 

 of Wight, during the prevalence of a south-west wind, so as to 

 obtain a quantity of air which should be free from the taint of 

 the metropolis. I cite this fact to illustrate the spirit in which 

 I have met the objections w r hich Professor Magnus has raised 

 from time to time against my experiments. Those objections 

 demand, have received, and will continue to receive the utmost 

 consideration from me. In all their forms (and these have been 

 very various) I have endeavoured to meet them by the produc- 

 tion of clear and irrefragable facts. This course it is my inten- 

 tion to pursue with reference to Professor Magnus's last paper. 



Yours very truly, 



John Tyndall. 



Royal Institution, May 1867. 



