44 Prof. Tyndall on the Passage of Radiant Heat 



as a radiant is known, the difficulty experienced by Melloni dis- 

 appears. The former hypothesis, however, though probably cor- 

 rect in ascribing the effect to radiation, was incorrect in ascribing 

 it to the radiation of " the air." 



Dr. Hooker encourages me to hope that this newly discovered 

 action may throw some light on the formation of hail. The 

 wildest and vaguest theories are afloat upon this subject. But 

 the same action which produces serein must, if augmented, freeze 

 the minute rain, and the aggregation of the small particles thus 

 frozen would form hail. I cannot think the hail that I have 

 had an opportunity of examining to be due to the freezing of 

 drops of water, each hailstone being merely the ice of the drop. 

 The " stones " are granular aggregates, the components of which 

 may, I think, be produced by the chill of radiation. I will not, 

 however, dwell further on this subject, but will now commit the 

 entire question to those who are more specially qualified for its 

 investigation. 



VI. On the Passage of Radiant Heat through Dry and Humid Air. 

 By John Tyndall, F.R.S., ^c* 



IT is known to the readers of the Philosophical Magazine, 

 that Professor Magnus and myself have arrived at different 

 conclusions regarding the action of dry air, and of the aqueous 

 vapour diffused throughout our atmosphere, on radiant heat. 

 Last autumn I had the pleasure of meeting my eminent 

 friend in London ; and soon after his arrival it was agreed upon 

 between us to subject the points on which we differed to a 

 searching examination. We accordingly met on several occa- 

 sions in the laboratory of the Royal Institution, where every 

 result that I had previously announced was reproduced in the 

 presence of Professor Magnus. Facts were placed before him 

 which he professed his inability to explain ; but, like a cautious 

 philosopher, he reserved his opinion. It was, however, proved 

 that the results observed by us in common could not be ascribed 

 to any defect of method or error of observation which it was 

 then possible to point out. I wished very much to subject the 

 most recent experiments of Professor Magnus to a similar 

 examination, and he evinced an equal desire to show them to 

 me. He began his arrangements, but it was not my good 

 fortune to see them accomplished. In fact, coming to London 

 as a visitor to the International Exhibition, the numerous other 

 claims upon his time and attention were amply sufficient to pre- 

 vent him from carrying out his own wishes and gratifying mine. 

 In the latest Number of PoggendorfPs Annalen, Professor 

 Magnus has published a paper " On the Diathermancy of Dry 

 * Communicated by the Author. 



