342 Prof. Tyndall on some Physical Properties of Ice. 



Such, if I mistake not, are the properties of radiant heat which 

 modern physics have revealed ; and I think they render it evident 

 that the hypothesis of M. Agassiz and the Messrs. Schlagintweit 

 was accepted without due regard to its consequences. 



But as the sun does reach the lower strata of the atmosphere, 

 it may be thought that we are protected from its effects by the 

 radiation from the air neutralizing its absorption. The fact 

 however is, that air may be heated to an intense degree without 

 showing any measureable amount of radiation. The hot current 

 from an Argand chimney produces, according to Melloni, no 

 sensible effect upon a delicate thermo-electric apparatus. Hence 



which the element hydrogen appears to exercise upon radiant heat, and the 

 longer waves of hght. Wherever hjdrogen enters into a compound, I think 

 it may be predicted that it will destroy the longer undulations. For the 

 present I will avoid speculation u]ion this subject, and confine myself to 

 facts. If we examine the list of solid substances whose power to transmit 

 radiant beat instantaneouslj- have been examined by Melloni, we find that 

 those at the lower endof the hst, that is, the most imperfectly diathernianous 

 substances, all contain hydrogen. In no single case, where this element 

 occurs, is the substance capable of transmitting rays from a source of 

 752° F., while in every case where it does not occur, the power of trans- 

 mitting rays from this source is manifested in a greater or less degree. 

 Amber, gum, citric acid, alum, sugar-candy, tartrate of potash and soda, 

 are the substances which exercise this destructive agency upon the longer 

 undulations, ice being the most non-diathermanous transparent body 

 hitherto examined. Turning to Melloni's list of liquids, the same fact 

 reveals itself. There is a sudden fall in the power of transmission at the 

 place where hydrogen enters the list. Protochloride of phosphorus transmits 

 62 per cent, of the rays incident upon it, but the next substance, hydro- 

 carbiu'Ct of chlorine, transmits only 37 per cent. From this point to the 

 end of the Table the substances named all contain hydrogen, the list being 

 closed by distilled water, which transmits only 1 1 per cent, of the calorific 

 rays from an Argand lamp. 



To the same element, I believe, is to be referred the difference between 

 Sir W. Herschel and Secbeck as to the place of maximum heat in the solar 

 spectrum. Herschel found this jdace to be beyond the extreme red ; but 

 Seebeck, with a prism of sulphuric acid, found it to be in the orange, while 

 with a prism of water he found it in the yellow. Melloni has shown that 

 this is due to the destniction of the less refrangible rays by the two liquids, 

 the sulphuric acid being regarded by him as acting in a manner analogous 

 to water. In both the water and the sidphuric acid used by Seebeck, I 

 believe the hydrogen to be the agent which gives the observed character 

 to the results. The colour of ice and water is also a necessarj' consequence 

 of this hostihty of the element hydrogen to the instantaneous passage of 

 the longer undulations. 



The mathematical theory of undulation is, perhaps, perfect; but the 

 physics of the process, that is, the real afiections of light and heat in their 

 passage through bodies, are wholly unknown to us. Cases, therefore, like 

 the foregoing, which single out a particular substance as exhibiting a special 

 deportment towards light and heat, are, I think, of great value. Increased 

 knowledge will probably enable us to connect these efiFects with the other 

 properties of this substance, and thus establish physical relations which 

 are now unknown to us. 



