164 Prof. Tyndall — Contributions to Molecular Physics. [Mar. 17, 



must be extra-red also. The absorption by diy air of the heat emitted by 

 a platinum spiral raised to incandescence by electricity was found to be 

 insensible, while that by the ordinary undried air was 6 per cent. Sub- 

 stituting for the platinum spiral a hydrogen flame, the absorption by dry 

 air still remained insensible, while that of the undried air rose to 20 per 

 cent, of the entire radiation. The temperature of the hydrogen flame 

 was as stated, 3259° C, that of the aqueous vapour of the air was 20° C. 

 Suppose, then, the temperature of our aqueous vapour to rise from 20° C. 

 to 3259° C, we must conclude that the augmentation of temperature is 

 applied to an increase of amplitude^ and not to the introduction of periods 

 of quicker recurrence into the radiation. 



The part played by aqueous vapour in the economy of Nature is far 

 more wonderful than hitherto supposed. To nourish the vegetation of 

 the earth, the actinic and luminous rays of the sun must penetrate our 

 atmosphere, and to such rays aqueous vapour is eminently transparent. 

 The violet and the extra- violet rays pass through it with freedom. To 

 protect vegetation from destructive chills, the terrestrial rays must be 

 checked in their transit towards stellar space, and this is accomplished 

 by the aqueous vapour diffused through the air. This substance is the 

 great moderator of the earth's temperature, bringing its extremes into 

 proximity, and obviating contrasts between day and night which would 

 render life insupportable. But we can advance beyond this general 

 statement now that we know the radiation from aqueous vapour is inter- 

 cepted, in a special degree, by water, and reciprocally, the radiation from 

 water by aqueous vapour ; for it follows from this that the very act of noc- 

 turnal refrigeration which produces the condensation of aqueous vapour 

 upon the surface of the earth — giving, as it were, a varnish of liquid water 

 to that surface — imparts to terrestrial radiation that particular character 

 which disqualifies it from passing through the earth's atmosphere and 

 losing itself in space. ^ 



And here we come to a question in molecular physics which at the 

 present moment occupies the attention of able and distinguished men. By 

 allowing the violet and extra-violet rays of the spectrum to fall upon sul- 

 phate of quinine and other substances. Professor Stokes has changed the 

 periods of those rays. Attempts have been made to produce a similar 

 result at the other end of the spectrum — to convert the extra-red periods 

 into periods competent to excite vision — but hitherto without success. 

 Such a change of period the author believed occurs when a platinum wire 

 is heated to whiteness by a hydrogen flame. In this common experiment 

 there is an actual breaking-up of long periods into short ones — a true 

 rendering of invisual periods visual. The change of refrangibility here 

 effected differs from that of Professor Stokes, first, by its being in the 

 opposite direction, that is from lower to higher ; and secondly, in the 

 circumstance that the platinum is heated by the collision of the molecules 

 of aquepus vapour, and before their heat has assumed the radiant form. 



