620 BITNSEN MEMORIAL LECTURE. 



reflected and {ib.sovl)ed aecordino- to the same laws as the visibh^ rays, 

 and that their intensity diminishes as the scjuare of the distance. The 

 question as to ' whether energy is expended in the act of photochemical 

 combination for which an equivalent amount of lioht disappears, or 

 whether the action, like that of the liberation of a spring, is brought 

 a))out by the chemical rays without any appreciable loss of light,' is 

 decided in favor of the first view. The phenomenon is termed l)y the 

 authors photochemical extinction. 



''A second very remarka))le phenomenon, first pointed out hy the 

 authors, is that of chemical induction. This refers to the fact that the 

 action of light on the sensitive mixture of chlorine and hydrogen does 

 not begin in its full intensity, but that it slowly increases, until after 

 the lapse of a certian time it attains its regular and maxinuun rate. A 

 satisfactory exi)lanation. nuu-h less a theory, of induction is as yet 

 wanting. Lastly, it was provtnl that j)h()tochcmical action depends 

 solely u])on the quantity of the incident light, and is altogethcM- inde- 

 pendent of the time during which the insolation ttikes place. 



"The great and important influence which photochemical action 

 exerts in organic nature, especially in plant assimilation, renders the 

 application of photochemical measurements to meterological and 

 climatic phenomena of special interest. But the difficulties which 

 surrounded such an a])plicati()n were enormous. In the fiist place, it 

 was necessary to find a luiit of al)S()lute measui'emcnt for the chemic- 

 ally active rays. A flame of carbonic oxide which emits chemically 

 active rays of great intensity, burning in air under carefully specified 

 conditions, satisfied the n'quirement. It was found that whilst the 

 variation of the chemical action of the light reflected from a clouded 

 sky was subject to no recognisal)le law. that obtaining when the sky 

 was (•loudl(\ss and when direct suidight was employed at once exhil)ited 

 distinct relations. Thecurvt's of daily intensity thus obtained before 

 and after noon were seen to be synnnetrical throughout the day. In 

 direct sunshine these curves, of course, rise much higher than is the 

 case in difl'usi> daylight; moreover, the considerable variation due to 

 change of latitude was precisely calculated. 



"The dependence of the chemical action on the wave length of the 

 incident light was carefully studied, the result being that the most 

 intense action was exerted i)y the rays l)etween the lines G and H of 

 Fraunhofer; the curve falls sharph' toward the red end of the spec- 

 trum, whilst it extends in the more refrangible portion far into the 

 ultra violet. Strictly speaking, this oidy applies to the mixture of 

 chlorine and hydrogen, still experiment has shown that the same thing 

 is to some degree true of many other sensitive substances, although the 

 distri'^ution of the chemical activity in the spectrum is a difi'erentone. 



"This .short account of the photochemical researches is far from 

 doing them justice. ' In no other research in this domain of science 

 do we find exhibited such an amount of cheuiical, physical, and mathe- 

 matical dexterity, of ability in devising experiments, of patience and 

 perseverance in carrying them out, of attention given to the minutest 

 detail, or of lireadth of view as applied to the grander meteorological 

 and cosmical phenomena of nature'. — Ostwald.'' 



And now comes Bunsen's introduction: 



"The measureless store of energy which nature has amassed in the 

 sun's body flows in an unceasing current as solar ra3"s throughout the 

 universe. 



