662 SCIENCE PROGRESS 



according to Draper, was an increase in the affinity of 

 chlorine to hydrogen acquired by the former element during 

 its exposure to light. He describes an experiment which 

 confirms this view. Two similar tubes were inverted over 

 salt water and half filled with chlorine in the dark. One of 

 these was exposed to sunlight, the other being left in the 

 dark; an unequal volume of hydrogen was then added to 

 both by lamplight. On placing the tubes side by side and 

 exposing them to diffuse daylight, the liquid in the one con- 

 taining the chlorine which had previously been exposed to 

 light was observed to rise instantly, whereas the liquid in the 

 other tube remained motionless for some time. The change 

 in the chlorine is described as permanent or, at any rate, 

 very lasting and quite unlike any transient effect due to a 

 temporary elevation of temperature. 1 In the same paper 

 several instructive experiments are described proving that 

 rays of the same class are operative in bringing about the 

 change in the properties of the chlorine and in causing the 

 interaction of chlorine and hydrogen. In both cases, the 

 maximum of action was found to be in the indigo space 

 and to decrease gradually from that region towards both 

 ends of the spectrum. The explanation offered for these 

 facts is somewhat difficult to understand, as it rests upon 

 an abstraction which was current at that time but has 

 since fallen into disrepute. Solar light was regarded as com- 

 posed of at least three imponderable agencies — light, heat and 

 tithonicity or the power to bring about chemical changes. 

 Each agency was alike dispersable by a prism and therefore 

 the solar spectrum was in reality three superimposed spectra 

 of the imponderable agencies. As Draper expresses it, during 

 the pause in -the first moments of illumination, the tithonicity 

 is being absorbed and rendered latent by the chlorine, the 

 electronegative properties of this gas being exalted at the 

 same time, the chlorine the while acquiring a disposition to 

 enter into combination with the hydrogen. But after the 

 initial absorption of tithonic rays is over, equal quantities 

 of the rays give rise to equal effects. 



1 We take this opportunity of correcting a mistake which has crept into 

 the literature of this subject. It is erroneously supposed that Draper stated 

 that solarised chlorine will combine with hydrogen in the dark. This, how- 

 ever, is not the case. 



