280 Dr. A. Richardson on the Measurement of the 



In column I. the intensity of the light is given as measured 

 by the actinometer. In column II. the expansion of the 

 chlorine, as shown by the movement of the index from zero, 

 is given in centimetres ; the zero-point, which was determined 

 from time to time by shading the tubes, was found to remain 

 nearly stationary throughout. The calculated intensity, in 

 terms of the movement of the index through 1 centim., is 

 given in column III., and from the mean value so obtained 

 the intensity is recalculated in terms of the observed ex- 

 pansion (column IV.). In making these observations great 

 difficulty was experienced owing to the constant variation in 

 the intensity of the light ; and as the maximum movement 

 of the index was only reached after some time, these sudden 

 variations, which were registered accurately by the actino- 

 meter, did not produce a corresponding change in the position 

 of the index. The close agreement between the results ob- 

 tained directly by the actinometer and those calculated from 

 the expansion of the gas lead, however, to the conclusion that 

 the change in volume in chlorine gas is directly proportional 

 to the actinic intensity of the light to which it is exposed, 

 although the friction of the acid in the gauge tends to smooth 

 out the more sudden variations. 



By the aid of such a differential apparatus as that above 

 described, to which the name of "chlorine actinometer" may 

 be applied, the intensity of the light can be read directly from 

 the gauge, and it is thus found possible to study other light- 

 effects under constant conditions of intensity by making 

 exposures whenever the index stands at a given point. 



It was next necessary to inquire how far dilution with air 

 influenced the amount of expansion of chlorine when exposed 

 to light. To do this a differential apparatus, shown in fig. 1, 

 Plate III., was used, in which the gas to be exposed was con- 

 tained in the tubes A and B, 15 centim. in length and about 

 3 centim. in diameter. Two glass stop-cocks, and D, were 

 sealed on to A, the lower one, D, being connected with a 

 graduated pipette E containing air and dipping under strong 

 sulphuric acid contained in F. The graduated capillary tube 

 G was filled with strong sulphuric acid, as also the bulbs 

 H and I, the movement of a short column of air serving to 

 indicate any change in volume in either tube. The tube A 

 was first filled with chlorine, whilst B contained dry air 

 throughout the experiment. Both tubes were then exposed 

 to light and the expansion of the chlorine determined, the 

 intensity of the light being at the same time noted by means 

 of the chlorine actinometer ; the tubes were then shaded and 

 the zero-point found. The stop-cocks C and D were then 



