278 Dr. A. Richardson on the Measurement of the 



a differential apparatus was taken, consisting of two bulbs of 

 160 cubic centim. capacity each and connected by a capillary 

 tube containing strong sulphuric acid ; any change in pressure 

 of the gas in either bulb was indicated by the movement of an 

 index of air contained in the capillary. 



The bulbs were first filled with dry air and placed in a glass 

 tank through which a current of cold water circulated ; under 

 these conditions it was found that no movement of the index 

 took place on exposure to bright sunlight. The air in one of 

 the bulbs was then replaced by dry chlorine, and both bulbs 

 were again exposed under precisely similar conditions ; an 

 immediate expansion of the chlorine took place, causing the 

 index to move through a distance of from 20 to 30 centim., 

 but on shading the bulbs the index returned to zero. On 

 interposing variously coloured glasses between the bulbs and 

 the source of light, it was found that with cobalt glass the 

 index receded 15 to 20 centim. from the zero-point, whilst 

 even faintly yellow glass as well as ruby glass caused it to 

 return to zero. It was further found that the radiations from 

 a cannon-ball at a temperature just below a red heat produced 

 practically no movement of the index when the bulbs were 

 exposed in the air at a distance of from 1 to *5 metre. 



Budde's observations being thus confirmed, it was next 

 necessary to determine how far the expansion of chlorine is 

 proportional to the actinic intensity of the light to which it is 

 exposed. In order to do this a series of measurements of the 

 expansion of the gas were made, whilst at the same time the 

 intensity of the light to which the gas was exposed was 

 determined by means of an actinometer. 



The expansion of the gas was measured in a differential 

 arrangement, consisting of two tubes of 55 cubic centim. 

 capacity and 10 centim. in length ; these were connected 

 with a horizontal gauge graduated in '5 centim., provided 

 with a small bulb at each end. These bulbs and also the 

 gauge contained strong sulphuric acid, a short column of air 

 being introduced to serve as index. 



The tubes to be exposed were suspended in a box, which 

 could be placed at any required angle so as to face the sun, 

 and when filled with dry air were found to be equally heated; 

 the bulbs containing the acid were in all cases protected from 

 the light. One tube was then filled with dry chlorine and the 

 acid up to the index was saturated with the gas. 



The chlorine used in this and subsequent experiments was 

 prepared by the action of hydrochloric acid on potassium bichro- 

 mate ; traces of hydrochloric acid were removed by passing 

 the gas through U-tubes containing solutions of chromic acid, 



