394 Dr. A. R. Leeds on the Action of Light 



sun's rays, or, in case of solutions exposed in tubes of colour- 

 less glass and of uniform bore, in the same ratio as the increase 

 of volume, or the dilution. In the absence of air, no decom- 

 position takes place either in light or darkness. When the air 

 has been entirely removed by a long-continued current of car- 

 bonic acid, the solution may be exposed for days to the sun 

 without undergoing change. If the carbonic acid be replaced 

 by a stream of oxygen, decomposition begins, and, in the case of 

 potassium-iodide solution a thousand times dilute and exposed 

 to the sun, may attain to 6 mgrms. of liberated iodine per 

 hour. 



When these principles had been experimentally established, 

 they were applied in the first instance to an actinometric 

 measurement of the solar ray. The solutions were contained 

 in comparison-tubes, which when filled to the depth of 150 

 millims. held 100 cubic centims. In each were placed 1 cubic 

 centim. of potassium iodide of 10 per cent., 1 cubic centim, of 

 sulphuric acid, 5 cubic centims. of starch-water, and sufficient 

 water to make up the volume to 20, 40, 60, 80, or 100 cubic 

 centims. They were supported on a frame kept normal to 

 the sun's ray. The results are summarized in the accom- 

 panying diagram ; and it will be noted that the curves of the 

 more concentrated are regularly circumscribed by those cor- 

 responding to the more dilute solutions. The determination 

 was interrupted at 2 p.m. by the sun's clouding over ; but when 

 resumed two days later, numbers were obtained which intro- 

 duce no sudden breaks in their appropriate curves. 



In the subsequent experiments the acids employed were some 

 prepared with especial care by myself, and of such strength 

 that 1 cubic centim. of the sulphuric was equivalent to 25 

 cubic centims. of a normal soda solution, 1 cubic centim. of the 

 hydrochloric to 10*7 cubic centims., and 1 cubic centim. of the 

 nitric acid to 12*6 cubic centims. of the normal soda. Similar 

 remarks apply to the soluble iodides, of which 1 cubic centim. 

 of a 20-per-cent. potassium-iodide solution was used in each 

 of the subsequent trials, and 1 cubic centim. of solution of the 

 remaining iodides, these solutions being chemically equivalent 

 to a 10-per-cent. solution of the potassium iodide. In every 

 case 1 cubic centim. of the acid was likewise employed, and 

 the liquid made up to 100 cubic centims. Advantage was 

 taken of an exceptionally brilliant day; and the actinometric 

 measurement was repeated with the above reagents, the results 

 being as given in the subjoined Table (p. 396). The figures in 

 the vertical columns are milligrammes of iodine set free du- 

 ring each half hour of the day. It having been found that the 

 amounts of iodine set free in the absence of starch much ex- 



