718 



BEPORT — 1893. 



Report on the Action of Light on the Hydracids of the Halogens in the 

 Presence of Oxygen. — See Reports, p. 381. 



5. On the Iodine Value of Sunlight in the High Al]js. By Dr. S. Rideal. 



During the past ■winter, at St. Moritz, in the Engadine, I had an opportunity 

 of determining the intensity of the light as measured by the liberation of iodine 

 from an acidulated solution of potassium iodide on the lines formulated by the Air 

 Analysis Committee of Manchester. St. Moritz is at an altitude of about 7,000 feet 

 above the sea level, and a succession of bright, sunny days can usually be relied 

 upon, even in the depths of winter. The experiments in England, which have 

 been carried out chiefly in towns, have not given a maximum value for the quantity 

 of iodine that can be liberated by sunlight in one hour ; and as the atmosphere in 

 St. Moritz is not only free from haze, but is also remarkable for its exceptionable 

 dryness, higher values than those likely to be obtained elsewhere were to be ex- 

 pected. Also, since the daily meteorological conditions of the place are carefully 

 taken and recorded in the ' Alpine Post,' the observations may possibly be of addi- 

 tional value. 



The instructions laid down by the Air Analysis Committee were carefully 

 followed, and the solution of iodine taken as a standard was titrated with great 

 care. The hyposulphite solution was checked against the standard iodine solution 

 from time to time, and was kept in the dark when not in use. The only previous 

 values obtained in Switzerland in the winter are those of Professor Oliver, who 

 tells me that his winter average at Grindelwald for one hour's sunlight was repre- 

 sented by 1-6 c.c. of the thiosulphate solution, equal to 8 mgms. of iodine per 



