Dr. Andrews on Ozone in the Atmosphere. 315 



liar retarding influence lias been suggested which the heating and 

 luminous rays are supposed to exert upon the more refrangible 

 portions of the spectrum. The fallacy of these statements has 

 been fully proved by a series of direct measurements of the che- 

 mical intensity of sunlight under the equator, ,made at Para by 

 Mr. T. E. Thorpe. The curves of daily chemical intensity given 

 in the paper show that the activity of the chemical rays in the tro- 

 pics is very much greater — on one day fifty-five times as great as 

 in our latitudes ; and these measurements prove that the reported 

 failures of photographers cannot at any rate be ascribed to a di- 

 minution in the chemical intensity of sunlight. The following 

 numbers give some of the daily mean chemical intensities at Para 

 compared with the same days in Kew : — 



Daily Mean Chemical Intensity. 



18G6. Kew. Para. Ratio. 



April 6 28-6 242*0 8-46 



„ 7 77 3010 39-09 



„ 9 5-9 3264 55-25 



„ 11 25-4 233-2 9'18 



„ 20 38-9 3850 990 



„ 24 83G 3(52-7 4-34 



The measurements were made at Para in the middle of the rainy 

 season, and at very frequent intervals -during the day; the curves 

 show the enormous and rapid variation in intensity from hour to 

 hour which the chemically active rays undergo under a tropical 

 sun during the rainy season. 



" On the Identity of the Body in the Atmosphere which decom- 

 poses Iodide of Potassium with Ozone." By Thomas Andrews, 

 M.D., F.R.S. 



It w r as assumed for many years, chiefly on the authority of Schon- 

 bein, that the body in the atmosphere which colours iodide-of-po- 

 tassium paper is identical with ozone ; but this identity has of late 

 been called in question; and as the subject is one of considerable 

 importance, I submitted it lately to a careful investigation, the 

 results of which I beg to lay briefly before the Society. The only 

 property of ozone, hitherto recognized as belonging to the body in 

 the atmosphere, is that of setting free the iodine in iodide of potas- 

 sium ; but as other substances, such as nitric acid and chlorine, 

 which may possibly exist in the atmosphere, have the same pro- 

 perty, no certain conclusion could be drawn from this fact alone. 



One of the most striking properties of ozone is its power of oxi- 

 dizing mercury, and few r experiments are more striking than that 

 of allowing some bubbles of electrolytic oxygen to play over the 

 surface of one or two pounds of mercury. The metal instantly 

 loses its lustre, its mobility, and its convexity of surface ; and when 

 moved about, it adheres in thin mirror-like films to the sides of the 

 containing glass vessel. The body in the atmosphere acts in the 



