56 Royal Society : — Messrs. Roscoe and Thorpe on the 



(1) Air taken from town, (Taken at a height of 2 metres from 

 ground.) 



(NH 4 ) 2 CO., as grammes (NH 4 ) 2 C0 3 in parts 



Date of Experiment. per 100.000 litres of air by "weight per 



at 0° C. and 760 mm. barom. 100,000 of air. 



1869. September 30 1-1294 -8732 



October 4 -62117 -4801 



6 -5251 -4059 



„ 8 -62117 -4801 



November 26 1-0729 -8293 



28 1-1000 -8503 



(2) Air from country. (Taken at a height of 2 metres.) 



(NH 4 )., CO.^ as grammes (NH 4 )., C0 3 in parts 

 Date of Experiment. per 100,000 litres of air per 100,000 of 



at 0° C. and 760 mm. barom. air. 



1869. December 6 *7620 -5890 



„ 8 -7826 -6085 



9 -6601 -5102 



11 -6635 -5121 



1870. February 12 '7639 -5904 



The direction of the wind does not seem to have any influence 

 on the ammonia found ; immediately after heavy rain, however, the 

 quantity falls somewhat below the average, but the air is again re- 

 stored to its normal condition after a lapse of two or three hours. 



Attempts were made to make the method more delicate still by ab- 

 sorbing the ammonia in pure water and then distilling, but the nitro- 

 genous organic matter suspended in the air was found to interfere 

 with the results. 



When the air is passed through cotton-wool before entering the 

 absorption-tubes, it is found to be entirely deprived of its ammonia 

 by the filter. This is also the case with air artificially charged with 

 ammonia to a large extent. This absorption is not due to the pre- 

 sence of hygroscopic moisture, since cotton-wool, when absolutely 

 dry, is capable of taking up 1 1 5 times its own bulk of dry ammonia 

 (confined over mercury) at 10°*5 C. and 755*7 millims. barom., the 

 gas being again slowly evolved when the wool is left in contact with 

 the air at 100° C. 



All other porous substances that were tried for filtering agents 

 were found to possess this property more or less ; even freshly ignited 

 pumice-stone is not entirely without absorptive effect upon the gas. 



March 31.— Lieut-General Sir Edward Sabine, K.C.B., 

 President, in the Chair. 



The following communication was read : — 



" On the Relation between the Sun's Altitude and the Chemical 

 Intensity of Total Daylight in a Cloudless Sky." By Henry E. 

 Roscoe, F.R.S., and T.' E. Thorpe, Ph.D. 



In this communication the authors give the results of a series of 



