AND THE ACTINIC CONSTITUTION OK TltR ATMOSPHERE. 17 



that they were uot such as to [)r()ve it. This conclusiou diminishes my regret at 

 having misLaid the data. 



INFLUENCE OF OXIDIZABLE SUBSTANCES SUSPENDED IN THE AIR. 



With this second questiun 1 iiave been more successful, since comparative ex- 

 periments can here be made with far more precision than elsewhere. All that is 

 necessary is to expose in one and the same place two vessels containing the same 

 oxalic solution ; one being made to float upon water contained in a deep crystal- 

 lizing pan in such a way that a layer of stagnant aqueous vapor may be kept 

 above the surface of the lirpiid which it contains. The other vessel floats iu like 

 manner, and under the same conditions, on the surface of some turpentine or of 

 any other essential oil. It is always found that combustion is far less advanced in 

 the second than in the first vessel. As I said before, I lost the relative figures of 

 the results obtained by the experiments made in 1886 and 1887. But Mr. Elf- 

 ving, professor at the University of Ilelsingfors (Finland), to whom I had men- 

 tioned the results thus obtained, began once more to experiment with essence of 

 turpentine, and I will here quote the I'esults as he reported them to me in a letter 

 which I have fortunately preserved : 



" I have repeated and confirmed your experiments on August 30, 1888, from 

 8 A.M. to 4 P.M. with a clear sky. There wei'e 53 pei' cent of the oxalic acid burnt 

 above the water, and 39 per cent above a bath of essence of turpentine. The next 

 day, which remained clear from 9 a.m. till noon, the figures were 47 per cent and 

 20 per cent for the same length of exposure. It is certain, therefore, that the 

 presence of oxidizable suljstances in the air possibly diminishes the consuming 

 power of the sun." 



Mr. Elfving has confirmed this conclusion by the following experiment, which 

 I have, in my turn, repeated and confirmed. It consists in sifting, so to speak, the 

 rays of the sun through a solution of sulphate of quinine, which absorbs a part of 

 the chemical radiations before they can react on the oxalic solution. Another sift- 

 ing apparatus, of the same thickness, but consisting of pure water, furnishes a 

 standard of comparison. The latter, rigorously, might be neglected, for the quan- 

 tity of watery vapor or of liquid or solid water which the rays of the sun have 

 traversed before reaching the level of the soil, exceeds by far the thickness of the 

 supplementary screen of liquid ; the absorption due to water, is, moreover, very 

 feeble. In my experiments I suppressed this complication. Mr. Elfving used two 

 glass bell jars with double walls, of which one contained water, the other a solu- 

 tion of sulphate of quinine. He wrote to me on June 17th : 



