16 ATMOSPHERIC ACTINOMETRY 



leads us to ask if the (|uestiuii of altitude may not perhaps be of importance. Two 

 identical vessels, containing the same solution and exposed during the same time 

 at different heights in the atmos2)here — will they or will they not undergo the 

 same degi-ee of oxidation 1 



UfFLUENCE OF ALTITUDE. 



I begin with the last question, because the documents which helped me to 

 solve it wei'e lost, together with those which gave the results of the experiments 

 already mentioned, and I must therefoi-e be very brief in my treatment. 



In order to solve this problem I installed myself at the foot of the Puy-de- 

 Dome in the little village of Orcines, and I made a nundjer of experiments simul- 

 taneously in the garden of the house in which we lived, and on the terrace of the 

 obsei'vatoiy on the Puy-de-Dome, where M. Ilumandon kindly undertook to expose 

 and to i-emove again at certain fixed houi's the vessels containing the oxalic acid 

 which had been I'endered sensitive. The two stations are distant fi'om each other 

 4 kilometres in a diiect line. The vertical difference amounts to about 400 metres. 

 The incline, therefore, between the two stations does not count for much, and they 

 cannot be considered as being upon the same vei-tical line. Experiments made on 

 the tt)]i and at the foot of the Eiffel tower would have been more satisfactory in 

 this i-espect. Hut at the Eift'el tower I shoidd have had to apprehend encounter- 

 ing difficulties of another kind, especially the want of honiogeneousness between 

 the layei's of the atmosphere. For the lower ones which had swept populous parts 

 of the city could, in that amount, no longer be considered e(piivalent to the upper 

 paiis. What tempted me to choose the station of the Puy-de-Dome was exactly 

 this IiDmogeneonsness of the whole region so far as its vegetation was concerned. 

 The Puy-de-Dome is surrounded to a great distance by a dry, almost deserted 

 counti-y, covered with wt)ods and largely with heathei-, while some portions are 

 absolutely bare in places where [)ozzolanes cr<)[i out of the soil or in those immense 

 overflows of lava of recent date, which ai'e called " clu'irt's,"' and wliicli dcl'v any 

 attempt at cultivation. One of these "cheires" ci'opped out close to the house in 

 whii-h I ilwclt, and I imagine that, on the whole, there was no reason why the air 

 on the top an<l that of tlie table-laixl from which the mountain I'ises, should be 

 heterogeneous. In spite of these favorable circumstances I did not find that the 

 combustion at the observatory was very different from that at Orcines. It exceeded 

 the latter a little, on an aveiage, l)ut with exceptional results in one or the other 

 direction, so as to prevent any positive conclusion. I remend)er that my estimates 

 showed an increase of altitude accompanying an increase of actinic intensity, but 



