AND THE ACTINIC CONSTITUTION OF TITK ATMOSPTIKRE. 29 



makes partial compensation. J5esides, the higher figures obtained at Ilelsingfors, 

 persist at the thi'ee positions of the French station : at Paris, on tlie coast of the 

 Channel, and on the mountain of Puy-de-Donie. 



There can, therefore, remain no possible doubt on this {XMut : the actinic 

 intensity of light in northern counti'ies, close to the soil, is greater than in our 

 tempei-ate zone at the same hours of the day. It would no d<mbt be found still 

 weaker if we approached more nearly to the equator. This conclusion was alto- 

 gether unforeseen. 



The fact once establivshed, explanation becomes necessary. This greater ac- 

 tivity of combustion which the air has in noithein regions, might be ascribed to 

 ozone, rendered more abundant there by the dischai'ges which constitute the 

 aurora borealis, and more active by the action of light. 1 have begun to study 

 this sul)ject, but my experiments, intemipted by winter and my I'eturn to Paris, 

 are not yet completed, and I shall not be able to take them up again till next 

 spring. I believe, however, that I may already say that ozone can have but a 

 very secondary influence on the [>henomenon, and that if light is more active 

 within the same length of time at the north than in France, the reason is that it 

 has lost fewer of its chemical radiations in passing thiough the atmosphere, because 

 the latter is pooi'er in oxidizable substances. I know, of course, that theie are in the 

 north those pine forests, of which 1 have spoken befoi'e, and that jjei'haps, if 

 Helsingfors were built in the heart of the woods, instead of being a large city on 

 the sea-coast, the points of dift'ei'euce would be somewhat less. But there would 

 always remain the fact that the quantities of vapoi- diffused through the air 

 increase with the temperatui'e, and that, for one and the same aspect of the fauna 

 and the floi'a of the earth, the equatorial atmosphere will always be more heavily 

 loaded than that of the temperate zones, which in its tuin will again be more so 

 than that of boreal regions. 



Whatever finally the explanation of the fact may be, the important point is to 

 show that it exists, and that there is a difference in light, so fai- as its (piality is con- 

 cerned, at the same hours of the day, at the north and in the heart of Europe. 

 But this is not all. After having examined this question of quality, we have to 

 look next at the question of qnaiitity. The days which are useful to vegetation at 

 the noi'th are longer than with us — what now is the influence of the d nidation of light 

 on the chemical phenomenon which serves us as a means of measurement? Is the 

 effect thus produced proportionate to the length of exposure to the sun ? Does it in- 

 crease more or less slowly? Such are the first questions which we have to consider. 

 I believe they are new, l)ecause up to this day, both as regards meteoi'ologic 

 instruments and in theoretical speculations, it has always been held that the effect 



