AND THE ACTINIC CONSTITUTION OF THK ATMOSPHERE. 39 



i;i(liati(in of two coiiserutive days. It may tlius be said tliat, sjipnkiim- u-eiicnillv, a 

 day at the north is woi'tli t-\\o of oiirs as regards aftiuir [lower. 



Not only does the actinic effect of a fine day increase more raiudly than tlic 

 length of the day itself, hut it may actually spread itself over tlie next day. aihl 

 the day after that, an<l thus make u|), in some degree, for the absence of the sun. 

 In like manner, a "fine morning may lender condnistion luore rapid even though 

 the evening l)e dark and stormy. It is enough that tlic li(|uid shall have been 

 made sensitive; and as this sensitization is the more rajiid as tiie actinic intensity is 

 greater, the atmospheric condition of northern countries favors them in this respect 

 beyond us, and a new superiority is thus attained thi-ough the superposition and 

 mutual emphasis of the other two causes. 



Finally, the sensitiveness produced l>y a fine day continues for several days. 

 A number of ])ad days, following each other, is conse([uentl\ no( a jieriod of 

 inertness and loss; it draws upon the store wliicli was collected during fine 

 weather. On the other hand, we have seen that the sensitiveness which was 

 accpiired in the sun, did not increase without limit, and that it reached quite rap- 

 idly a maximum beyond which it did not go. A succession of fine days, therefore 

 does not develop actinic phenomena to an extreme. We here meet once more 

 with the system of balancing which weakens great effects, increases small ones, 

 and which has been pointed out with regard to so many other manifestations of 

 the forces of nature. 



Summing up the matter, then, it woidd seem that we have hitherto missed our 

 way, in considering the chemical action of solar light as independent of locality and 

 proportionate to time of isolation, or as furnished or measured by meteorological 

 instruments. The first of these notions was purely instinctive and was suggested 

 especially by the uniformity which was ascertained to exist at different points of 

 the globe in so many other grand meteorological phenomena (such as the composi- 

 tion of tlie air, the average barometric elevation, the mean distribution of nebulos- 

 it)', etc.) Instead of such a unifoi-mity, we find, on the contrary, actinic climates, 

 limited in jjoint of surface, for they betray the local influence of the surface of the 

 soil — limited also in point of duration, for they are due to two kinds of clouds 

 which are subject, like the others, to the influences of place and season. 



Misjudging thus local influences, only the first cause has been thought of, and 

 all eff'oi'ts had Ijeen directed towards measuring the duration of insolation. On 

 this point, I think I have shown that the wrong way had been taken. The actinic 

 force of a day is not the same for the same day, in different parts of the globe, and 

 its effect increases moiv ra})idly than its length ; such is the principal lesson of this 

 Memoir. 



