VII. Photo-chemical researches.—Part V. On the direct measurement of the chemical action of sunlight

Abstract

The photo-chemical action exerted by direct sunlight and by diffuse daylight upon a horizontal portion of the earth’s surface, varies with the time of year and with the latitude of the place, and constitutes an important link in the chain of physical relations which connects the organic with the inorganic world. In former communications made to the Royal Society we have endeavoured experimentally to determine the distribution of these chemical actions on the earth’s surface, as varying with the time of day and year, and with the geographical position of the place, when the sky is perfectly unclouded. The methods of measurement there adopted are, unfortunately, not applicable to the determination of the variations in photo-chemical intensity when, as is most frequently the case, the transparency of the atmosphere is more or less obscured by clouds, mist, or rain. To enable us to estimate the alterations which occur in the amount of the chemically active rays falling on the earth’s surface, we must, therefore, have recourse to a mode of measurement totally different from that employed in our former investigations.