The relation between the sun’s altitude and the chemical intensity of daylight was more accurately determined by the writer from a long series of observations made by Roscoe’s method under a cloudless sky near Lisbon in the autumn of 1867. The fact was confirmed that the direct sunlight is robbed of almost all its chemically active rays at altitudes below 10°, and that although the chemical intensity for the same altitude at different places and at different times of the year varies according to the varying transparency of the atmosphere, yet the relation at the same place between altitude and intensity is always represented by a straight line. The differences in the observed actions for equal altitudes, which may amount to more than 100 per cent. at different places, and to nearly as much at the same place at different times of the year, serve as exact measurements of the varying transparency of the atmosphere. As illustrating the wide differences in the daily march of chemical intensity at various places, it was found that, when light of unit intensity acting for 24 hours is taken as 1,000, the value of the mean chemical intensity at Kew is represented by the number 94·5, that at Lisbon by 110, and that at Pará by 313·3.[10] Roscoe’s hope that measurements of the chemical intensity of daylight might become part of the regular work of meteorological observations has, unfortunately, not been realized. Observations of the kind, no doubt, consume much time, and if properly conducted require the whole service of a skilled assistant. But considering the enormously important part played by chemically active light in the economy of nature, and more particularly in the phenomena of vegetable life, it cannot be doubted that a sufficiently long-continued series of observations, systematically carried out on a well-considered plan, at observatories distributed over the earth’s surface, would afford most valuable information concerning the facts of solar energy, and incidentally serve to elucidate many important collateral questions. With the assistance of Mr. Horace Darwin, Roscoe made attempts to devise an automatic arrangement which should minimize the labour of observation, but in the absence of any certain assurance that such an instrument would be utilized, the trials were discontinued.[11] It has been thought desirable, for the sake of continuity, to describe Roscoe’s work on chemical photometry, arising out of his association with Bunsen, so long as he continued to pursue that subject. A subsequent paper will, however, be mentioned later. We must now revert to his work when he returned from Germany. On leaving Heidelberg to settle again in London, as already stated he engaged Dittmar as research assistant, and they jointly studied, by Bunsen’s methods, the absorption of hydrochloric acid and ammonia in water,[12] proving that these gases do not obey Dalton and Henry’s law.