82 Dr Stark (yn the Influence of Colour on 



till the influence of the sun at once converts the northern winter 

 into summer, without the intervention of spring. 



III. — On the Influence of Colour on the Deposition of Dew. 



As connected with the preceding investigations, it may be 

 mentioned, that I had projected a train of experiments to ascer- 

 tain the proportions in which dew was deposited on variously- 

 coloured substances. Dr Wells found the deposition of dew 

 influenced by the radiating power of the substance employed ; 

 but neither he nor any of the philosophers who have treated of 

 the subject, seem to have been aware of the modifying effects 

 of colour on the absorption and radiation of heat. My avoca- 

 tions have hitherto prevented me from following out these ex- 

 periments ; but to shew that the influence of colour may be ex- 

 tended to moisture, and of course to dew, I shall give from my 

 notes the result of two experiments made to ascertain this point. 



Jan. 16. 1833. — I exposed last night (the temperature ranging 

 from 28° to 30°, and with a dense fog) ten grains of black wool, 

 the same quantity of scarlet wool, and an equal weight of white 

 wool, on a black board, which was placed on the leads on the 

 top of the house. When taken in this morning, and carefully 

 weighed, the black wool had gained 32 grains, the scarlet wool 

 25 grains, and the white wool 20 grains, deposited on the wool 

 in the form of hoar-frost. 



A few nights afterwards, after a slight thaw, and when to- 

 wards night the temperature fell to 31°, I again exposed four 

 colours of wool, ten grains of each, in the same manner. By 

 next morning, the black wool had gained 10 grains, the dark 

 green 9/(j grains, the scarlet 6 grains, and the white 5 grains. 



Dr Wells had indeed made experiments on the deposition of 

 dew, with equal quantities of black and white wool ; and in four 

 out of five experiments, the black wool was found to have ac- 

 quired a little more dew than the white : " whence,'" says he, " I 

 concluded that it had also radiated a little more heat. But I 

 afterwards remarked, that the white wool was somewhat coarser 

 than the black, which might have occasioned the diff^erence of 

 their attraction for dew."" On another night, he made an expe- 

 riment with pasteboard covered with white paper, and paste- 



