281 



increase of temperature of the screens themselves, Mr. Powell is led 

 to conclude that the rise of the focal thermometer, where screens are 

 used, is not attributable to any new property acquired by the heat 

 in its passage through the first screen, when two are used, or to any 

 direct radiation through the glass where one only is employed, but 

 is simply the effect of secondary radiation from the heated screens ; 

 and that this cause must have operated extensively, is evident from 

 the circumstance, that the reflectors were placed in some of the ex- 

 periments at 15 inches from each other, in others only at 12. In the 

 latter case, the first screen was found to have acquired in some cases 

 as much as 23 (centigrade) of temperature above that of the ambient 

 air, its distance from the heated ball being 2 inches. 



The author next proceeds to examine the interception of heat by 

 glass of extreme thinness ; in which case, according to Mr. Ritchie's 

 experiments, heat from non-luminous sources appears capable of ra- 

 diating through that medium when transparent, but not when ren- 

 dered opake. His experiments were made with fragments of a large 

 glass bulb blown to extreme tenuity, and either left transparent or 

 blackened with soot ; but their results proved unfavourable to Mr. 

 Ritchie's conclusion, no difference having been observed between the 

 effects of thin and thick glass sufficient to warrant any difference in 

 their mode of transmission. 



The Bakerian Lecture. On the Relations of Electrical and Chemical 

 Changes. By Sir Humphry Davy, Bart. P.R.S. Read June 8, 

 1826. [Phil. Trans. 1826, p. 383.] 



The author prefaces the experimental results and investigations in 

 this lecture with a brief historical statement of the origin and pro- 

 gress of electro-chemical science, with a view to correct the erro- 

 neous statements which have appeared in this country and abroad. 

 In this the first origin of this branch of knowledge is stated to be the 

 discovery of the decomposition of water by the voltaic pile by Messrs. 

 Nicholson and Carlisle in 1800. This was followed by the experi- 

 ments of Cruickshank and of Dr. Henry, and by several papers by 

 the author himself, the chief contents of which are stated, and in 

 which the appearance of acids, oxygen, and azote at the positive, 

 and of alkalies, sulphur, and metals, at the negative pole, is shown. 



The experiments of Hisinger and Berzelius in 1804 are placed next 

 in order, which establish similar results; and in 1806, on the oc- 

 casion of the agitation of the question respecting the formation of 

 muriatic acid and fixed alkali from pure water, the author presented 

 to this Society his Bakerian Lecture on the chemical agencies of elec- 

 tricity, in which he drew the general conclusion, that the combina- 

 tions and decompositions by electricity were referrible to the law of 

 electrical attractions and repulsions, a theory in which, he observes, 

 he has hitherto found nothing to alter, and which, after a lapse of 

 twenty years, has continued, as it was in the beginning, the guide 

 and foundation of all his researches. 



