162 Physical and Chemical Properties of the [S£?tv 



like it by metallic mirrors, results afterwards confirmed by the 

 experiments of Scheele and Pictet. Mr. Leslie and Count 

 Rumford examined, with particular care, the influence which 

 the nature of the substance, and the state of the surface, has 

 upon the radiation of heat, when it enters into bodies, or escapes 

 from them. Finally, M. Prevost, of Geneva, has included all 

 the phenomena of the radiation of heat in an ingenious theory, 

 which, viewed only in a systematic point of view, as is done by 

 the author, enables him to connect all the phenomena together 

 by laws. Very lately M. Delaroche has added a new fact to 

 these results, which seems to exhibit a gradual passage from heat 

 to light. The rays of invisible heat traverse glass with difficulty 

 at a temperature below that of boiling water; but they traverse 

 it with a facility, always increasing with the temperature, as it 

 approaches the point when bodies become luminous: so that it 

 would appear from these experiments that the modification, 

 whatever it be, which must be impressed upon the invisible 

 rays, to render them more and more capable of passing through 

 glass, makes them approach more and more to the state in which 

 they must be when they penetrate our eye, and occasion the 

 sensation of vision.- M. Delaroche has found, likewise, that the 

 rays of heat which have passed through a plate of glass are pro- 

 portionally more adapted to pass through a second plate. This 

 is a new proof of the peculiar state of these rays, and of the 

 modification which they acquire. 



The results which we have stated relate to the motion of heat. 

 Its chemical action, compared with that of light, has been 

 equally studied. 



MM. Gay-Lussac and Thenard have proved that all the 

 changes of colour produced by light may be imitated and pro- 

 duced by heat, and by an elevation of temperature not exceeding 

 212°. Other phenomena previously observed, showed that irr 

 that comparison of the actions of heat and light in heating 

 bodies or producing chemical changes in them, there is a great 

 difference in the rays of different colours. M. Rochon had 

 announced that the heat produced by the different rays of the 

 spectrum was unequal. Dr. Herschell afterwards found that the 

 heating powerincreasesprogressively from the violettothered end 

 o. the spectrum. He even fixed the maximum effect beyond the 

 red ray; so that, according to his experiments, the most heating 

 rays of the spectrum were entirely, or nearly, invisible. Dr. 

 "Wollaston, and Messrs. Ritter and Beckmann, havingexamined' 

 the opposite, or violet end of the spectrum, found that it like- 

 wise possessed peculiar properties; and that there exist, beyond 

 the violet, invisible rays, which possess, in greatest perfection, 

 the power of determining chemical combinations. The experi- 

 ments of Dr. Herschell, though confirmed by several phitosa- 



