Copy 1, Volume 1
A system of chemistry. In four volumes / By Thomas Thomson.
- Thomas Thomson
- Date:
- 1817
Licence: Public Domain Mark
Credit: A system of chemistry. In four volumes / By Thomas Thomson. Source: Wellcome Collection.
46/592 page 28
![Book I. some how or other, in a peculiar vibration of its particles I Division I. Q^hers, on the contrary, think that it is a distinct substance. Each of these opinions has been supported by the greatest philosophers; and till lately the obscurity of the subject has been such, that both sides have been able to produce exceedingly plausible and forcible arguments. The recent improvements, however, in this branch of chemistry, have gradually rendered the latter opinion more probable than the former: and a discovery, made by Dr. Herschel, has at last nearly put an end to the dispute, by demonstrating, that we have the same reason for considering heat to be a substance, as we have for believing light to be material. Discovery 1. Dr. Herscliel had been employed in making observa- caSc on the sun by means of telescopes. To prevent the inconvenience arising from the heat, he used coloured glasses; but these glasses, when they were deep enough coloured to intercept the light, very soon cracked and broke in pieces. This circumstance induced him to examine the heating power of the different coloured rays. He made each of them in its turn fall upon the bulb of a thermome¬ ter, near wEicli two other thermometers were placed to serve as a standard. The number of degrees, which the thermometer exposed to the coloured ray rose above the other two thermometers, indicated the heating power of that ray. He found that the most refrangible rays have the least heating power; and that the heating pow'er gra¬ dually increases as the refrangibility diminishes. The violet ray therefore has the smallest heating power, and the red ray the greatest. Dr. Herschel found that the heating power of the violet, green, and red rays, are to each other as the following numbers: Violet =16; Green = 22*4; Red = 55. It struck Dr. Herschel as remarkable, that the illuminating power and the heating power of the rays follow such dif¬ ferent laws. The first exists in greatest perfection in the middle of the spectrum, and diminishes as we approach cither extremity; but the second increases constantly from the violet end, and is greatest at the red end. This led him to sus])ect that perhaps the heating power does not stop at the end of the visible spectrum, but is continued beyond it. He placed the thermometer completely beyond the boundary of the red ray, but still in the line of the sjicc- trum; and it rose still higher than it had done when ex-](https://iiif.wellcomecollection.org/image/b29325717_0001_0046.jp2/full/800%2C/0/default.jpg)
