Essays Chiefly on Chemical Subjects Author:William Irvine Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: before the propofed inferences be admitted. There are other rays befides thofe directly from the fun,- which pafs through the prifm, and are to be found beyond t... more »he red ray. Something mould be allowed for reflection, and fomething, as Mr. Leflie thinks, for inaccuracy. The ffect of a ftream of air, in arrefting the progrefs of radiant caloric, is alfb very remarkable, and much againft Dr. Herfchell's hy- pothefis. The notion of invifible rays of heat has been long, entertained: the novelty of Dr. Herfchell's theory confifted in placing them among the folar beams, as feparate from the rays of light. It is clearly fup- pofed that the invifible and heating rays are pure caloric. If it could be imagined that they were any modification of light, we fhould be difpofed to account for the appearance of heat in the old way, at lead no proof of the contrary doctrine would have been adduced. Dr. Hutton and other philofophers have doubted as to the neccflity of vifibility to the exiftence of light, and, as it feems to me, on very good grounds. It is extremely probable, that either the air or the combuftible fubftance contains light invifibly, which the procefs of combuftion afterwards developes The compofitions called pyrophori can be deprived of their power of emitting rays of light by expofing them to fevere cold. This luminous quality can however be reftored to them, by a very moderate rife of temperature. In this cafe, if light and heat be really different, itcanfcarcely be doubtedthat the light exifted in the frozen pyrophorus in art invifible ftatc. Hence, it is only light in a peculiar ftatc that can be feen. A certain intenfity of light, moreover, is neceflary for exciting vifion. It is well known that fome animals can fee diftinctly where the human eye has no perception. The human ...« less