280 



fined, together with a portion of calcareous rock ; yet though such 

 direct proofs are wanting, he considers the collateral evidence as 

 quite satisfactory in proving that a true solvent power over calca- 

 reous matter is exerted by those animals. 



An Account of some Experiments relative to the Passage of radiant 

 Heat through glass Screens. By the Rev. Baden Powell, M.A. 

 F.R.S. of Oriel College, Oxford. Communicated March 9, 1826. 

 Read June 1, 1826. [Phil. Trans. 1826, p. 372.] 



The object of this paper is to examine the correctness and the con- 

 sequences of a conclusion of De la Roche, that if radiant heat be in- 

 tercepted by two transparent screens, the additional diminution of 

 effect occasioned by the second is proportionally much less than that 

 produced by the first, and so for any number of screens. This M. 

 De la Roche explains by supposing the heat passing through the first 

 screens to acquire thereby a kind of polarization, which enables it to 

 pass the rest more easily. The author, however, observes, that when 

 the temperature of the source of heat is above incandescence, the 

 fact admits of more simple explanation, by regarding the heat as 

 consisting of two portions, the one incapable of penetrating glass, 

 and therefore wholly stopped by the first screen ; the other capable 

 of so doing, and therefore readily transmitted through any number 

 of screens. 



If, however, he observes, the same effect is produced at tempera- 

 tures below luminosity, this explanation fails; and either De la 

 Roche's idea of polarization, or some other, must be resorted to ; and 

 it is to the examination of this case that the experiments detailed in 

 his paper are devoted. 



His apparatus consisted of two tin reflectors, opposed to each 

 other, and having in the focus of one a blackened thermometer, and 

 in that of the other an iron ball heated to redness, and cooled till it 

 ceased to be visible in the dark. The indications of the thermometer 

 were observed ; first, for the direct effect ; secondly, with one glass 

 screen interposed; thirdly, with two; sometimes a mercurial, some- 

 times an air thermometer was used. The indications were noted 

 after each half minute of exposure, till the thermometer ceased to 

 rise. The temperatures acquired by the screens were also noted. 

 The conclusions drawn by the author from a great number of such 

 experiments are, 



First, That the fact observed by M. De la Roche is verified when 

 the source of heat is below luminosity, as well as above it. For in 

 all the trials a rise was observed to take place in the focal thermome- 

 ter, much smaller with one than with no screen, and very small in- 

 deed with two. The diminution, however, occasioned by the second 

 screen was proportionally much less than that occasioned by the first. 



On analysing more minutely, however, the progress of the rise of 

 the temperature from half minute to half minute, and comparing it, 

 in the cases where screens were used, with the observed progressive 



