94- 



observations which appear to him to confirm what he had advanced 

 on a former occasion touching the dispersive powers of the eye. 



On the Composition of Emery. By Smithson Tennant, Esq. F.R.S. 

 Read July 1, 1802. [Phil. Trans. 1802,^. 398.] 



The ultimate results of the experiments made on this substance, 

 which it seems had never before been properly analysed, are that 

 25 grains contain 12^ grains of argillaceous earth, 2 of silex, and 8 

 of iron ; that 1 grain was not dissolved, and that the remainder, 

 being 1| grain, was lost in the process. Another process gave the 

 same components, but in somewhat different proportions. These in- 

 gredients being very similar to those found by Mr. Klaproth in Dia- 

 mond spar, it is thought that emery is essentially a substance of the 

 same nature, with perhaps a somewhat greater proportion of iron. 



Quelques Remarques sur la Chaleur, et sur V Action des Corps qui I'in- 

 terceptent. Par P. Prevost, Professeur de Philosophic ti Geneve, fyc. 

 Communicated by Thomas Young, M.D. F.R.S. Read July 1, 1802. 

 [Phil Trans. 1802,;?. 403.] 



The remarks here brought forward relate chiefly to Dr. Herschel's 

 experiments on the solar and terrestrial rays that occasion heat, pub- 

 lished in the Philosophical Transactions for the year 1 800, and are 

 meant to rectify some anomalies which appear in their results. The 

 paper consists of two parts : the first being the observations on Dr. 

 Herschel's experiments, and some new ones, with inferences deduced 

 from them ; and the second the exposition of a theory, which the 

 author thinks may reconcile all contradictions. 



In the first part he sets out with briefly stating the manner in 

 which Dr. Herschel conducted the experiments he made, in order to 

 estimate, by the indications of different thermometers, the quantity 

 of heat transmitted through various substances, compared with the 

 heat afforded by direct rays from different luminous bodies, or more 

 properly sources of heat. Here the author soon starts a difficulty 

 concerning the mode of estimating the intercepting power of the 

 substances used in the experiments. As these experiments consist 

 of a series of observations made progressively at intervals of one mi- 

 nute between each other, it follows that the ratio Dr. Herschel adopted 

 between the heat produced by direct rays, and those transmitted 

 through coloured media, is not, as he imagined, a constant proportion, 

 he having uniformly deduced his inferences from the differences be- 

 tween the initial and the final degrees of heat ; whereas, had he at- 

 tended to the intermediate observations, he would have found that 

 each of them would have afforded a different ratio. 



Having maturely considered this subject, the author, adverting to 

 this circumstance of the various proportions of heat progressively 

 yielded in these experiments, observes, that it can hardly be con- 

 ceived why the faculty of transmitting and intercepting heat should 



