Mr. D. Forbes on Evansite, a new Mineral Species. 341 



upon the face of his pile, it required all Melloni's acuteness to 

 nurse the calorific action up to a measurable quantity. Such 

 experiments, however, demonstrate, not that the two agents are 

 dissimilar, but that the sense of vision can be excited by an 

 amount of force almost infinitely small. 



46. Here also we are able to offer a remark as to the appli- 

 cability of radiant heat to fog-signalling. The proposition, in 

 the abstract, is a philosophical one; for were our fogs of a 

 physical character similar to that of the iodine held in solution 

 by the bisulphide of carbon, or to that of iodine or bromine vapour, 

 it would be possible to transmit through them powerful fluxes 

 of radiant heat, even after the entire stoppage of the light from 

 our signal lamps. But our fogs are not of this character. They 

 are unfortunately so constituted as to act very destructively 

 upon the purely calorific rays ; and this fact, taken in conjunc- 

 tion with the marvellous sensitiveness of the eye, leads to the 

 conclusion that long before the light of our signals ceases to be 

 visible, their radiant heat has lost the power of affecting, in any 

 sensible degree, the most delicate thermoscopic apparatus that 

 we could apply to their detection. 



Roval Institution, October 1864. 



XL. On Evansite, a new Mineral Species. 

 By David Forbes, F.R.S., 8f<?.* 



I^HIS mineral was brought from Hungary in the year 1855 

 by the late Mr. Brooke Evans of Birmingham \, and was 

 then reported to be found in some abundance as an incrustation 

 in drusic cavities which occurred in the brown iron ores. It 

 was regarded as pertaining to the mineral species allophane J, 

 with which it agrees in many of its physical properties, as hard- 

 ness, colour, specific gravity, &c, as well as in the percentage of 

 loss sustained upon heating the mineral to redness. 



The specimen I received from Mr. Evans was labelled Allo- 

 phane from Zsetcznik, Gomar Comitat, and was very beautiful 

 in appearance, consisting of an agglomeration of small stalactites 

 with reniform and globular excrescences on brown haematite, many 

 of these excrescences much resembling artificial or natural pearls, 

 having both the figure and characteristic pearly lustre of such. 



I doubted the identity of the mineral with allophane ; and a 



* Communicated by the Author. 



t After whom the species is now named. 



X A considerable number of specimens had been given by Mr. Evans to 

 private collections in England all labelled " allophane," and I understand 

 that many more had likewise been distributed in Germany under the same 

 name. 



