142 Messrs. A Smithells, H. M. Dawson, and H. A. Wilson. 



reaction would take place between sulphuric acid, and an iodide, in the 

 presence of a reducible substance, at a higher temperature. 



Conclusions. 



It would appear that the only evidence on which the assumption 

 that gases of a permanent character, such as hydrogen, carbon mon- 

 oxide, nitrogen, helium, and argon, exist in the free state in the mineral 

 substances from which they are evolved on heating, rests on certain 

 observations with regard to the cavities which can sometimes be 

 detected by microscopic examination. 



The cavities may be either apparently empty or they may contain 

 liquid, and when the mineral is warmed the liquid disappears at a 

 temperature which is a few degrees below the critical point of carbon 

 dioxide or of some hydrocarbon. The fact that the critical temperature 

 of the liquid is a little below the point corresponding to carbon dioxide, 

 in the case of a mineral containing that substance, is not, however, of 

 very great significance as pointing to the presence of a permanent gas. 

 A small quantity of methane would produce the same result.* 



Further, although it can be shown that compact minerals do enclose 

 carbon dioxide and hydrocarbons, gases which can easily be liquefied, 

 the analogy cannot be extended to gases such as hydrogen and helium 

 in connection with minerals like chlorite, mica, and cleveite, which 

 exhibit many cleavages. 



On the other hand, there is, as I have endeavoured to show, a con- 

 siderable amount of evidence in favour of the theory which I have put 

 forward : That in the majority of cases where a mineral substance 

 evolves gas under the influence of heat, the gas is the product of the 

 decomposition or interaction of its non-gaseous constituents at the 

 moment of the experiment. The results of such experiments cannot, 

 therefore, serve as basis for speculation as to origin and history of the 

 substances in question. 



" The Electrical Conductivity and Luminosity of Flames contain- 

 ing Vaporised Salts/' By ARTHUR SMITHELLS, H. M. DAWSON, 

 and H. A. WILSON. Communicated by Sir H. E. ROSCOE, 

 F.K.S. Received October 24, Read November 17, 1898. 



(Abstract.) 

 1 . Object of tJie Investigation. 



No general consensus of opinion appears to exist as to the mode by 

 which the metal of an alkali salt is liberated when the salt is vaporised 



* Kuenen, ' Pliil. Mag.,' 1897. 



