200 Proceedings of the British Association for [Sept. 



David Brewster at last meeting. Sir David found that this 

 mineral possesses different refracting povt^ers at different 

 depths of the crystal, and he concluded, that it consisted 

 of distinct layers, and that 'if subjected to experiment it 

 would afford the result of a compound substance. His re- 

 sults refer to particular species only ; but the composition 

 of the species vary, and are as represented here 

 C 



+ 3 AS2 + 6 Aq. 





where Mr. J. conceives that it is easy to see the cause of 

 the difference, for the refractive power of chabasite is posi- 

 tive, and that of quartz negative ; thus accounting for the 

 double refracting power observed by Brewster. 



Dr. Thomson remarked that the observations of Brewster 

 probably referred to one species of chabasite. But there 

 are two species, the one containing soda and the other lime 

 as a base. He, therefore, considered that until both species 

 were examined, no inference whatever could be drawn. 



9. Dr. Daubeny stated, that according to the opinion of 

 Von Buch, carbonate of magnesia must have been sublimed 

 in many instances by volcanic action, although as far as 

 Dr. Daubeny was aware, it was not agreeable to the results 

 of chemists. A curious fact illustrative of the truth of Von 

 Buch's opinion, occurred to Dr. Daubeny in Italy. He 

 visited a locality where there was an upper stratum of lava, 

 containing cavities. In one of these Colonel Robinson dis- 

 covered a large quantity of carbonate of magnesia. Dr. 

 Daubeny found a quantity coating the upper surface of the 

 lava. 



Dr. Dalton observed that there could be no doubt as to 

 the sublimation of carbonate of magnesia, as Dr. Henry had 

 informed him that a quantity of this salt was always driven 

 off whenever the heat was carried beyond a certain height. 



10. Dr. Dalton stated the results of his examination of 

 the spirit distilled from caoutchouc. He found it to depress 

 the barometer like sulphuric ether. It passes through 

 water without diminishing its volume, thus differing from 

 ether. It is absorbed by water like olefiant gas. It con- 

 sists of 2 olefiant gas. 10 vols, when burned give 40 

 carbonic acid, and require 60 of oxygen. It appears to 

 be the same as a substance described by Faraday. It differs 



