394 Proceedings of the British Association, 



Tuesday, Wth August, — 6. Mr Davy detailed some experi- 

 ments which he had made upon the preservation of tin-plate by 

 the agency of zinc. When exposed for some days to the action 

 of water, the plate by itself soon becomes slightly corroded, but 

 is completely preserved by the zinc, the latter at the same time 

 oxidizing. Hence, the plate might be employed in place of 

 copper for many purposes, where salt water comes in contact 

 with vessels. Several metals, he had ascertained, are not pro- 

 tected. 



7. Mr Graham described the constitution of certain salts, in 

 continuation of the papers which he has published upon this 

 subject. 



8. Mr Johnston made some observations on the optical pro- 

 perties of chabasite, in reference to those made by Sir David 

 Brewster at last meeting. Sir David found that this mineral 

 possesses different refracting powers 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 results refer to particular species 

 only ; but the composition of the species vary, and are as repre- 

 sented here, 



S||J + 3AS 2 + 6Aq. 



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

 the difference, for the refractive power of chabasite is positive, 

 and that of quartz negative; thus accounting for the double re- 

 fractive power observed by Brewster. Dr Thomson remarked, 

 that the observations of Brewster probably referred to one spe- 

 cies 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 oc- 

 curred to Dr Daubeny in Italy. He visited a locality where 

 there was an upper stratum of lava, containing cavities. In one 



