VOL. LV.] PHILOSOPHICAL TRANSACTIONS. IQS 



wise made, at the same time, to enlarge the apertures of longer object glasses, 

 where convex eye-glasses were used, by the same method; but these not succeed- 

 ing in the same manner, the method of making them with one lens of crown 

 glass, and one of white flint glass, was continued. 



As I could not see any good reason why the method, which was practised with 

 so much success, when concave eye-glasses were used, should not do with convex 

 ones, I determined to try some further experiments in that way. After a few 

 trials, I found it might be done; and in a short time I finished an object glass of 

 5 feet focal length, with an aperture of 3-|- inches, composed of two convex 

 lenses of crown glass, and one concave of white flint glass. Thinking that the 

 apertures might be yet admitted larger; I attempted to make one of 3-l feet focal 

 length, with the same aperture of 3-|- inches, which I have now completed, and 

 am ready to show the same to the r. s., if desired. The difficulty of procuring 

 good glass of so large a diameter, and of the thickness required, added to the 

 great exactness of the surfaces, in order to correct the aberration in such large 

 apertures, has prevented me from attempting to extend them any farther in that 

 length. 



Fill. Of a Salt found on the Peak of Tenerife. By W. Heberden, M. D., 



F. R. S. p. 57. 



In the account of a journey up the Peak of TenerifFe, by Dr. Tho. Heberden, 

 printed in the Philos. Trans., vol. 47, N° 57, mention is made of a sort of salt, 

 as well as of brimstone, with which some parts of the peak are covered. There 

 is no difficulty in conceiving how brimstone may be forced up by subterraneous 

 fires; and it is no uncommon thing to find it in other places: but it is not so 

 easy to understand how a salt, of so fixed a nature as this is, should be sublimed 

 to such a height, without being cooled and fixed long before it arrives at the sur- 

 face of the earth, where no sensible heat is perceived. Neither could Dr. W. 

 H. explain how it happens, that a substance so easily melted in water, is not dis- 

 solved and washed away, as fast as it can be produced, by the dews, and rains 

 and melted snow. 



By means of his brother Dr. Tho. Heberden, Dr. W. H. had procured a par- 

 cel of this salt collected from the peak, a specimen of which, together with some 

 of the sulphur, he presented to the Society, both which, though so very pure, 

 were just as they were taken up. His brother informed him, that the salt is 

 found not far from the verge of the crater, and that it is called, by the Spanish 

 inhabitants of the island, sahtron ; which is the name given by them to salt-petre; 

 and that it is sold for about 5 pence a pound. It appears to be the natron or 

 nitrum of the ancients, or, as it is sometimes called, the fossil alkali, which is 



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