and Chem ical Composit ion of Minerals. 7 
Water of Crystallisation, t 297 
Sulphuric Acid, - 25.5 
Oxide of Nickel, - - 39-5 
Oxide of Copper, - — - <5.3 
100.0 
a result which removes the only exception which I had found to 
the generality of the law respecting sulphates with a single base* 
The Nitrite of Lead , formed by boiling the nitrate with me- 
tallic lead, crystallises in fine regular octohedrons of a yellowish 
colour, and is destitute of double refraction *. Mr Herschel 
afterwards examined this salt as made in a similar manner by 
himself ; but he found that it crystallised in long, flat, yellow 
needles, and had two axes of double refraction. It must there- 
fore have been a different substance from that which I used ; 
and I suspect it to be the Quadro-nitrite of Lead . 
The advantages of optical analysis will appear in a still more 
striking point of view, from a memoir on the Mesotypes which 
will soon be ready for publication. The examination of this in- 
teresting class of the Zeolites, was suggested to me in 1818, by 
Mr Warburton, to whom I had occasion to mention the results 
which I had obtained respecting the connexion between the opti- 
cal structure and the chemical composition of minerals. This 
acute philosopher informed me, that Dr Wollaston had detected 
Lime in the Iceland Mesotype ; whereas the Auvergne mineral 
contained Soda in place of lime ; and he suggested this as an ex- 
cellent test of the application of optical analysis. I lost no time in 
examining the structure of the mesotypes, with which I was am- 
ply supplied from the cabinets of Sir George Mackenzie, Mr 
Allan, and Mr Ferguson of Raith, which are always liberally open 
for the purposes of science, and I thus obtained results of which 
neither Mr Warburton nor myself could have had the most 
distant anticipation. The substance which II ally calls Mesa - 
type, includes no less than six separate mineral species, all of 
which are distinguished from one another by optical characters 
of the most marked and beautiful kind. Mineralogists speedily 
recognised a new mineral in the Mesotype epointee of Hauy. 
* 'the crystals from which I obtained this result, were made for me by Mr 
Badams of Birmingham. See Vhil. Trans. 1818, p. 25i. 
