VOL. LII.] PHILOSOPHICAL TRANSACTIONS. SSQ 



hurt; and in 4 or 5 minutes time, he perceives this flame to decrease, and fall 

 into the earth. In the day-time the flame is of a different colour, and not 

 much unlike the flame which arises from a furnace. There are several mines 

 discovered in this county by these mineral fires, where there were no symptoms 

 of such mines before : but it is generally observed that they abound with mar- 

 casite and pyrites. These mineral flames, arising from ignited pyrites, are fre- 

 quently discovered in the bottom of mines and coal-pits ; and are often detrimen- 

 tal, and sometimes destructive to the miners; which made the late Dr. Wood- 

 ward and others imagine that they were vapours arising from an abyss. 



From what has been said Mr. S. draws the following conclusions. 



1 . That all subterraneous fires, even those of Hecla, Vesuvius, and ^tna, 

 together with those observed in the mines and coal-pits, are caused by the heat 

 and fixing of pyrites and marcasites. 2. That the waters of our hot baths derive 

 their heat from passing over a bed of ignited pyrites. Indeed the solid contents 

 of those waters do evidently prov^e this assertion, being nothing more than such 

 particles of the pyrites as are soluble in water. 3. That these mineral flames will be 

 more or less subtle, according to the minuteness of the particles of the combus- 

 tible matter, and the quantity of phlogiston which they contain. 4. That the 

 convulsive motions and tremblings of the earth are caused by the heat of the 

 burning pyrites expanding the air contained in its bowels. This is clearly proved 

 by their causing, immediately after, an eruption of the earth, which generally 

 discharges a dark coloured, cinder-like and frothy matter. And, 5. That those 

 places, where the earth contains the greatest quantity of pyrites and marcasites, 

 will be most liable to these convulsive motions and tremblings, no other natural 

 cause contradictory. 



XXII L Additional Observations on some Plates of IVhite Glass found at Her- 



culaneum, and on the Use of Glass in fVindows. By /. Nixon, A. M., F. R. S. 



p. 123. 



In a paper which Mr. N. presented to the Society about 2 years before, he 

 offered his thoughts on some plates of white glass found in the ruins of Hercu- 

 laneum. He now adds some more observations, with a view partly to explain 

 and support what he then delivered, and partly to communicate such new infor- 

 mations as he has since received relating to the same subject. 



Mr. N. observed, on the authorities produced by Mons. Renaudot, that glass 

 plates were not applied for magnifying objects in optical experiments, till the 

 beginning of the 13th century: but on reviewing his dissertation, Mr. N. finds 

 he sinks the antiquity of that usage a century lower than this. Mons. Renaudot 



3 7 2 



