[ 236 ] 
ExtraB from an Efay^ entltuled. On the Ufes of a 
Knowledge of Mineral Exhalations when applied 
to dilcover the Principles and Properties of Mineral 
Waters, the Nature of Burning Fountains, and 
ofthofe poifonous Lakes, which the Ancients called 
Averni j which was read before the Eoyal Society in 
April 1741, and to which the Author refers in the 
foregoing Experiments. 
A More accurate Hiftory of fubterraneal Exhala- 
tions would alfo further contribute towards the 
improvement of the Medical art, by leading us to a 
more perfedl knowledge of the genuine principles of 
feveral kinds of mineral waters j and by teaching us 
how to apply them rightly, and how to account, in 
a rational manner, for their operations and effedts 
upon the human body. 
For it hath long been obferved (i), that the chief 
energy and virtue of feveral mineral waters doth not 
xefide in the more grofs, faline, earthy, or metalline, 
particles, which they imbibe in their fubterraneous 
paffages, but in a far* more fubtile and volatile prin- 
ciple whei'ewith they long remain imbued when 
clofed in bottles ; but when they are expofed in open 
veffels, this volatile principle foon takes its flight,^ 
leaving the waters dead and inert, and deprived ot 
their former falutary qualities. 
This fubtile and fugitive principle Is, therefore, 
fitly called the Spirit of mineral fountains. To ex- 
trad which fpirit in the form of a liquid, many have 
j[i) Vide Bechcri Phyfica Subterranea, fe£l. il. cap. iv. 
employed 
