iv PREFACE TO THE THIRD VOLUME. 



and perfect the abftrufer dodlrines of pneumatic fcience, and direct 



their application to mofl: ufeful purpofes of human life. 



Modern experiments have proved, that the mineral fpirit, which 

 imparts to chalybeate-waters all that pungent tafte, vivacity, and 

 volatile principle, on which their virtues chiefly depend, is no other 

 th^n fixed aif\ which endues water with the power of difiblving not 

 only calcarious earths, but even iron. Dr. PriejUy has ftiewn an 

 eafy mode of extracling this air from grollly- powdered chalk, juft 

 covered with water, with a fmall addition of fpirit of vitriol, com- 

 monly called the vitriolic acid. He has (liewn it capable of opera- 

 ting medecinally in putrid diftempers; and the experiments tried 

 by Dr. Perchal, and other able gentlemen of the faculty, have con- 

 firmed its amazing efficacy in feveral cafes. 



Such difcoveries lead us on to a moll interefting feries of enquiry j 

 for they fucceflively prefent to our view fome new tefHmony of the 

 divine Wifdom, in the contrivance (and moft beneficent purpofes 

 towards us, in the den:ination)of many wonderful phaenomena pecu- 

 liar to our fyftem. In this purfuit, tempefts, volcanos, lightning 

 and earthquakes, begin to lofe their horror j and while they appear 

 remedial in thofe diforders to which the material conftitution of 

 our world is liable, we cannot but refpedl them as necefTary and 

 propitious, in the fame manner as we regard thofe valuable fpecifics, 

 which bring a recruit of found health to our own diflempered 

 frames. 



The inceflant vitiation of our atmofpherej by the breath and pu- 

 trefadion of animal, and the decay of vegetable bodies, by fires, 

 and by volcanos, made it reafonable to conclude, that fome pro- 

 vifion there muft be in nature for corredting this depraved flate, 

 and reftoring the air to purity. Dr. Hales feemed to think no other 

 agent neceflary for this purpofe than motion ; that, frcfti common 

 air, impelled with velocity into that which was flagnant, confined, 

 rancid, and peftilential, or into flinking water, fweetened them by 

 difperfing and carrying off the volatile, offenfive particles, with 

 which they had been furcharged, and which loft their ill quality 

 upon being diluted with, and abforbed into the vaft tide of the at- 

 mofphcre. He indeed fuppofed that the acid fteams of vinegar and 

 Ailphur, having a kind of eledive attraction towards the alkaline 



effluvia^ 



