PREFACE TO THK THIRD VOLUME. iii 



reiuoved, may be injurious to a perfon conftantly abiding there at all 

 hours, and more efpecially during the night, when they may hover 

 near the earth, for want of wind to difperfe, or of heat to rarefy 

 them. Such fpots are likewife commonly firft fet with plantane 

 luckers, which are of quick growth, extremely porous, and adapted 

 to purify the air; infomuch that walks, or plantations of them, are 

 remarkably healthful for reiidence. 



This fixed air, wK\ch, per fe, or uncompounded, is thought to be 

 one of the greateft antifeptics in nature, and may be received into 

 the lungs and bowels in a confiderable quantity, not only without 

 danger, but with eminent advantage in many purulent diforders; is, 

 neverthelefs, under feme modes of compofition and application, de- 

 flruulive to vitality, and affirmed to be the fame as mephitic air. 

 Philofophers, however, feem not to be as yet entirely agreed in this 

 charadleriftic : for fome alledge, that it is of a denfity or fpecific 

 gravity much greater than that of common or atmofpheric air ; 

 whereas the experiments made upon the air of a well, in which a 

 lighted torch was inftantly cxtinguifhed ; on the air of the Grotto 

 de Cani in Italy, and that of the cavern of Pyrmont f.«], feem to 

 indicate, that the mephitic does not differ from common air, either 

 in gravity, humidity, or elafticity. Until further difcoveries there- 

 fore {hall decide this variety of opinion, the definition which Dr. 

 Dobfon has given us may fafely be admitted. 



" Fixed air (fays he) is the general term by which this fubjedt is 

 *' diflinguiftied ; and when it produces any noxious effeds, either 

 " in confequence of the procefs by which it is procured, or the 

 *' manner in which it is applied, it is then properly to be called 

 " mephitic air [^]." 



Among the late advances towards an improved (yftem of natural 

 philofophy, there are none which refledl more honour on the hu- 

 man faculties, than thofe which have penetrated as it were into the 

 invifible world, and (by the teft of experiment, aided by rational 

 inductions) brought forth to our perception fome of thofe flupen- 

 dous agents, whofe fubtility had efcaped our fight, and whofe acti- 

 vity and power confounded and perplexed our judgement. Of thefe 

 difcoveries none are worthier our^attention, than what tend to explain 



[a] Percival on the vapours of charcoal, p. loo. [b] P. 94, 



a 2 and 



