PREFACE TO THE THIRD VOLUME. v 



effluvia^ which render the air produdive of malignant diflempers, 

 the gaol-fever and the plague, fermented with, and neutralized 

 them. 



He had alfo difcovered, that the upper and nether waters of the 

 ocean, by the unequal preffure of its furface, when formed into vafl: 

 waves and furges, were blended together; which contributed to the 

 keeping its lower waters fweet ; and that in rivers, the furface 

 being an inclined plane, the upper parts are continually defcending 

 and re-afcending, fo as to form a perpetual inteftine motion, which 

 preferves their water from becoming putrid. But this ingenious 

 philofopher was not fo compleatly happy in his refearches, as to 

 conjedtuie the means which nature has provided for carrying out 

 of the atmofphere thofe heterogeneous particles, that are inceffantly 

 loading it; and which, if retained and fufFered to float in it, muft 

 foon infed the whole mafs, and render it unfit for fupporting 

 animal life. 



We owe to Dr. Prieflly the fuggeftion of two grand refources 

 for this falutary end J the firft he afligns to ihz 'vegetable kingdom, 

 the next to ihc fea, and other large colleftions of water; not how- 

 ever excluding Dr. Hales's principle of ventilation from a fhare in 

 this important office. 



He finds that the effluvia of vegetables are endued with the power 

 of reviving common air, that has been vitiated, or fouled, by fire or 

 refpiration. That the aromatic vapours of plants, are not neceflary 

 participants in the office of refloring this purity ; for that vegetables 

 o*^ an offenfive fmell, or thofe of no fmell at all, but are of a quick 

 growth, prove the very beft for this purpofe. 



That plants thrive wonderfully well in putrid air, and in air that 

 has been fpoiled by animal refpiration, in proportion to their vigour 

 and the found ftate of their leaves and branches. Thus the air, 

 which is deftrudive to animal life, is falutary and nutrimental to 

 vegetable ; and what is poifon to the former, is food for the 

 latter. 



In regard to the fecond refource, he tells us, that as well the air 

 corrupted by tht breath of animals, as that which is impaired by 

 other putrid mat\er, is in a good meajure fweetened by the feptic 

 particles infufing tuemfelves into water; hence he deduces, that the 



fea, 



