l6 Plants forbade do not afford good Air. [Book VIII. 



mold, &c. find a proper nutriment in them, and by 

 converting a considerable part of the noxious effluvia 

 into their own fubftance, arreft it in its progrefs to cor- 

 rupt the atmofphere. 1 



The fame vegetables which afford vital air very 

 plentifully in the light of the fun, afford in the fhade 

 air lefs pure than that of the atmofphere. This ftrik- 

 ing effect of light on vegetables is a ftrong argu- 

 ment in favour of the opinion, that the motion of the 

 juices of the vegetables is performed by veffels, which, 

 like thofe of animals, poiTefs irritability, and are excited 

 to action by ftimulating lubftanceS. 



The effect of vegetation in producing the vital air, 

 which was afforded in the preceding experiments, 

 leemed in fome meafure dubious to Sir Benjamin 

 Thompfon, who extracted vital air, by immerfing in 

 water a variety of Jubilances, as raw filk, cotton, wool, 

 eider down, hare's fur, fheep's wool, ravdlings of linen, 

 and human hair as related in a former book. He 

 was led, from the refult of thefe trials, to fufpect that 

 the ;?nre air was merely feparated from the water, and 

 th.n any fubitance which would act by a capillary at- 

 traction, fo as to feparate ti.e component parts of the 

 water, would effect the production of pure air. He 

 therefore procured a quantity of (pun glafs, which con- 

 fifts of minute tubes., which he immerfed in water, but 

 the quantity of pure air produced was very trifling. 

 Hence he concludes, that there is fomething in thole 

 fubftances which operates in producing pure air, and 

 fhat it is not merely a mechanical reparation of the 

 component parts of water. 



The light of lamps produced the fame effect as the 



^iun's light, air in great quantities was produced, and 



perfectly pure. Vegetables will alfo, with any ftrcng 



light, produce vital air as well as with the light of the 



fun. 



