368 Mr. Henrfs Conftderatiom on different Materials, 



dyes ; for, thofe that can withftand its aftion, 

 will endure every other hardftiip without injury. 



Again, fome chemifts have confidered iron, as 

 the colouring matter of vegetables, as it cer- 

 tainly is, in many inftances, of minerals. But 

 this theory does not neceffarily exclude phlo- 

 gifton. For it is fuppofed that the vari^n 

 colours which plants exhibit, may depend on 

 the various Hates o( phlogijl-ication in which the 

 iron exifts. Thus iron, when dilTolved in vi- 

 triolic acid, is green. Apply fuch a degree of 

 heat, as may drive off a part of the remaining 

 inflammable principle, and the acid, it becomes 

 yellow i and, on carrying the procefs ftill farther, 

 it defcends farther to red, and purple. It muft 

 be allowed that iron enters as a component part 

 into moft plants : and that its calces are capa- 

 ble of exhibiting great variety of colours. But 

 ftill it is to phlogifton or light, that we are to 

 look up as the real caufe of colour j this being 

 the aftive, while the martial calces, can, at 

 moft, be regarded as the paflive principle. 



Since the formation of the antiphlogiftian 

 theory, indeed, by which the exiftence of phlo- 

 gifton is denied, the various colours of plants 

 have been accounted for, from the different pro- 

 portions of dephlogifticated air, they may retain. 

 Plants, when expofed to the aftion of the iun'St 

 rays, have their water decompofed and part with 

 this pure air, yielding it in proportion to the 



quantity 



