VOL. LV.] PHILOSOPHICAL TRANSACTIONS. 187 



made ; and I have cited Becher's opinion, that the green or blue colour in glass 

 is an indelible mark of its vegetable origin. 



This observation of the constancy of that colour in glass made of vegetable 

 ashes, and its being caused by iron, led me to conjecture, that the colour of the 

 entire vegetables arises also from the iron, so universally diffused throughout 

 their substance in their growth. Green is the colour which iron assumes con- 

 stantly, when dissolved by the acid in the air ; that metal thus dissolved being a 

 true green vitriol of iron : and as this ferruginous or vitriolic matter is univer- 

 sally disseminated through the leaves and branches of plants, those parts of it 

 which are at the surface will, by their contact with the air, assume the colour 

 peculiar to its salt or vitriol. 



Most vegetables, when they grow in such a manner as to be defended from 

 the contact of the air, are prevented from becoming green. This happens to 

 the root of trees, and as much of their stem as is covered with earth : grass 

 growing under stones, or other bodies, that accidentally lie on it, is white; not 

 having the least green, but as the air has access to it : and it is a method com- 

 monly used by gardeners, to cover with earth those parts of plants which they 

 would preserve white: by that means hindering them from being tinged green 

 by the contact of the air, as the parts exposed to it are: though it appears from 

 experiment that the presence of light, as well as of air, is necessary to the pro- 

 duction of the colour of plants. 



Besides the iron dissolved at the surface of plants by the air, that which is 

 contained in the inside of them, may be kept in a state of solution, when it 

 meets with a proper quantity of acid ; and it is remarkable that the inside of 

 most fruits, and other parts of plants, remain green no longer than they con- 

 tinue in an acid state. The quantity of iron contained in plants will not appear 

 too small to produce their colour, when it is known that one grain of vitriol, of 

 which only a small part is iron, the rest being acid and water, is able sensibly to 

 communicate a green colour to ten thousand grains of water. Lemery men- 

 tions this great divisibility of iron as an argument of its being able to pass into 

 the smallest parts of plants. Mem. Acad, anno 1706. 



A circumstance which strongly confirms that the colouring matter of vege- 

 tables, and a ferruginous vitriolic substance, are of one and the same kind, is, 

 that the vitriol of iron, which is green, passes through the same colours, while 

 its moisture is evaporating, which vegetables do, when by withering they undergo 

 the same sort of change : the vitriol deprived of its water by calcination grows 

 first yellow and then red ; and Sir Isaac Newton has observed that, " when 

 vegetables wither, some of them turn to a greenish yellow, and others to a 

 more perfect yellow or orange, or perhaps to red, passing first through the 

 aforesaid intermediate colours : which changes seem to be effected by the exhaling 



B B 2 



