186 fHILOSOPHICAL TRANSACTIONS. [anNO 1765. 



its specific gravity being diminished, appears in a stone described by Dr. Grew in 

 the Museum of the Royal Society: this gem is a kind of emerald, which, when 

 expanded by heat, becomes blue, and remains of that colour till cold, in which 

 state it returns to its usual colour, which is green. 



Tin is not capable of being vitrified, or imparting any colour to glass; nor are 

 any preparations of it of any primary colour. 



MERCURY. 



1. There is no body of an intermediate weight between gold and mercury; 

 and it is probable that a great part of the difference between their specific 

 gravities depends on the fluidity of. the one and the solidity of the other. Mer- 

 cury is not capable of communicating any colour to glass, being so volatile that 

 it will not bear the degree of heat necessary to incorporate it with the glass in 

 fusion. But it is well known that its calx, either prepared per se, or by dis- 

 solving it in an acid, and evaporating the menstruum, is red. — 2. A solution of 

 mercury tinges the skin, &c. red, as gold does. 



PLATINA. 



The specific gravity of platina being nearly equal to that of gold, it seems 

 necessary to examine whether the colour of its preparations correspond with 

 those of gold. On looking into a dissertation written by Dr. Lewis on that 

 metal, in the Philos. Trans., I find that the precipitates and crystals obtained 

 from solutions of platina, are red : and that a solution of that metal in aqua 

 regia, to perfect saturation, is of a dark red, though, when diluted, yellow ; 

 in the same manner that " a red liquor (as Sir Isaac Newton observes) in a conical 

 glass, looks of a pale and dilute yellow, at the bottom, where it is thin ; and a little 

 higher, where it is thicker, orange ; where it is thicker still, it becomes red ; and 

 where it is thickest the liquor is deepest and darkest." Newton's Opt. p. l6o. 



Having gone through these experiments and facts, which seem to show that 

 the metals invariably exhibit colours in the order of their densities, when 

 melted with glass, under the circumstances above mentioned ; and that the other 

 preparations of the same metals, for the most part, assume the same colours ; 

 it seemed probable that the cause, on which the colour of natural bodies de- 

 pend, may sometimes be conjectured from the chemical analysis of such sub- 

 stances. This I have attempted with regard to the colour of plants. It is 

 known, from the experiments of Lemery and others, that all earth is impreg- 

 nated with iron ; that the ferruginous matter is received into the roots of plants 

 in their growth, and makes part of their substance, and is universally dissemi- 

 nated through them ; and that iron may be separated by a magnet from the 

 ashes of all vegetables. 



It has been already observed, that the green colour of the glass used in 

 making bottles, is caused by the iron contained in the materials of which it is 



