VOL. LV.] PHILOSOPHICAL TRANSACTIONS. 188 



glass-houses, under a muffle, to a strong fire, for the space of half an hour ; 

 and found that they were all become blue. 



If the crocus of iron be added in too great a proportion, it continues to ad- 

 here together, and remains unmixed, or at least imperfectly mixed, with the 

 glass, retaining for that reason the colour natural to it when undissolved ; or if 

 it be in a smaller quantity, though yet in too great a proportion to be dissolved, 

 it will make some intermediate colour between the ruddy and the blue, which 

 last it always imparts when in a sufficient degree of fire, and a proper proportion. 

 The necessity of a due proportion of metal to the glass has been already instanced 

 in gold, which, if in too large a proportion to be dissolved by the glass, instead 

 of imparting a red colour to it, runs together in its metallic form. 



Henckel has given us a method of making a beautiful blue glass by this 

 means. It consists merely in mixing iron with the matter of which the purest 

 glass is composed, and exposing it to a violent fire. Gellert observed also, that 

 iron imparts to glass this colour. Mr. Lehman obtained the same colour from 

 emery, which is a kind of iron ore, or ferruginous stone, by mixing it with a 

 vitrifiable earth ; which colour he attributes to the iron contained in it. Neri 

 mentions a sky colour imparted to glass by Bohemian granates, which he con- 

 stantly practised at a manufactory in Flanders. It is well known that iron is 

 the metal contained in those stones ; that they obey the loadstone ; and that, 

 being calcined with a proper heat, they yield a considerable quantity of iron. 



I exposed in a crucible to a glass-house fire, for the space of 30 hours, part 

 of a flint-glass retort, in which a native green vitriol of iron had been distilled, 

 and which had been corroded and tinged by it ; by this means it became coloured 

 of a fine transparent blue, not distinguishable from that which cobalt imparts to 

 glass. Iron vitrified per se is converted into a blue glass. In short, it is indu- 

 bitable that iron is the only metal, which will, without any addition, impart to 

 the matter of glass a blue colour ; for copper will not communicate that colour, 

 without the addition of a considerable quantity of salts, or some other matter 

 that attenuates it ; and the other metals cannot by any means be made to pro- 

 duce it at all. 



Having showed that the metals exhibit colours, invariably in the order of 

 their densities, when melted with glass in a due proportion, without any other 

 ingredient, and exposed to a sufficient heat ; I proceed to show that the other 

 preparations of the metals, viz. their solutions, precipitates, crystals, &c. do 

 for the most part exhibit the same colours, in the order of their densities, 

 though not so invariably as their glasses ; some small variation of colour hap- 

 pening in the more imperfect metals, probably from a change of density in their 

 different preparations. 



GOLD. 



1. Gold precipitated from aqua regia, and washed with hot water, or boiled 



