182 PHILOSOPHICAL TRANSACTIONS. [aNNO IjOb. 



to remain in the furnace a few minutes after the copper is added, the mass will 

 turn out green instead of red : in effect, the preparation of copper recommended 

 on this occasion, is exactly the same as that used in tinging glass green. 



IRON, 

 being of all metals the most imperfect, is subject by various means, to be cal- 

 cined or reduced to a ruddy crocus, similar to the rust tliat arises from its being 

 corroded by the acid in the air. In this state it requires a considerable degree of 

 heat to dissolve and incorporate it with glass : till that heat is applied, it retains 

 its ruddy colour ; by increasing the heat, it passes through the intermediate 

 colours till it arrives at its permanent one, which is blue : this being effected in 

 the same degree of heat in which we have examined the other metals, that is, 

 the greatest that the glass will bear without losing all colour whatever. The 

 green, with which the glass used for bottles and chemical vessels is tinged, is 

 occasioned by the iron contained in the vegetable ashes and sand of which that 

 glass is composed. When the pots, in which the matter has been kept in 

 fiision, are nearly emptied, the glass remaining at the bottom is always blue ; 

 this is caused by its continuing longest exposed to the fire, and in so small a 

 quantity that the fire has a greater effect on it. The whole mass acquires the 

 same colour, if too much sand be added in proportion to the ashes ; for in that 

 case the materials being more difficult of fusion, the workmen are obliged to 

 apply a greater heat, and to continue it longer. It is known, from the experi- 

 ments of Lemery and others, that the vegetable ashes contain iron. To exa- 

 mine whether that metal be also contained in the sand used in making this glass, 

 and how far the colour of the glass depends on it, I made the following experi- 

 ments : 



Exper. 1 . Having procured some of the sand used in making green glass, I 

 melted 1 parts of it with one part of borax, and one part of nitre , and found 

 that it produced a glass similar in colour to that which is made with the same 

 eand fluxed with pot ashes. Hence it appears, that the colouring matter was 

 contained in the sand. — Exper. 1. I mixed 3 parts of this sand with one of pow- 

 dered charcoal, and exposed it for some hours to a red heat. When this mixture 

 was cold, I separated from it, by a magnet, small grains of iron, weighing about 

 ^ part of the sand. — Exper. 3. I melted sand thus deprived of its iron, 

 with half its weight of borax, and the same quantity of nitre ; and found that 

 it produced a perfectly colourless and transparent glass. — Exper. A. To 2 parts 

 of the white sand used in making crystal glass, and one of borax and nitre, I 

 added a 20th part in weight, of the grains of iron which I had extracted from 

 the sand by exp. 2, and having vitrified this composition, I found that it was 

 become exactly similar, in colour, to that commonly used in making green glass. 

 — Exper. 5. I exposed several pieces of green bottle glass made at different 



