VOL. LXVI.] PHILOSOPHICAL TRANSACTIONS. 105 



glass. This glass is composed of sand, kelp, calcarious earth, and lixiviated 

 vegetable ashes. 



Crystallizations frequently occur also in the glass of which common bottles 

 are made, the materials used in the composition of which are nearly the same 

 as those abovementioned for broad-glass, with the addition sometimes of the 

 scoria of iron furnaces. Of this kind is the specimen marked N° 5, in which 

 the crystals are not enveloped in a medium of transparent uncrystallized glass, 

 for the whole piece is an opaque, crystallized substance ; but they are prominent 

 from the surface of the mass. The form of the crystals is that of the blade of 

 a two-edged sword, vAhose point is truncated. In no other glass had Mr. K. 

 seen such perfect crystals as in those 2 kinds abovementioned, broad and bottle- 

 glass, which being more fluid and less tenacious, when melted, than any other, 

 the minute particles of which crystals consist, more easily concrete, and apply 

 themselves to each other with less resistance from the medium. Perhaps also 

 the greater proportion of calcarious and other earthy particles may dispose these 

 glasses to crystallize more than others, which contain a larger quantity of saline 

 and metallic fluxes. 



Flint-glass, when long exposed to a dull red heat, acquires a cloudiness, 

 which probably proceeds from a number of small white particles, concreted by 

 means of crystallization : but these crystals are too minute for observation. 

 Mr. K. suspects also that the opaque whiteness, given to glass by 

 arsenic, is the effect of a crystallization, to which this substance disposes 

 certain kinds of glass ; for the opacity given to such glass by arsenic, being 

 greater than the opacity of the arsenic itself, cannot be communicated to a large 

 proportion of transparent glass, merely by the mechanical interposition of this 

 opaque, and sometimes only semi-opaque substance. 



Mr. Reaumur has observed, that some kinds of glass, by long exposure to 

 certain degrees of heat, acquire a white opaque crust on their surface ; and that 

 this change of colour and texture, by a longer continuance of the heat, pene- 

 trates farther, till at length the whole substance of the glass is converted into a 

 white, opaque body, which, from some supposed resemblance to porcelain, has 

 been distinguished by the name of Reaumur's porcelain, but is really nothing 

 else than glass indistinctly crystallized. 



Some of the properties of glass are considerably changed by crystallization ; 

 its transparency is destroyed, and it acquires an opaque or semi-opaque white- 

 ness, its density is increased, for the density of a piece of crystallized glass was 

 found, by experiment, to be to that of water as 2676 to 1000 ; whereas the den- 

 sity of a piece of uncrystallized glass, which had been contiguous to the former, 

 and consequently had been composed of the same materials, and exposed to the 

 same heat and other circumstances, was to the density of water, as 2662 to 



VOL. XIV. P 



