106 PHILOSOPHICAL TRANSACTIONS. [aNNO 1776. 



1000. The brittleness of glass is diminished by crystallization ; for crystallized 

 glass is less apt to crack by change of heat and cold. 



Crystallization is always accompanied or preceded by an evaporation of the 

 lighter and more fluid parts of the glass; for Mr. K. found that, by exposing a 

 piece of transparent glass till it was entirely crystallized, a 58th part of its weight 

 was lost by evaporation : and he was induced to believe, from other trials, that 

 glass, which contains too large a proportion of saline fluxes, is less capable of 

 crystallizing than other harder glasses, till it has lost its superfluous quantity of 

 such fluxes by evaporation. A doubt may therefore arise, whether the change 

 of properties, induced by crystallization, be merely the effects of altering the 

 texture, that is, the arrangement of the minute integrant parts of glass ; since 

 this change is always accompanied with a loss of the lighter parts of the crystal- 

 lizing substance. But though a superfluous quantity of saline or other fluxes 

 may impede the crystallization, yet that the change of properties induced by crys- 

 tallization is principally or solely the efl^ect of an alteration of texture, is evident 

 from this observation ; that a piece of crystallized glass, when exposed to a heat 

 considerably more intense than is sufficient merely for its fusion, and afterwards 

 hastily cooled, loses all its acquired properties, and is again reduced to the state 

 of transparent brittle glass, which however, by means of the evaporation it has 

 sustained of its lighter and more volatile parts, is rendered considerably harder, 

 denser, and less fusible, than it was before the crystallization. 



Many analogous instances might be adduced to show how much the properties 

 of bodies depend merely on the different arrangements of their integrant parts, 

 or on their modes of crystallization. Thus, for instance, cast-iron and steel, 

 when cooled suddenly, acquire a much finer grain or texture than when annealed, 

 or slowly cooled, and are also more hard, elastic, brittle, and sonorous. From 

 the above description of vitreous crystals we learn, that very different crystalliza- 

 tions occur in the same kind of substance exposed to different circumstances ; 

 and even that sometimes differently shaped crystals are found in the same piece 

 of glass ; in which case, the circumstances must have been the same. Perhaps 

 indeed the difference observable in the shape of the crystals, in the same piece 

 of glass, may only mark the different periods in the progress of crystallization ; 

 for the crystals represented by fig. 7, 8, and Q, which are found in the same 

 piece of glass as those represented by fig. 6, chiefly differ from these in their 

 altitude ; and perhaps the latter kind may have been composed of a number of 

 the former uniting by their bases. The wheel-like crystals also, fig. 1 1, seem to 

 consist of several of the semi-columnar ones, arranging themselves round a 

 common centre, like the spokes of a wheel. The globular crystals in the 

 specimen N° 4 have been already observed to consist of many needle-like crystals, 

 converging to one central point. 



