352 OS TWO XEM- CRYSTALLINE COMPOUNDS 



Curve of Variation in Composition. 



In order to compare together the composition of the crystals and that of the alloy in 

 which they foim, I have resorted to the usual method of Analytical Geometry, and in 

 the plate at the end of this memoir the lower horizontal line is the axis of abcissas, 

 and the vertical line at the extreme left the axis of ordinates. The first has been di- 

 vided into equal parts, which denote the per cents of zinc in the crystals, and the last 

 into parts of the same size, which stand for the per cents of zinc in the alloys. The 

 zinc rather than the antimony determinations have been selected for comparison, as 

 being generally more accurate, and as having been all made in exactly the same way. 

 The points determined by analysis are indicated with dots, and the double line drawn 

 through these dots is a curve, w^hich represents the relation of the composition of the 

 crystals to that of the alloy in which they form. In order to make clear the connec- 

 tion between the two, it will be well to discuss this curve, commencing with what may 

 be termed the two centres of crystallization, the alloys of 42.8 and 31.5 per cent of 

 zinc, and examining the effect produced on the crystals by diminishing or increasing 

 the amount of zinc in the alloy. 



It has already been stated, that the crystals of SbZng are obtained in their greatest 

 perfection from the alloy of 42.8 per cent of zinc. They are then comparatively large, 

 generally aggregated, and, as the three analyses already cited prove, have the same com- 

 position as the alloy. On increasing gradually the amount of zinc in the alloy up to 

 48.7, the crystals continued to have the composition of the alloy, and the only differ- 

 ence which could be observed in their character was, that they were smaller, and more 

 frequently isolated. Between these limits the whole mass of the alloy exhibited a 

 strong tendency to crystallize, and by pouring it as it cooled from one vessel to another, 

 it could be crystallized to the last drop. The portion a b of the curve is therefore a 

 straight line equally inclined to the two axes. On increasing the amount of zinc in the 

 alloy to 50.7 per cent, the amount of zinc found in the crystals was only 46.89 per 

 cent, and above this it was uniformly less than it was in the alloy; but no closer rela- 

 tion between the two could be detected, owing undoubtedly to the unavoidable irregu- 

 larity in the crystallizations of the alloys which contained more than 50 per cent of 

 zinc. This arose from a peculiar pasty condition which the fluid mass assumed at the 

 point of crystallization, apparently caused by the separation of the excess of zinc. 

 Definite crystals, however, were obtained even from the alloy of 60 per cent of zinc, 

 which contained 55 per cent ; above this, the crystals became less and less abundant, 

 and gradually faded out, although the alloy even of 86 per cent of zinc exhibited a 



