356 ON TWO NEW CRYSTALLINE COMPOUNDS 



cent (the calculated per cent of SbZna is 33.5), and j^ = 31.5 per cent, ^\hich is one 

 half of 43 + 20. Through this point draw a line m h, parallel to the axis of ordi- 

 nates, and intersecting the line a h hat c. The line m i is the tangent, and the line 

 b h the chord, of the required arc. On the line in i take c i= c k, and i is the point 

 at which the arc should touch the tangent. Erect a perpendicular on the tangent at 

 the point i ; take o i = half b h, and from o as a centre with a radius = o i describe 

 the arc h n i. Also, from the centre o let fall a perpendicular o g on the chord b h, and 

 produce it to a point d, making d g ^= o g. It will intersect the arc at n. From d, 

 as a centre with a radius d n, describe a second arc, m n intersecting the tangent at m. 

 Finally, draw from k a straight line k I parallel to b h, then the broken line I k m n h 

 will be the required curve. 



It will be noticed that the tangent, which has been laid down on the plate through 

 the points determined by analysis, is two tenths of one per cent in advance of the line 

 which would correspond to ShZn^. This position is essential to the equality of k c and 

 cijifwe retain as the value of the radius of the larger arc R = v 6 /«. If the analyses 

 should have given erroneously too much zinc, so that the true position of the line 

 should be at x = 33.5 per cent, then this equality would be destroyed, and the condi- 

 tions for finding the centre o w-ould be reduced to the co-ordinates of the point h, the 

 length of the radius, and the position of the tangent, from W'hich, by a very simple con- 

 struction, the cuiTe might be drawn. It should, however, be remarked, that the posi- 

 tion of the tangent in advance of the line x =: 33.5 is in accordance with the fact, 

 already noticed, that the crystals of SbZuj have, throughout, a proneness to an excess 

 of zinc, caused apparently by the influence of SbZus ; but it is also true that the ten- 

 dency of the error in the zinc determinations is in the same direction. 



Before discussing the conclusions to which the facts already stated seem directly to 

 point, it will be well to see how far the variation in composition corresponds to a vari- 

 ation in the properties of the two compounds. Three classes of properties have been 

 examined in this connection ; namely, Specific Gravity, Crystalline Form, and Afiinity 

 for Oxygen, which wull be treated of in order. 



Specific Gravity. 



The specific gravity of all the crystals analyzed, as well as that of the antimony and 

 zinc used in the investigation, has been taken with the greatest care. The determina- 

 tions were made with a nicely constructed specific gravity bottle, as this method was 

 found susceptible of greater accuracy than any other when the temperature of the 

 water was observed with precision. The small double cone of silver recommended by 



