OF ARTS AND SCIENCES. 73 



but, with the solutions of the far less soluble antimonious iodide (in 

 disulphide of carbon) the phenomena were not perceptible. It is 

 evident, from these facts, that the phenomena of supersaturation are 

 not confined to aqueous solutions, or to substances which take into 

 their crystalline structure a portion of the solvent, like water of crys- 

 tallization. 



The second method of obtaining crystals of antimonious chloride, is 

 the familiar process of pouring off the still fluid portion, after the 

 melted substance has partially solidified. Since, in consequence of 

 the low temperature at which it hardens and the large amount of 

 latent heat evolved in the process, the chloride sets comparatively 

 slowly, the crystals form under these circumstances with great perfec- 

 tion, and are left clear and brilliant when the fluid is poured off. 



It is very easy to obtain, by either of these methods, very perfect 

 crystals, with very brilliant faces ; but to measure these crystals is a 

 difficult problem, which we have as yet been able to solve only imper- 

 fectly. Antimonious chloride is so very hygroscopic, that, during the 

 short time required to isolate the crystals and mount them in tightly 

 corked glass-tubes (and under such protection the measurements were 

 made), the faces so far lost their lustre as to render the reflected 

 image of the goniometer signal indefinite. Hence, the results given 

 below are to be regarded as only approximate, and may be in error to 

 the extent of even a degree. The angles of the vertical prism are 

 probably the most accurate, because the crystals could be most quickly 

 mounted with this dome parallel to the axis of the tube. The two 

 domes present were measured on different crystals, and the angles given 

 are the results of what were considered the most favorable observations. 

 When the crystal was once mounted, its position could not be shifted ; 

 for the antimonious chloride attacked the wax used, and this circum- 

 stance added to the difficulties attending the necessary manipulations in 

 measurements with the goniometer. 



The crystals of antimonious chloride are trimetric, and have the 

 same general habit whether obtained by the one or the other of 

 the two methods just described. The chief difference is a greater or 

 less degree of elongation in the direction selected as the vertical axis, — ■ 

 a difference which is shown by Figs. 1 and 2 of Plate I. The crystals 

 were examined with a polarizing microscope arranged as a stauroscope ; 

 and we observed that, when the crystals were resting on either of their 

 planes, so that the light passed between two opposite and parallel sur- 

 faces, the principal optical sections were, as nearly as could be observed, 

 either parallel or normal to the prismatic edges. This, although not 



