98 PROCEEDINGS OF THE AMERICAN ACADEMY 



tunately, in ttieir natural state, these crystals were not well adapted to 

 the purpose, and the best we have hitherto obtained have been too 

 small,, and the material too easily altered, to admit of the preparation 

 of sections. We shall return to this work at the first opportunity, and 

 we hope for better success in the future. For the present, we can only 

 say that the axes of elasticity in the plane of symmetry make a small 

 angle with the basal plane and its normal. With a minute and 

 imperfect section, the rough measures which were alone possible 

 under the microscope gave for the value of this angle about 8^. And, 

 furl her, that we were unable to see the interference figure through 

 the basal planes under conditions which were far more favorable than 

 those under which the hyperbolas were seen through the orthorhombic 

 plates : for the tabular monoclinic crystals we used in these observa- 

 tions were both larger and thicker than the plates ; and, in order to 

 secure an accurate comparison we examined the different crystals in 

 succession. We also examined several cleavage sections parallel to 

 the basal planes, with no better success ; probably because the angle 

 between the optical axes is sufficiently great to determine total reflec- 

 tion. The high index of refraction of the substance would naturally 

 produce this effect at a very moderate value of the optical angle, and 

 unfortunately the cr}stals at once lost their transparency when im- 

 mersed in the liquids generally used in such cases. 



The above results, although partial, are important as indicating a 

 marked difference between the two classes of crystals we have last 

 studied. Nevertheless, a comparison of measurements will show that 

 the dimensions of the basal section of the monoclinic crystals differ 

 by only two or three degrees from those of the orthorhombic plates 

 before described. Thus, we have 62° 44' in place of 60°, and 

 100'' 16' in place of 98° 13'; and if, from the angle of the hemi- 

 octahedral plane on the basal section, we calculate the length of an 

 assumed rectangular vertical axis, the value obtained (0.867) does not 

 differ very greatly from the values of the corresponding axis for the 

 chloride, bromide, and hexagonal iodide of antimony, as given on page 

 113. Moreover, on recrystallizing the monoclinic iodide from a solu- 

 tion in pure sulphide of carbon, we have in two instances obtained — 

 mixed with the hexagonal iodide, which is always the chief product — 

 microscopic rhombic plates showing all the planes of the octahedron, 

 and whose angles (as nearly as they could be measured under the 

 microscope) were 60° and 120°. 



As it seems to us, the natural inference from the facts we have 

 developed is the tlieory already intimated. According to this theory, 



