86 PROCEEDINGS OP THE AMERICAN ACADEMY 



The yellow iodide generally forms but a small portion of the product of 

 the sublimation ; but, by carefully regulating the temperature and gas cur- 

 rent, the proportion can be largely increased, and it can then be picked 

 out among the scales of red iodide with which alone it is mixed. To 

 obtain, however, in this way, a sufficient quantity of the yellow iodide 

 for analyses wholly free from its more abundant red associate, would 

 have been a very tedious task ; but this was not necessary, in order to 

 establish the perfect isomerism of the two substances. Of the material 

 used in the following analyses, more than nine-tenths consisted of the 

 yellow iodide of antimony ; and as the results agree so closely both 

 with theory and with the parallel analyses of the associated red iodide, 

 which could be easily and perfectly isolated, there can be no question 

 whatever that the two have the same percentage composition. These 

 analyses are reproduced from our previous paper, in order to bring 

 together all the facts bearing on the present discussion. 



Comparative Analyses of Rkd and Yellow Iodide of 

 Antimony. 



Red Hexagonal. Yellow ORxnoRHOMBic. 



. No. Weight taken. % of Iodine. No. Weight taken. % of Iodine. 



1. 1.1877 76.110 1. 0.4610 76.161 



2. 2.3201 76.040 2. 0.3496 76.161 



Mean value 76.075 76.161 



Theory for Sbl^, when Sb = 120 and I = 127 76.047 



As additional evidence of the isomerism of these remarkable sub- 

 stances, we may here, in anticipation of a fuller discussion of the subject, 

 mention a fact which would be by itself conclusive. At a very mode- 

 rate elevation of temperature, the yellow iodide of antimony is com- 

 pletely converted into the red modification, and under such conditions 

 that there can be neither loss nor gain of material in the process. 



In measuring the angles of the rhombic plates as they lay in a nor- 

 mal position under the microscope, we found that we could obtain the 

 sharpest results by projecting the image on a sheet of paper by means 

 of a camera lucida. We were then able to adjust a straight edge to 

 one after the other of two edges of the crystal, and draw the corre- 

 sponding lines, whose angular divergence we then measured with a 

 protractor. These measurements gave, for the acute angle of the 



