310 F. R. Fraprie — Chromates of Caesium. 



pyramid the faces of which could not be measured on account 

 of etching. Many crystallizations were made before well-ter- 

 minated crystals were obtained, as the slightest rise in temper- 

 ature of the solution caused etching, owing to the considerable 

 change in solubility of the salt, and the pyramid faces were 

 first attacked. By removing the crystals at a time when the 

 temperature of the solution was falling, very perfect crystals 

 were finally obtained and the pyramid measured on a two- 

 circle goniometer. With the exception of some indeterminate 

 line edges in the prism zone on one or two etched crystals, no 

 other forms were observed on these crystals. The angle from 

 the pole of the prism zone (corresponding to the absent basal 

 pinacoid) to the pyramid was found to be 39° 23'. 



Many attempts were now made to crystallize isothermally at 

 from 50° to 70° in a thermostat. The solubility increases so 

 markedly at high temperatures that the solution always got 

 down to very small bulk before crystallization commenced, and 

 crystals could only be got by allowing the solution to go to 

 dryness. These crystals were naturally not ideal for measure- 

 ment, and only one crop was obtained, at 70°, which gave crys- 

 tals with measurable pyramids. These gave the same value 

 as those crystallized at ordinary temperature. 



The hypothesis of compressible atoms* suggested that crys- 

 tals grown under pressure might exhibit a change of habit or 

 angles due to unequal atomic compression, — a change which 

 might be preserved by the viscosity of the solid after the pres- 

 sure was removed. Such a change, if demonstrated, might be 

 the cause of the irregularities to be observed in the angles of 

 minerals. Accordingly experiments under pressure were next 

 made in a Hempel bomb, to which could be attached an oxy- 

 gen generator. As the manometer was graduated only to 

 twenty -five atmospheres, the highest pressure used was about 

 twenty-eight atmospheres, most of the runs being made at 

 twenty-three or twenty -five. The bomb was tight enough so 

 that the diminution in pressure during the course of an experi- 

 ment rarely amounted to more than one atmosphere. About 

 forty experiments were made, some at the room temperature, 

 some at various temperatures in a thermostat. The method 

 was as follows : the caesium chromate was contained in a tall, 

 narrow, flat-bottomed glass tube, the solution being saturated 

 and in contact with some undissolved salt. This tube was 

 placed in the bomb on a leaden tripod which stood in concen- 

 trated sulphuric acid, used as a desiccating agent. As this on 

 one occasion got into the chromate, it was afterwards replaced 

 by sticks of caustic potash packed into the spaces between the 

 walls and a cardboard tube surrounding the chromate tube. 



*T. W. Richards, Proc. Amer. Acad. Arts and Sci., 1901, xxxvii, 1-17; 

 399-411. 



