iodine and Bromine above Critical Temperature. 425 



iron tube, and a current of electricity sent through the water 

 in the short arm. The absorption spectrum was watched for 

 an hour and a half, at the end of which time the apparatus 

 exploded, but up to the very end the fine fines lost nothing 

 in distinctness. The pressure was calculated from the time, 

 the current strength, and the capacity above the liquid, and 

 was found to have been about 250 atmospheres, or more than 

 double the critical pressure of bisulphide of carbon. This 

 indicates that the disappearance of the lines is due to the 

 density of the vapour rather than to its pressure. 



The quantitative investigation of these phenomena was 

 next undertaken. A powerful arc light was substituted for 

 the incandescent lamp, and a lens so arranged as to throw 

 an image of the " crater " on the heated tube. By this 

 arrangement, the spectra of much denser solutions could be 

 observed. A tube provided with a long capillary neck, of 

 the form shown in fig. 2, was constructed and carefully 

 graduated. The contents of this tube could be varied without 

 altering its volume by cutting off the tip of the capillary, and 

 by warming or cooling the tube cause the liquid to run out 

 or in; the tip could then be sealed once more. This operation 

 could be repeated about 60 times before using up the capillary, 

 only about 2 mm. being removed at each filling. A certain 

 amount of iodine and bisulphide of carbon being introduced, 

 the tube was sealed, heated, and examined. If the lines were 

 present in the spectrum, a little more of the bisulphide 

 was added, and this was continued until the lines just 

 disappeared, indicating complete solution. The density was 

 determined by noting the amount of fluid as measured by 

 the graduations, since, when the contents are homogeneous, 

 these values are proportional. Just before complete solution 

 the lines are so faint as to be invisible in the stationary 

 spectrum, but by moving the telescope to the right and to 

 the left, they could be detected, the eye being more sensitive 

 to a moving faint object than a stationary one. By using 

 this device, the density necessary to just cause the disappear- 

 ance of the lines could be determined with considerable 

 accuracy. 



The iodine was measured in the following manner. A 

 saturated solution in OS 2 was made at 12° and a capillary 

 pipette (p, fig. 2) was dipped into it. The fluid rose to a 

 certain height, which was marked. The iodine solution was 

 then washed out of the capillary into the small tube t by means 

 of a drop or two of CS 2 put into the wide top of the capillary. 

 This was immediately transferred to the graduated tube in 

 the manner described. The amount held by the capillary 



