OSMOTIC PRESSURES OF AQUEOUS SOLUTIONS OF CALCIUM FERROCYANIDE. 187 



Run. gr. 



hours at 30 C. gave +0'0004 

 30 +0-0005 

 90f 30 +0-0005 

 115 +0-0003 



gr- 

 + 0-0011 



+ 0-0007 

 + 0-0010 



+0-0001 



+ 0-0006 

 + 0-0008 

 + 0-0013 

 + 0-0008 



and 



It will be noticed that the gains are greater for the higher temperatures than for 

 the lower, and, further, the average gain shows a tendency to increase with increased 

 length of run. Both facts are explicable on the assumption that the gains are due 

 to mercury vapour, and it is noteworthy that, calculating from the approximately 

 known vapour pressure of mercury (LANDOLT and 

 BORNSTEIN'S tables) and estimating the amount of 

 air passed (from previous work), the gains are 

 found to be of the order of the number given from 

 these data. Exact concordance cannot be expected, 

 because in all experiments the inverted U -tubes 

 (fig. 3, p. 186) were brought down as close to the 

 exit and entry tubes as was practicable. 



It was therefore decided to test this question 

 by using a form of joint which avoided the use of 

 mercury. For this purpose Messrs. Johnson and 

 Matthey made inverted U -tubes of platinum- 

 iridium (see fig. 4) in such a way that they were 

 not rigid and, consequently, allowed the unavoid- 

 able expansion and contraction, when the whole 

 train of vessels was put into and taken out of the 

 bath, to take place without risk of breaking the 

 glass vessels or unduly straining the joints. 



A and B are the exit and inlet tubes, and the 

 inverted platinum-iridium U-tube C is ground into 

 these at D and E. The internally conical parts E 

 and F are for the ground-glass stoppers (not shown) 

 used when weighing the vessels. The upper part 

 of the U-tube is of thin tubing and is flexible. 

 This joint was tested, after some preliminary 

 experiments, as follows : 





E- 



C 



D- 



-B 



Fig. 4. 



Two sulphuric acid vessels in series were joined by one platinum-iridium joint and 

 air passed for 63 hours at 30 C. (rubber lubricant in ground joint). There was a gain 

 in the 2nd vessel of 0'0004 gr. Another run of 72 hours at 30 C. gave a gain 

 of O'OOOl gr. Three vessels in series, run of 116 hours at 30 C., with a dry-air 

 current, gave losses O'OOOl gr., O'OOOl gr., 0'0004 gr. 



2 B 2 



