OSMOTIC PRESSURES OP AQUEOUS SOLUTIONS OF CALCIUM FERROCYANIDE. 183 



from the 2nd solution vessel, is saturated with water vapour up to the vapour 

 pressure of the solution, and on emerging from the water vessel it is then saturated 

 up to the vapour pressure of the water. Hence the ratio of the loss of weight of the 

 vessels containing the solution to the sum of the loss of weights of solution and water 

 vessel equals the ratio of the density of the vapour over the solution to that over 

 water. As pointed out in the communication mentioned above, it is evident that if 

 the air on entering the train of vessels has been dried by sulphuric acid, and on 

 emerging from the water vessel it is again dried in a similar manner, the gain in 

 weight of the 4th vessel (the vessel containing the sulphuric acid) should equal the 

 sum of the losses of weight of the solution and water vessels. In that paper it was 

 shown that this did not obtain -there was a considerably greater loss of weight than 

 there was gain by the 4th vessel. This discrepancy can be shown to be partly caused 

 by the saturated air giving up some of its moisture* to the tube connecting the 3rd 

 and 4th vessels. 



As it was found to be impossible to weigh accurately the connecting tube, it was 

 sought to bring the deposited moisture into the sulphuric acid by making the internal 

 diameter of this tube only 1/5 mm., so that the velocity of the air current would 

 sweep the moisture forward. 



Fig. 1 (p. 184) shows the first form tried and is self-explanatory. 



The use of the joint in this form, and other modifications of it which need not be 

 further described, caused a considerable improvement, as shown by the following test 

 experiments : 



Air was passed very rapidly for 24 hours through two vessels in series, joined by 

 the above-mentioned form of joint. The 1st vessel contained water and the 2nd 

 sulphuric acid. The temperature of the bath was constant at about 30 0. 



The water lost 2'8425 gr. 



The sulphuric acid gained 2'8393 gr. 



The "dipping" tube (i.e., the tube connecting the water and sulphuric acid vessels) 

 was found to have gained 0'0048 gr. 



The difference between the losses and gains being, therefore, +0 - 0016 gr. 



A repetition of the experiment with a slow air current gave a difference of 

 -0-0003 gr. 



* There are three plausible explanations of the phenomenon : (1) That there are eddy currents (possibly 

 caused by inequalities of surface) in the junction tube and the consequent changes of temperature bring 

 about deposition ; in this connection it is noteworthy that more moisture is deposited on a dirty tube. 

 (2) That the surface of any glass vessel is always slightly hydrated, and if the glass be soluble a solution 

 of lower vapour pressure is formed, and, consequently, more moisture is deposited ; evidence in favour of 

 this view is afforded by the well-known fact that less moisture is deposited on lead glass, which is stated to 

 be less soluble than soda glass ; on the other hand, we find that we get the usual deposit on platinum- 

 iridium tubes. (3) That the air, containing ions, is really supersaturated in as far as the ions may carry a 

 watery envelope ; the latter is deposited on the walls of the tube where the ions come in contact with it. 

 We are at present engaged in experiments to test this hypothesis. 



