Displacement in their own Mother -liquor 199 



in it everything that he requires, including his balance. The 

 window of the room must face the north, and the precautions 

 generally to be observed are similar to those prescribed by 

 Bunsen for the practice of his gasometric method. 



The salts used in this research were the chlorides, bromides 

 and iodides of potassium, rubidium and caesium. The rubidium 

 and caesium preparations were from the works of Schuchardt 

 in Goerlitz and were of the highest degree of purity. The 

 potassium salts were also unexceptionable as regards quality 

 and were supplied by Merck. All of these salts dissolve easily, 

 and most of them abundantly, in water. They also crystallise 

 with great readiness. 



The first operation is to prepare a hot solution of the salt 

 such that, after standing over night, or for such length of time 

 as may be deemed sufficient, it shall furnish about 60 c.c. of 

 mother-liquor and about 15 c.c. of crystals. In the case of the 

 potassium salts there was no difficulty, as their solubility at 

 all temperatures is well known. The solubility of the rubidium 

 and caesium salts had to be determined, at least approximately, 

 in each case, in order to economise the costly material. The 

 following simple method furnished the required information 

 easily and expeditiously. A suitable vessel, beaker or flask, 

 is weighed empty, and then with 25 grammes of distilled water, 

 of the temperature of the air. The salt is then gradually 

 added and the mixture stirred with the thermometer. In the 

 case of every one of these salts the temperature falls rapidly 

 and by as much as from 15 to 20. The salt is added as 

 rapidly as it is taken up by the water. When the fall of tem- 

 perature slackens, a minimum is soon reached, while some salt 

 still remains undissolved at the bottom of the vessel. It is 

 continually Stirred; the temperature rises slowly while 

 the salt gradually passes into solution, until, at a certain tem- 

 perature the amount of salt remaining un<lis>lve<l is such 

 ,i further rise of one degree of temperature will evidently 

 cause it to disappear. The vessel is now weighed and, as a result . 

 we have the weight of salt dissolved in 25 grammes of water at 

 about the last observed temperature. \Vith a little cart it is 

 easy to arr.u I his temperature >h;ll !> in the neighbour- 



