204 



MR J. Y. BUCHANAN ON THE 



100 Grams of Water Dissolve 



Grams 



98 



164 



264 



225 



51 



157 



93 



121 



156 



222 



of 



RbBr 



KbI 



Rbl 



CsCl 



Csl 



Csl 



CsBr 



CsBr 



CsBr 



CsBr 



at°C. 



12 



20 



boiling 



25 



12 



107 



7-5 



24-5 



50 



93-5 



With this information there is no difficulty in preparing the sohition which shall, 

 after allowing for unavoidable loss in preparation, give the required amounts of mother- 

 liquor and of crystals. The water is warmed and the pure salt is added while the 

 temperature is raised to that of ebullition, or to any lower temperature that may have 

 been selected. When the salt has all passed into solution, the liquid is poured into a 

 flat crystallising dish and crystallisation begins immediately. The area of the dish 

 should be such that the layer of solution shall not be more than half a centimetre thick. 

 The mother-liquor is then everywhere in close touch with the crystals. The dish is 

 then put away in a cupboard for the night. 



In the morning, the temperature of the contents of the crystallising dish and that 

 of the air are taken very carefully. The mother-liquor is then poured ofi" clear into 

 a stoppered bottle, while the crystals are collected, allowed to drain, and dried in the 

 ordinary way. The temperature which the mixture had when separated is noted as 

 that at which the crystals and the mother-liquor are in equilibrium ; and it is exactly 

 at this temperature that they have to be brought together again in order to determine 

 the specific gravity of the salt. It is at this temperature also that the specific gravity 

 bottle is weighed when filled with distilled water and with mother-liquor respectively. 

 In fact the temperature of equilibrium and of separation is the only temperature used. 



§ 123. In Table I. the experimental details are given in full in the case of one salt, 

 namely, caesium chloride. For the other salts the results only are given, and they are 

 collected in Table II. 



All the weights as given represent the weight in vacuo. 



The specific gravity bottle which was used was one of the common and convenient 

 form which has a thermometer for a stopper and a lateral capillary tube for the adjust- 

 ment of level. Its nominal capacity was 50 cubic centimetres. On three occasions 

 one of 25 c.c. capacity was used for determining the displacement of the mother-liquor. 



The molecular concentration (m) of the mother-liquor is determined by titration with 

 tenth-normal silver nitrate solution. This solution was made with the greatest care and 

 contained exactly 17 grams of silver nitrate in one litre, at the ordinary temperature 

 of the laboratory at the time. The burette used was divided into tenths of a cubic centi- 

 metre and had a capacity of 50 c.c. The determination of the halogen was not made 

 until the specific gravity had been determined, and, if the concentration was not already 

 known within narrow limits, a preliminary titration was made, after which the volume 

 of mother-liquor was weighed which would certainly require 40zbl c.c. for titration. 

 The capacity of the burette from to 40 c.c. was determined by weight with great 

 care. The concentration is stated in gram-molecules salt per 1000 grams of water. 



