SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 221 



closed at the top, solutions having a specific gravity of 1 '2 could be experimented on, 

 but the accessory weight required was so great as almost to disturb the equilibrium of 

 the instrument. In order to meet this difficulty, the stem of the hydrometer was left 

 open, so that the internal weight or ballast could be varied at will. With the open 

 hydrometer so constructed, saturated and even supersaturated solutions of very soluble 

 salts have been experimented on, and results of the highest interest have been obtained. 



The most noteworthy case is that of calcium chloride in supersaturated solution. 

 In it a very remarkable state of unrest was observed before crystallisation took place. 

 When the crystallisation of this solution is finished, the sum of the volumes of the 

 crystals and the mother-liquor is less than that of the original supersaturated solution. 

 The state of unrest which precedes the actual appearance of the first crystal consists in 

 a rhythmic series of isothermal expansions and contractions, which cease the moment 

 the first crystal appears and heat is liberated. The supersaturated solution exhibits 

 veritable symptoms of labour before giving birth to the crystals and becoming itself a 

 mother- liquor. The details of this remarkable phenomenon are to be found in Section XV. 



It now only remains for me to discharge the pleasant duty of acknowledging my 

 obligations to the gentlemen who have acted as my assistants in the experimental work 

 and in the preparation of this memoir. The work has been hard and continuous, having 

 extended to nearly ten years, and it is impossible for me adequately to express my 

 thanks to these gentlemen for the intelligence, skill, and perseverance with which they 

 have all devoted themselves to it. 



The secretarial work connected with it has been very heavy, and it has been managed 

 with great ability and success by Mr W. Gr. Royal-Dawson, to whom my best thanks 

 are due. The pages of Tables in the memoir will suggest to anyone who is familiar 

 with such work the amount of labour which has been expended in their preparation and 

 verification. 



The experimental work has for nearly three years been in the hands of Mr S. M. 

 BoswoRTH, B.Sc, who has carried it out in a room in the Davy-Faraday Laboratory, 

 which was admirably suited to the purpose. My thanks are especially due to Sir James 

 Dewar and the Managers of that Institution for their generosity in putting it at my 

 disposal. Mr Bos worth's name appears several times in the text in connection with 

 some of the more remarkable features chronicled, more particularly in connection with 

 the state of unrest occurring in the supersaturated solution of calcium chloride before 

 crystallisation. It was owing to his confidence in the exactness of the readings of the 

 hydrometer which he observed in this solution, and in the reality of the discrepancies 

 which he observed, that the state of unrest was not only noticed but measured. Mr 

 BoswoRTH was preceded as my assistant by Mr H. F. Fermor, now of the Metro- 

 politan Water Board, to whom a large part of the experimental work recorded in the 

 Tables is due. His work was of the highest order, and justified his selection for the 

 responsible office which he now holds. Before him, my laboratory assistant was Mr H. 

 Royal-Dawson, brother of Mr W. Gr. Royal-Dawson, and he, like all the gentlemen whom 



