220 MR J. Y. BUCHANAN ON THE 



people the hydrometer is associated with a rough-and-ready method of ascertaining the 

 specific gravity of liquids in public works, and in other similar places, where its use is 

 commonly entrusted to a workman ; and the idea of readiness, if not of roughness, is, it 

 may be said, habitually associated with the instrument and its use. An important part 

 of this paper is devoted to showing how the hydrometer has to be used if the best 

 results of which the instrument is capable are to be obtained from it. It will be seen 

 that experimental skill and perseverance, and constant attention to many minute pre- 

 cautions, are necessary. If this care is taken, the results will be good ; if it is not taken, 

 they will be bad. 



When the hydrometric method is practised in the manner here specified, it is possible 

 to obtain the specific gravity of liquids with greater accuracy than by any other means. 

 Hence, in the case of saline solutions it is possible with it to carry the exact determina- 

 tion of the specific gravity of the solutions of a salt to much higher dilutions than is 

 possible by other methods. It was to experiment on solutions of such high dilution 

 that their specific gravities have hitherto escaped experimental determination, that 

 this systematic research was originally undertaken. It will be seen that the results 

 obtained fully justify the time and labour expended on them. It has hitherto been 

 the general experience that, when two equal quantities of a salt are dissolved seriatim 

 in a quantity of water, the diminution of the total volume of the salt and the water 

 produced by the dissolution of the first quantity is greater than that produced by the 

 further dissolution of the second quantity. It has been proved in this memoir that 

 for the solutions of many salts there is a concentration below which this law is reversed. 

 It is the first time that this has been unequivocally demonstrated. In the case of 

 some salts which, when dissolved so as to furnish solutions of moderate concentration, 

 exhibit considerable contraction, they at high dilutions exhibit an expansion, which 

 may cause the volume of the solution to exceed the sum of the volumes of the salt 

 and the water. 



A similar and very remarkable feature of saturated solutions is shown in the 

 case of the salts of the ennead MR. In the saturated solutions of the salts of potassium 

 and rubidium the sum of the volumes of the salt and water is greater than that of the 

 solution produced, while in the case of the solutions of the caesium salts the reverse is 

 the case. From this it follows that increase of pressure must assist the crystallisation 

 of the solutions of the caesium salts, and hinder that of the solutions of the potassium 

 and rubidium salts. 



The main purpose of this investigation was to determine the specific gravity of 

 solutions of moderate concentration and of high dilution. In order to use the same 

 hydrometer for these different classes of solutions, its weight was altered by the use of 

 accessory weights attached to the top of the stem. It occurred to me during the course 

 of the investigation that, V)y carrying this principle further, the use of the hydrometric 

 method, in all its delicacy, might be extended to solutions of any degree of concentration 

 by increasing the additions made to its weight. It was found that for our hydrometers, 



