SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 19 



the lengths a and b. It follows, therefore, that the expression (a — b) represents the 

 weight of a volume of the liquid equal to that of the solid body, and that the specific 

 gravity of the solid body, referred to that of the liquid as unity, is 



D = 



a-b' 



When a solid body is placed on the platform M, the hydrometer always sinks 

 deeper in the liquid ; therefore a is always positive. When a solid body consists of a 

 substance which is denser than the liquid, then, when it is attached to the hook K and 

 is immersed with the instrument in the liquid, it causes the hydrometer to sink deeper 

 in it, and b is also positive in this case. When the substance of the solid body is less 

 dense than the liquid, a is positive as before ; but when the body is attached to the 

 hook, and is immersed with the hydrometer in the liquid, it exerts a pressure wpivards, 

 which causes the hydrometer to emerge and expose a part of the stem below the point 

 0. On this part of the scale the numerals have the negative sign, and the weight of 

 the volume of liquid displaced by the solid body is, as before, (a — b) ; and its specific 

 gravity is, also as before, 



a-b 



The identity of the expressions of the experimental data in determining the specific 

 gravity of substances so dissimilar as, for instance, a stone and a cork never failed to 

 arrest the attention of the students. 



It will be noticed that, by using this method, the specific gravity of a solid body 

 is obtained ivithout any deter^nination of weight having been made, either' in the 

 production of the instrument or in its use, and that the only measurements made are 

 those of length. 



§ 3. Usefulness of the Hydrometer in the Study of Mineral Waters. — But the 

 hydrometer and its uses had always had a fascination for me. I began to pay particular 

 attention to the subject in Wiesbaden, when working as a student with Fresenius, and 

 afterwards (1866-67) as an assistant in his private analytical laboratory. During this 

 period I became much interested in the mineral waters which abound in the (then) 

 Duchy of Nassau and the neighbouring parts of the Rhineland, and especially in the 

 Kochbrunnen of Wiesbaden, perhaps the most celebrated of them all. I had great 

 curiosity to investigate the variations, if any, in its concentration at different times and 

 seasons ; but, as a student, I had to follow the plan of instruction laid down, and, in the 

 private laboratory of the final referee in Germany regarding all matters of dispute or 

 arrangement which could be decided by chemical analysis, the important and responsible 

 work entrusted to me made it impossible for me to occupy myself with anything 

 else at the same time. During the voyage of the Challenger, I many times made 

 up my mind, on my return to Europe, to visit Wiesbaden and use the hydrometer 

 in carrying out a systematic investigation in this sense ; but my intention has not 

 been realised. 



