200 PHYSIOLOGICAL PHYSICS. [Chap. xix. 



the balance is disturbed. Restore it accurately by 

 weights placed in the pan to which the body is sus- 

 pended ; these weights represent the weight of the 

 displaced volume of water. Let the weight be repre- 

 sented by 2. Plunge, next, the same solid body in the 

 liquid to be examined ; find, as before, what weights 

 are required to restore the balance ; this gives the 

 weight of the same volume of the displaced liquid as of 

 the water, and let it be represented by 3. Thus the 



weights of the two equal volumes of water and of the 



g 



other liquid can be immediately compared : - = 1 '5. 







It is to be observed that the general principle of 

 these methods is the comparison of the weight of the 

 body with the weight of an equal volume of water. 

 Special adaptations must be made when the body 

 is soluble in water. A very simple method, for which 

 only one weighing is required, has been recently 

 devised by Dr. J. J. Dobbie and Mr. Hutcheson, 

 of the Chemical Laboratory, Glasgow University. A 

 tube is taken of a bore similar to that of an ordinary 

 burette. At its lower end is united a tube of fine 

 bore, the two forming a U tube. In the middle of 

 the wide tube a zero mark is placed, and the fine 

 tube is marked off into cubic centimetres. Let water 

 be placed in the tubes up to the level of the zero line. 

 Then drop in the solid body whose sp. gr. is to 

 be determined, its weight in air in grammes having 

 previously been determined. It will displace some 

 water, which will rise above the zero line. The top 

 of the wide tube is now closed with an accurately 

 fitting indiarubber cork, connected with a stop-cock. 

 The cock is opened, and by blowing through it the 

 level of the liquid in the wide tube is depressed below 

 the zero line. The level is now permitted to rise 

 till it is exactly at the zero line, and the stop-cock 

 is closed. In the narrow tube there is now read off 



