204 PHYSIOLOGICAL PHYSICS. [Chap. xix. 



be so loaded that when immersed in distilled water 

 it sinks to the level of a mark placed near the 

 extremity of the stem. Call this zero. Then let 

 a solution be made of 15 parts of salt in 85 parts 

 of water, both by weight, and immerse the instru- 

 ment in the solution. Mark 15 at the level to which 

 it sinks. Provided now that the stem is quite 

 regular, the space between zero and 15 may bo 

 divided into equal parts, and this regular marking 

 may be continued down the stem, say to 100. Each 

 subdivision ought to represent an equal volume. To 

 the instrument so made the name SALIMETER is 

 applied, because it will give the density of any saline 

 fluid in relation to that of distilled water. For fluids 

 lighter than water the hydrometer is so loaded that in 

 distilled water the surface of the water is only up to 

 a level with the bottom of the stem, which is marked 

 0. Thus in GAY-LUSSAC'S CENTESIMAL ALCOHOLI- 

 METER, zero is at the bottom of the stem, the level of 

 distilled water. In pure alcohol the alcoholimeter 

 sinks to the top of the stem, which is marked 100. 

 Other marks down the stem indicate the level of 

 liquid containing different percentages of alcohol and 

 water, the levels having been determined by experi- 

 ment with each instrument. 



The densimeter of Rousseau is of great value 

 in scientific work, affording as it does a means of 

 estimating the density of a fluid of which only a small 

 quantity may be available. It is shown in Fig. 99. 

 The stem A is divided off bv marks into intervals, 



v f 



which correspond to equal volumes ; e.g. i^th of a 

 cubic centimetre. The stem carries a little tube c, 

 into which is placed one cubic centimetre of the fluid 

 to be measured. The method is as follows : 



The densimeter is placed in distilled water at four 

 degrees centigrade, and into the tube c is placed one 

 cubic centimetre of distilled water. This makes it 



