Fluid Pressure 93 



stem which would be necessary to provide for the 

 inclusion of the greatest and least values likely to 

 be met with, while at the same time giving readings 

 sufficiently exact for general purposes. Thus a set 

 of hydrometers with the following ranges would be 

 capable of utilisation for finding the specific gravity 

 of most liquids : 



0-7000-800; O'SOO 0-900 ; 0'900 I'OOO; I'OOO 

 1-200; 1-2001-400; 1-400 1-600 ; 1'600 2'000. 



As an example of the use of one of these instruments, 

 we will describe the determination of the specific 

 gravity of salt solution in 



EXPERIMENT 32. Make a 10 / solution of common 

 salt by dissolving 50 g. of salt in water and making 

 the total volume of the solution up to 500 c.c. 1 Pour 

 this liquid into a tall glass jar. Select a hydrometer 

 with a range from I'OOO to 1- 100 or 1-200. Float this 

 in the salt solution, taking care that it floats freely, 

 out of contact with the inner wall of the jar. Examine 

 the part under the liquid to see that no air-bubbles 

 are clinging to the hydrometer. Remove any that are 

 noticed with a long-handled camel-hair brush. When 

 the hydrometer is floating steadily, take the reading at 

 the level of the liquid. It will be noted that the liquid 

 creeps up the stem, forming a curved surface similar 

 to that assumed in the burette and other measuring 

 vessels. Allowance must be made for this effect of 

 capillary attraction by adding a correction usually one- 

 half the value of a single scale-division to the reading 

 taken at the point where the liquid surface appears 

 actually to meet the stem. Or, if preferred, the reading 



1 Strictly, 50 g. of salt should be dissolved in 450 g. of water. The 

 above is, however, the conventional laboratory method, and will be 

 adopted throughout this book. 



