40 THE APPLICATIONS OF PHYSICAL FORCES. .[BOOK i. 



alcohol, it shows in hundredths the proportion of the volume of the 

 spirit to that of the water. Thus, when the instrument immersed 

 in a mixture marks 70, it shows that this mixture really contains 

 70 parts of pure alcohol and 30 parts of water. 



Gay-Lussac, in order to graduate this hydrometer, immersed it 

 successively in mixtures containing 0, 10, 20, 30 . . . . 100 parts of 

 pure alcohol, a delicate and laborious operation, because the mixture 

 of the two liquids produces a lowering and a rise of temperature, so 

 that it w^s necessary to wait until the mixture had cooled to a 

 uniform temperature (that of 15 C.) and to calculate the new pro- 

 portion of the two volumes. 



In the United Kingdom spirit is valued for revenue purposes 

 according to the quantity which it would make if brought, by the 

 addition or abstraction of water, to a strength termed "proof": 

 proof-spirit being defined by law (58 G. iii, c. 28) to be such spirit 

 as at the temperature of 51 Fahrenheit shall weigh yf of an equal 

 measure of distilled water. 



S ikes' hydrometer and its accompanying tables are the means 

 adopted for the purpose of ascertaining the strength of spirit, and 

 calculating the quantity at proof for the purposes of revenue in 

 this country. It acts, like the saccharometer, upon 

 the principle of weighing the bulk of liquid displaced 

 by the instrument when floating in it. 



The instrument consists essentially of the follow- 

 ing, as shown in the diagram : B c is a hollow 

 brass ball, surmounted by a flat stem, A B, and loaded 

 below by a short conical stem, c I), terminated by the 

 pear shaped bulb D. 



By means of nine weights, ten principal divisions 

 on the stem, and five subdivisions to each, it has a 

 scale divided into five hundred parts, and ranges 



ig. 19. Sykc's 



Hydrometer. from a strength of 70 per cent, over-proof at 47 

 Fahrenheit, or a density of '8156, down to water, or density 1000. 

 One of these weights, w, is represented above. It (the weight) is 

 furnished with a slit, so as to allow of it being slipped on to the 

 narrowest part, c, of the lower stem. 



The instrument is so adjusted that it indicates the volumes of 

 water that must be added to or taken from 100 volumes of the 



