Specific Gravities. 315 



liquid and consequently its proportion of alcohol. An instru- 

 ment so constructed, is called a hydrometer.] 



432. In making use of the hydrometer, and in experiments 

 generally upon specific gravities, there are several particulars to 

 be taken into consideration. 



(1). Since all bodies expand with heat and contract with 

 cold, it will be perceived that the specific gravity of bodies is 

 modified by temperature. Thus bodies are specifically heavier 

 in winter than in summer, and the same spirit would indicate dif- 

 ferent proportions of alcohol at different seasons, regard not be- 

 ing had to this circumstance. Accordingly a thermometer is a 

 necessary appendage to a hydrometer, and the correction for 

 temperature is applied by means of a moveable scale, containing 

 the degrees of the thermometer, sliding upon another scale on 

 which are placed the numbers of the several weights, including 

 the stem. 



(2). When two substances are mixed together, there is often 

 a mutual penetration of parts, whereby the specific gravity is 

 increased, and there would seem, by the foregoing methods, to be 

 a greater proportion of the heavier ingredient than actually ex- 

 ists. Thus a pint of water and a pint of alcohol do not make a 

 quart of liquid. The defect is sometimes a 40th part. On the 

 other hand, the bulk in certain cases is augmented by compound- 

 ing. A cubic inch of tin mixed with a cubic inch of lead will make 

 a compound exceeding two cubic inches. No accurate allowance 

 can be made for such changes ; consequently the methods above 

 given, become in a degree defective, and the results to a certain 

 extent uncertain. 



(3). The number of ingredients may exceed two, or the na- 

 ture of one or more of the ingredients may be unknown. In all 

 such cases, the problem is in its nature indeterminate. 



433. Questions sometimes occur, especially in chemical re- 

 searches, the reverse of those we have been considering, namely, 



t There is a great variety of instruments of this kind, all depend- 

 ing on the same general principles. They are known also by a va- 

 riety of names, as areometer, gravimeter, alcoholometer, pese-liqueur^ 

 essay-instrument^ &c. 



