28 INTRODUCTION. 



contained in the weight of the substance in question. If we 

 say that mercury has a specific gravity or density of 13.6, or 

 that alcohol has a specific gravity of 0.79, we mean that equal - 

 volumes of water, mercury, and alcohol represent weights in 

 the proportion of 1, 13.6, and 0.79, or 100, 1360, and 79. 



The standards or units chosen for comparing the specific 

 gravity of gases are either atmospheric air or hydrogen. 



In order to obtain the specific gravity of any liquid, it is only 

 necessary to weigh equal volumes of water and the liquid to be 

 examined, and then to divide the weight of the liquid by the 

 weight of the water. 



A second method by which the specific gravity of liquids may 

 be determined is by means of the instruments known as hydrom- 

 eters, or, if made for some special purposes, as alcoholometers, 

 urinometers, alkalimeters, lactometers, etc. 



Hydrometers are instruments usually made of glass tubes, 

 having a weight at the lower end, to maintain them in an up- 

 right position in the fluid to be tested as to specific gravity, and 

 a stem above, bearing a scale. The principle upon which their 

 construction depends is the fact, that a solid substance, when 

 placed in a liquid heavier than itself, will displace a volume of 

 this liquid equal to the whole weight of the displacing substance. 

 The hydrometer will consequently sink lower in liquids of 

 lower specific gravity than in heavier ones, as the instrument 

 has to displace a larger bulk of liquid in the lighter than in the 

 heavier liquid in order to displace its own weight. 



Weight of gases. We have so far considered the gravity of 

 solids and liquids only, and the next question will be : Do gases 

 also possess weight, are they also attracted by the earth ? The 

 fact that a gas, when generated or liberated, expands in every 

 direction, might indicate that the molecules of a gas have no 

 weight, are not attracted by the earth. A few simple experi- 

 ments will, however, show that gases, like all other substances, 

 have weight. Thus a flask from which the atmospheric air 

 contained in it has been removed will weigh less than the same 

 flask when filled with atmospheric air or with any other gas. 



Barometer. A second method, by which the fact that atmos- 

 pheric air possesses weight may be demonstrated, is by means 



