FUNDAMENTAL PROPERTIES OF MATTER. 33 



a temperature of 25 C. (77 F.) ; that for gases is either atmospheric 

 air or, more generally, hydrogen at a temperature of C, (32 F.). 



Specific weight is generally expressed in numbers which denote how 

 many times the weight of an equal bulk of water is 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. 



Since all liquids and solids expand or contract with change of tem- 

 perature, it is very important to note the temperature in taking the 

 specific gravity of substances. For example, the specific gravity of 

 alcohol is less at 25 C. than that at 15 C., because alcohol expands 

 with rise of temperature. Likewise, at a given temperature, say 25 C. 

 it is greater when compared with water at 25 C. than when com- 

 pared with water at 4 C. or 15 C., because a volume of water 

 weighs less at 25 C. than it does at 15 C. or 4 C. Since the 

 change in volume of solids with change in temperature is much less 

 than in the case of liquids, the difference in specific gravity at differ- 

 ent temperatures is much less noticeable for solids than for liquids. 



Density. Density, in physics, is defined as the mass or quantity 

 of matter in a unit volume of the substance. In the^metric system 

 the mass of a unit volume of water at 4 C. (39.2 F.) is 1 gramme, 

 that is, the density of water at 4 C. is unity. At any other temper- 

 ature the density of water is less than one. It can be seen that when 

 the specific gravity of a substance is determined by comparison with 

 water at 4 C., the number expressing this specific gravity is identical 

 with the number expressing the density of the substance. 



When the comparison is made with water at any other temperature 

 than 4 C., the figures for the specific gravity and the density of a 

 substance are not identical, although the difference between them is 

 usually very small. 



The specific gravity of solids heavier than water is generally determined by 

 first weighing the substance in air and then while suspended in water. The 

 body will be found to weigh less in water because it displaces a volume of water 

 equal to its own, and loses a weight equal to that of the water displaced. Con- 

 sequently the relation between the loss in weight and the weight in air is also 

 the relation between the weights of equal volumes of water and the substance 

 examined. If, for instance, a body is found to weigh 6 grammes in air and 2 

 grammes in water, the loss being 4 grammes, then the relation between the 

 weights of equal volumes of water and the substance is as 4 to 6, or 1 to 1.5; 

 the latter being the specific gravity of the substance examined. 



The specific gravity of a solid soluble in water is determined by weighing it 

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