34 SCIENCE PRIMERS. [MATERIAL 



light rather carelessly. We call things that are easily 

 lifted light, and things that are hard to lift, heavy. 

 We say that sand, which is blown about by the wind, 

 is light, and that a block of wood is heavy, and yet 

 we have just seen that sand is heavier, bulk for 

 bulk, than wood. In order to get rid of this 

 double meaning, the weight of a volume of any 

 liquid or solid, in proportion to the weight of the 

 same volume of water at a known temperature and 

 pressure, is called its specific gravity. Water being 

 taken as i, anything a volume of which is twice as 

 heavy as the same volume of water is said to have 

 the specific gravity 2 : if three times, 3 ; if four and a 

 half times 4^5, and so on. Thus the specific gravity of 

 any liquid or solid expresses its density in proportion 

 to that of water under the same conditions. Sawdust, 

 oil, and spirit, have a less specific gravity than water, 

 while treacle, sand, and quicksilver have a greater 

 specific gravity. In this sense, the former three sub- 

 stances are light, while the latter three are heavy. 



25. Things of Greater Specific Gravity 

 than Water sink in Water ; Things of Less 

 Specific Gravity float. 



Here are two tumblers of water. Throw some 

 sand or iron-filings into one and some sawdust 

 into the other. What happens ? The sand and 

 the iron-filings sink to the bottom, the sawdust 

 floats at the top. We may stir them up as we 

 like, but the one will tumble to the bottom and 

 the other, as obstinately rise to the top. Thus that 

 which is lighter than the water floats, and that 

 which is heavier (bulk for bulk) sinks. So, if we pour 



