24. =The Meaning of Heavy and Light—Specific Gravity.= We are in the habit of using the words =heavy= and =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 1, 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 substances 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 into one and some sawdust into the other. What happens? The sand sinks to the bottom, the sawdust floats at the top. We may stir them up as we like, but the sand will tumble to the bottom and the sawdust, 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 some oil into the water, it floats, and if we pour some coloured spirit in carefully, it also floats; while treacle and quicksilver sink to the bottom, just as the iron-filings do. We saw that the iron-filings sank, because iron is heavier than water. Here is a piece of the thin tinned sheet-iron that they make tin boxes of. What will happen if we drop it into the water? It is heavier than water, bulk for bulk, and therefore it will sink as you see it does.