17. =Variety in the Properties of Matter.=--All matter has properties or qualities. Some of these are different in the different kinds of matter. Thus its three forms have different properties, as you saw in Chapter I. There is variety also in the properties of substances of the same class. Thus liquids are unlike each other in some respects. Some, for example, are lighter than others. Oil is lighter than water. Gaseous substances also differ in this and in other respects. But the variety in the properties of solids is greater than in those of gases or liquids. This will appear as I proceed. 18. =Divisibility of Matter.=--Any visible portion of matter can be divided into parts. Even if it be so small that you can see it only with a powerful microscope, it could still be divided if you could have an instrument sufficiently fine for the purpose. _Divisibility_, then, is said to be a _general_ property of matter; that is, a property belonging to all kinds of matter. 19. =Examples of Minute Division of Matter.=--There are numerous examples in which the division of matter is carried far beyond that which can be effected by any cutting instrument. Some of these I will notice: A gold-beater can hammer a grain of gold into a leaf covering a space of fifty square inches. So thin is it that it would take 282,000 of such leaves, laid upon each other, to make the thickness of an inch. And yet so even and perfect is this thin layer of gold, that when it is laid upon any surface in gilding it has the appearance of solid gold. A fifty millionth part of this grain of gold thus hammered out can be seen by the aid of a microscope which magnifies the diameter of an object ten times. But the division of gold is made even more minute than this in the manufacture of the wire of gold-lace. It is done in this way: A bar of silver weighing 180 ounces is covered with a layer of gold weighing an ounce. It is then drawn through a series of holes in a steel plate, diminishing in diameter, till it at length comes out a very fine wire 4000 feet long. Each foot of it then has only the one 4000th part of the ounce of gold, and yet the silver is well covered. A soap-bubble is a beautiful example of the minute division of matter. That thin wall which incloses the air which you have blown into it is composed of particles of the soap and of the water mingled together. It is supposed to be less than one millionth of an inch in thickness. The thread of the silk-worm is so minute that the finest sewing-silk is formed of many of these threads twisted together. But the spider spins much more finely than this. The thread by which you see him letting himself down from any height is made up of about 6000 threads or filaments, each coming from a separate hole in his spinning machine. A quarter of an ounce of the thread of a spider's web would extend 400 miles.