In stone and the metals, for instance, we move the particles with great difficulty.

A finger may be punched into putty with ease, but powerful machinery is required to disarrange the particles of matter forming iron.

Substances which crumble and break, instead of changing form, when force is applied to them, are called brittle. Chalk, stone, glass and cast iron are examples; but substances which permit a change in form are called malleable. Gold, wrought iron, lead, platinum and copper, are examples. In malleable substances, the small particles can be moved without separating so far as to be beyond the range of cohesive force. Some brittle substances become malleable when heated. Wax is such a substance, and glass, when heated, can be spun into the finest and most delicate threads.

Substances which can be drawn into wire are called ductile substances. Many of the metals are ductile to a great degree, while some are not at all so.

Gold is very ductile and very malleable. It can be drawn into the finest wire and can be hammered into sheets so thin and delicate that it takes two hundred and fifty thousand sheets to make a pile an inch high.

Almost all substances possess some peculiarity in structure; some variation in the size of molecules; some variation in the distance between

them; and, consequently, some variation in their cohesion or strength. Many substances may have their structure altered by various processes, and thus take a new nature.

If we examine a piece of soft steel, we notice that it is composed of rather coarse grains. If we strike this steel with some hard substance, we find that the particles are moved, and the steel is bruised and dented. By means of a file we can even pull particles away from the mass and reduce its size. We judge by this that. the cohesion of soft steel is not extraordinarily high. The particles do not strongly object to being moved out of their usual position. We may bend the piece of steel, and thus change the relative position of every particle, and the piece will stay bent, thus indicating that the particles are not tightly bound together. If we take this same piece of soft steel and heat it till red, and then suddenly cool it in water, or other cold matter, we will find that the molecular structure is greatly altered. If the piece be broken, it will be noticed that the grains are very fine and very closely compacted together. The cohesion of the particles has increased so very much that it is found almost impossible to separate them. Hard blows with the hardest of substances will not move the particles so as to form dents.

of

The hardest of files will not separate any the particles from the mass. It is now hard, and

its cohesion is so great that it will move the particles of almost all other substances against which it may be forcibly applied. This quality

fits hard steel for tools, etc.

If an attempt be made to bend this hard steel, it will be found that the particles are bound together so tightly and resist a change of position so powerfully, that they will not move until sufficient power is applied to separate them entirely. The piece of steel breaks instead of bending. Hardness is always accompanied by brittleness. If we heat this hard steel and cool it very slowly instead of suddenly, we again have soft steel instead of hard steel; malleable steel instead of brittle steel; particles far apart and loosely held, instead of closely packed and powerfully held together by cohesion.

Copper is exactly the reverse of steel in the way its cohesion is affected by sudden heating and cooling. Soft copper heated red and suddenly cooled becomes still softer and more ductile and malleable. Soft copper heated and slowly cooled becomes harder and more brittle.

Glass is a very hard and brittle substance. Its particles can be moved on each other only by a harder substance. Diamond is very much. harder, being, in fact, the hardest substance known.

Hard substances, owing to their brittleness, can often have their fine molecules broken away

by light blows. A highly polished piece of the hardest steel, if exposed to the action of a dusty wind, will become dull and rough by reason of the outside particles being hammered and cracked off by the millions of light blows received from the flying particles of dust. The diamond is so very hard, and its particles so extremely cohesive, that it withstands for many years the hammering action of the dusty air. A false diamond, made of the hardest of glass, and polished in the saine way and as highly as a real diamond, will reflect the light with great brilliancy and will appear like a diamond, but a few hours' exposure to the common air serves to break away and deaden its comparatively soft surface.

As before stated, the combination of two different substances often produces a substance unlike either. Thus copper and tin are both very soft, ductile, and malleable metals. Nine parts of copper and one part of tin, melted and mixed, form the alloy known as gun metal. This alloy, unlike either of the metals of which it is formed, is very hard and tough, and possesses comparatively no malleability or ductility. Of the two metals composing this alloy, tin is much the softer, yet an increase in the proportion of tin in the alloy hardens it. When the alloy is onethird tin and two-thirds copper, it is so hard that steel tools will not cut it, and so brittle as to fly to pieces when subjected to light blows.

In speaking of the molecular change in steel when suddenly cooled after being heated, it was stated that soft steel allowed its molecules to be moved around on each other by denting or bend. ing, and that the molecules will stay in any new position where put. Thus a blow on soft steel leaves a dent, or a new arrangement of the molecules. When the steel is hard, no dent can be made, because the molecules refuse to move.

If, instead of cooling the steel suddenly, it be cooled neither suddenly nor slowly (a process called tempering) we will have a new condition of the molecules, and new properties in the steel. It will be neither very hard nor very soft. If it be struck with a hard substance, the molecules will move, but will immediately return to their old positions. The steel can be dented, but the dent instantly disappears. When the steel is bent it instantly straightens again with considerable force, Steel in such condition is fitted for springs. When a material is composed of molecules which will allow themselves to be displaced, and still return to their original positions, that material is said to be elastic. Glass, though hard and brittle, is very elastic. Ivory is very much so; india rubber is elastic to a remarkable degree, and spring steel, and spring tempered metals generally, are very elastic. If a lead ball be dropped on the pavement, it will be flattened on the side which strikes; the flat part will