these atoms do not touch each other.

Around

and among the atoms there lies a delicate, subtile fluid called ether. The space occupied by this ether is very much greater than the space occupied by the atoms. The spaces between atoms are called pores, and all substances are more or less porous. In some substances there are large visible pores, large enough to admit common air, which is itself, like all other matter, made up of atoms separated by ether. We can see no pores in close-grained iron or close-grained wood, or lead, or mercury, or in some hard clay ware, but in cork and sponge, and some kinds of wood and some very open kinds of iron, the pores are large and visible. If a cylinder be cast of great strength, of the closest iron, and a piston be fitted tightly to it,. and water be put into the cylinder, and an immense pressure applied to the piston, water will find its way through the pores of the metal and will ooze in fine sweat from the outside. There are some substances which it seems hard to believe to be porous, such as glass, for instance, but science has determined that the pores form the bulk of all substances. In many substances the presence of pores and their great volume can be easily shown. A brick will hold a large amount of water and be no larger; the water simply enters the large pores. If a brick be powdered up very fine so that its particles can be packed

Liquids have

very close together, it will be found that there is very little of it; and it will also be found that the water which a brick will hold has a great deal more bulk than this closely packed brick. A sponge is another illustration. pores like all other substances. Liquids are made up of little atoms and larger pores, and the atoms and pores in different liquids are of different sizes. In some liquids the pores are large enough to admit the atoms of other liquids. In this case it will be seen that a liquid may actually hold more liquid without increasing in bulk. It would be in some degree like pouring water into sand. A gallon of water and a gallon of sand mixed will not make anything like two gallons of the mixture.

In the same way, strange as it might seem before the matter was somewhat understood, a gallon of water and a gallon of alcohol mixed will not make two gallons of mixture.

The small particles, of which all matter is composed, are drawn toward each other by some inner natural force, which we call cohesion. In some substances this cohesion is so small that we may easily pull the particles apart. In some others the cohesion is so great that enormous power is required to overcome it.

The particles composing a piece of clay may be pulled apart very easily; but two strong men pulling on an iron wire, a sixteenth of an inch

in diameter, cannot overcome the cohesion of the particles and break the wire. It will thus be seen that the strength of any material depends on the cohesive attraction between the particles.. The cohesion and consequent strength of some materials may be altered by working in various ways. An ordinary piece of bar iron may have a certain strength, but the operation of working the iron and drawing it into wire will sometimes. increase its strength five-fold. The various operations probably crowd the particles closer together, and thus allow the cohesive force to act at shorter distances.

Besides the force of cohesion, there is an opposite force called repulsion, which, when in action, tends to throw the particles of matter. away from each other. In fact, the two forces are always acting against each other in all substances. Were it not for repulsion, the atoms. would rush together and touch each other, and all matter would become solid. These two forces, always acting, are constantly changing their relative powers. The application of heat to substances increases the repulsion, the particles: move further apart, and the mass of matter becomes larger. This is the common action we speak of as expansion by heat. It will be seen that in reality the mass itself does not expand; the particles forming the mass are simply moved further apart. The force of repulsion, like the

force of cohesion, is enormous. In most substances, repulsion develops but slowly, while in others it is instantaneous. Thus certain substances, properly treated and mixed, form gunpowder, and the application of high heat to gunpowder will develop repulsion between the particles. They fly apart with immense force, and this force we often make use of in peace and

war.

A change in the internal forces of matter often changes the nature of the substance. A piece of ice one inch square, for instance, has great cohesion. It is hard to pull the particles asunder. The application of heat to the ice will separate the particles so far that the force of cohesion will no longer hold them in place. They will roll around and fall and slide over each other. By this application of heat, the substance has been changed from a solid to a liquid. There are, however, still the same number of minute particles, and their form and nature has not been changed. If we apply heat still further, we separate the particles still more, and finally they become pushed apart with a force which will lift fifteen pounds for each square inch which these particles act upon. This takes place when the thermometer indicates 212 degrees. It is the boiling point of the water, which now changes from water into a vapor, and the cubic inch of ice has been expanded into seventeen hundred

cubic inches of vapor.

If this vapor was con

fined in a vessel, instead of being allowed to expand freely, the repulsive force would be so great as to burst the vessel, if not of great strength. If we take the heat away from this vapor, the particles draw together again. This action is termed condensation, and when the heat has been reduced to below 212 degrees, we again have water, or the matter in a liquid state, and when the heat is reduced to below 32 degrees, we have a cubic inch of ice again, and our substance is once more in the solid form. No change has been wrought upon the particles of matter; they have simply, by changes in the forces of cohesion and repulsion, been separated from each other, and again brought near to each other.

Just such changes take place in many substances by the application of heat. Iron, at ordinary temperatures, is a solid; at a high heat it softens; at higher heat it turns liquid, and at still higher heat it vaporizes; and by the removal of heat it is brought back successively to liquid and solid.

In some substances the particles are held together with such a light cohesive force that they may be pushed apart with ease. When we can do this, we say the substance is soft, as putty or clay for instance; but when the particles insist on holding their usual position, and yield only to great force, we say the substance is hard.