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CAPILLARITY

If a small quantity of a liquid is placed on a smooth horizontal surface, we might expect it to spread out in a thin film of uniform thickness; this result seems to be a direct and necessary consequence of gravitation.

If experiments are actually carried out, however, the results will be found to differ with the nature of the surface and the liquid.

Thus a little drop of paraffin oil allowed to fall on the surface of still water spreads out in a film, which may be sufficiently thin to show interference colors.

Water on polished glass behaves in a similar way.

With mercury on glass, however, we obtain different results, for the mercury, instead of spreading out, gathers up into a pool, the latter being of circular outline. If then two of them come into contact they will unite together and again form a circular pool.

Instead of glass we may use nearly any non-metallic surface: wood, paper, etc. – and obtain similar results.

This is not an effect peculiar to mercury; a drop of water will not spread over a greasy plate, but gathers up like mercury on glass.

It is well to notice here that the liquids only spread out indefinitely provided they are capable of wetting the surface on which they are spread.

In other cases the liquids collect in drops or pools.

A study of these results suggests that the behavior of a liquid is not wholly controlled by its weight or by forces we have hitherto considered, but that there are other forces in action, which are dependent on the natures of the surfaces in contact.

These forces are called surface tensions.

FORCE

Mechanics is the branch of physics and engineering which deals with the interrelations of force, matter and motion.

We shall treat below the question of forces.

The term force, as used in mechanics, refers to what is known in every day language as a push or a pull.

We can exert a force on a body by muscular effort; a stretched spring exerts forces on the bodies to which its ends are attached; compressed air exerts a force on the walls of its container; a locomotive exerts a force on the train, which it is drawing.

In all of these instances the body exerting the force is in contact with the body on which the force is exerted, and forces of this sort are known to be contact forces.

There are also forces, which act through empty space without contact, these forces being called action-at-a-distance forces.

The force of gravitational attraction exerted on a body by the earth, and known as the weight of the body, is one of the most important forces in every day life.

Electrical and magnetic forces are also action-at-a-distance forces.

All forces fall into one or the other of these two classes, a fact that is found useful when deciding just what forces are acting on a given body. It is only necessary to observe what bodies are in contact with the one under consideration.

The only forces on the body are then those exerted by the bodies in contact with it, together with the gravitational force or the weight of the body.

Those forces acting on a given body, which are exerted by other bodies, are referred to as external forces.

Forces exerted on one part of a body by other parts of the same body are called internal forces.

RADIOCTIVITY

Uranium and thorium are the elements with the highest atomic weights. We can only imagine that these are survivors, other elements with still higher atomic weights having become extinct by disintegration.

Perhaps the inactive elements now existing are simply the products of the disintegration of many radioactive elements of which the earth was once composed.

It is interesting to note that according to the calculation, the heat given off by the disintegration of the radium known to exist in the earth is sufficient alone amply to account for the maintenance of its temperature.

A globe the size of the earth, possessing originally only heat energy, and cooling from a white-hot condition to the temperature of interstellar space, would have passed through the stage of habitable temperatures in a much shorter time than that which geological deposits and fossils show to have been actually available.

The discovery of the enormous but gradually released disintegration energy of radium enables us now to explain the prolonged period during which life has existed in the earth.

STRUCTURE OF A SUBSTANCE

A substance is composed of tiny particles called molecules. The molecules do not touch one another, because two opposite forces act between them – the force of attraction and the force of repulsion.

The magnitude of these two forces of attraction is much greater than that of repulsion. That is the reason why a much greater effort is required to crush a hard substance.

The force of attraction of the molecules in liquid bodies proves to be far less then in a hard substance. As a result, liquids offer little resistance when we try to break them up.

The swimmer makes use of this resistance however when moving his arms and legs in order to propel himself forward.

The turbine of a ship drives her forward by overcoming this resistance.

In gaseous substances the force of repulsion predominates. Hence their molecules strive to separate. That is why a gas, if placed in a vessel, fills it completely.

Due to the action of these two opposite forces there exists always a certain distance between the molecules and therefore they do not touch one another. The existence of this intermolecular space can be demonstrated by straining or stretching bodies.

Intermolecular space is present even in a gas, which, if subjected to exceedingly high pressure, ceases to contract its volume.

When a liquid evaporates, its molecules detach themselves from the common mass and begin to move freely in space.

The same thing happens to odoriferous substances. Their molecules detach themselves, begin moving freely and reach our organs of smell.

In conclusion it may be stated that molecules lead an independent existence, that all the molecules of a substance have the same properties, these properties being those of the substance itself.

ATOMIC STRUCTURE

At the beginning of the century scientists succeeded in breaking the tiny molecules of the atom by attacking it with even tinier particles propelled at enormous speeds. By penetrating the heart on the nitrogen nucleus with a fast moving one of helium (the so-called alpha particle) they converted the nitrogen into oxygen. The quantities used proved to be microscopic, but this was the actual transmutation of one element into another for the first time in history.

The old idea of an “indestructible atom” had died years before that when, in 1896, Becquerel and the Curies discovered radioactivity.

We now know that, far from being indestructible, uranium can slowly change into lead, the process being accompanied by the release of energy, which proves to be highly important.

The atomic structure is often likened to the solar system on a tiny scale; the centre, - the nucleus, is the “sun”, and the circulating electrons – the “planets”.

Using this analogy we can say that the ordinary chemical processes, we have so long known, affect only the “planets”, the “sun” remaining ever unchanged.

Now it is the very “sun” of the atomic structure, which is under attack.

We now at last know how some of the immense store of energy so securely held therein for perhaps thousands of millions of years, can be released.

Practically the whole atomic mass is held in the tiny nucleus, and this carries a positive electric charge.

The “planetary” electrons (they may be many or few) carry negative charges, but have exceedingly little mass.

Normally the sum of the negative charges on the electrons just balances the positive charge on the nucleus.

Ordinary physical and chemical properties of matter are determined by the number and disposition of the planetary electrons, and by the amount of the electric charge on the nucleus.

What is called the atomic number of an element is a measure of the electric charge on the nucleus, and therefore of the normal number of electrons circulating around it.

Hydrogen, the lightest, has only one; uranium, the heaviest, as many as 92.

Figures for the masses of nuclei run higher, thus with hydrogen as unity, the atomic mass of the most common form of uranium is 238.

Some uranium atoms however are known to have a nuclear mass of 235; this is said to be an isotope of the more common variety.

It has been discovered that some elements have isotopes – the forms in which the nucleus can have more than one mass.

THE REALM OF METALS

From the earliest times, man has gone to the crust of the earth to get raw materials for building the things he needed. He spent thousands of years in the Stone Age, living in caves and getting along as best as he could by using mostly rock and stone for making tools and implements of war.

Then came the momentous discovery that certain “rocks” would respond to fire and give forth a metal. In a few instances, uncombined or native metal simply melted and ran out.

In other instances, as we know now, compounds of the metals had to be reduced by carbon.

Copper and tin were two of the earliest metals obtained this way.

Then someone, somewhere succeeded in discovering these two metals when melted together to produce a new one that was much harder and stronger that either of them, thus making this unknown discoverer responsible for the Bronze Age.

There is little doubt that iron, too, was first obtained quite by accident. Then, when man learned to reproduce the accident at will, iron became the most important metal.

Thus we entered the Iron Age, an age from which we have not yet emerged.

True, we have learned how to obtain many additional elemental metals, how to produce thousands of alloys and how to manufacture dozens of other construction materials. But iron is still the key to an industrialized civilization.

However, important and interesting as it may be to learn how the various metals are obtained and prepared for use that is only half the story. Moreover, it is the less important itself, so far as most of us are concerned. As consumers of metals produced by others, our most immediate concern is how to take care of metallic structures and objects once they enter our possession.

We must wage a constant battle to retain possession of our metals. Processes, which result in the deterioration or “eating away” of metals, are commonly called rusting, tarnishing, and corrosion.

Fundamentally, these are processes in which metals oxidize and form compounds, in many cases the same compounds in which the metals normally occur in nature.

We try to prevent or reduce this sort of chemical action by electroplating, tinning, galvanizing, painting, or otherwise treating the metals that are susceptible to attack by the agencies of corrosion.

In conclusion we should mention one important aspect of “metal chemistry”, the question of how to continue meeting our needs for metals in the future.

After all, metallic ores are a natural resource which is neither inexhaustible nor renewable.

Chemistry is expected to be able to provide a satisfactory answer to this vital question.

ALLOYS

Generally speaking, metals are most useful in the form of alloys. Around 30 metallic elements serve modern needs, but over five thousand alloys are known, and hundreds of them are in common use. Sizable amounts of certain metals are converted into compounds of much importance. Brass, bronze, pewter, plumber’s solder, coinage metals, and stainless steels are examples of alloys. In general, alloys are formed when two or more metals are melted together and the mixture is allowed to cool and solidify. Alloys may also contain certain nonmetals, such as carbon, sulphur, arsenic, phosphorus, and silicon.

The main reason why so many different alloys are prepared is that modern industry requires metals with certain properties, or certain combinations of properties, which no one metallic element in the pure state can provide. Metallurgists attempt to regulate both the physical and the chemical properties of metals through the preparation of alloys. Various kinds of stainless steel illustrate alloys whose chemical properties enable them to resist the action of acids and the corroding agents of the atmosphere. Some of the metals used in automobile and aircraft engines, particularly in valves, must be able to withstand extremely high temperatures. Even now, future improvements in the gas turbine or jet engine may well depend in large measure on the development of new improved heat-resisting alloys.

In aircraft, the need is for metals that combine great strength with light weight. Many other industries have their own particular needs for metals with certain chemical and physical properties. All this should give you a pretty good idea of why alloys have been described as “metals made to order”.

Alloys may be classified in several ways. They are usually thought of in terms of composition, uses, or notable properties.

According to composition, we have ferrous alloys (those which contain iron), and nonferrous alloys (those which do not contain iron). The former group includes all the various kinds of steel – ordinary steels as well as alloy or special steels.

The nonferrous alloys are further subdivided according to the predominant or base metal. Thus we have alloys ‘in which aluminium, lead, copper, gold, silver, or any other metal except iron makes up the largest per cent of any constituent.

In terms of uses, there are bearing metals or antifriction alloys, coinage metals, and solders. Groups of alloys based upon distinctive properties include the lightweight alloys, low melting-point alloys, and electrical-resistance metals.