Marvels of Scientific Invention Part 6

Hydrogen can be liquefied in a similar machine, except that it needs a little preliminary cooling with liquid air. Liquid hydrogen is the coolest thing known approaching the region of absolute zero.

And now we can turn to the wonderful discoveries which have followed upon the manufacture of liquid air.

To make the story complete we need to go back to the time of Priestly and Cavendish, early in last century. They investigated the atmosphere and showed that it consisted of oxygen and nitrogen in certain invariable proportions, with under certain conditions a small proportion of carbonic acid. These facts were so well authenticated, and they seemed to explain everything so satisfactorily, that it was quite thought almost up to the end of the nineteenth century that there was nothing more to learn about the atmosphere.

Nevertheless there was an idea in the minds of some scientists that there must be another group of elements somewhere, the existence of which was then undiscovered, but it was never dreamed that these were in the air.

Soon after the weights of the atoms had been found a medical student named Prout in an anonymous essay called attention to the fact that there were curious numerical relationships between them. Speculation on the subject went on for many years, until in 1865 the great Russian chemist Mendeleeff published his conclusions. He had arranged the elements in the form of a table _in the order of their atomic weights_.

The table consisted of twelve rows of names forming eight vertical columns, and the remarkable thing was that all those elements which fell into any particular column, although their atomic weights were very widely different, had similar properties. This enabled him to _predict_ the discovery of certain new elements, for the table contained a number of blank s.p.a.ces. Three elements _have been found_ since, and their atomic weights and properties are just such as to fill three of the blank s.p.a.ces. One blank s.p.a.ce, it is thought, may be filled some day by the gas coronium, which like helium has been discovered in the sun, but unlike it has not yet been detected here. When it is, there is the place in the table which it may fill. The table then commenced with what is still called Group 1, but for reasons too complicated to explain here it appeared as if there must be a group before that, a group the chief characteristic of which would be the inactivity of the elements included in it. These were expected to be of various atomic weights, but these weights, it was antic.i.p.ated, would so occur in the intervals between the others that they would all fall into a new column to the left of "Group 1."

In the year 1892 Lord Rayleigh was investigating the question of the density of a number of different gases, including, so it happened, nitrogen. Now there are several ways of procuring nitrogen. One is to get it from the atmosphere by ridding it of the oxygen with which it is normally mixed. Another way is to split up some compound, such as ammonia, of which it forms a part, in such a way as to catch the nitrogen and leave the other elements with which it was combined elsewhere.

Lord Rayleigh tried both ways, and he found that the nitrogen from the atmosphere was denser than that derived from ammonia. Sir William Ramsey then carried the matter a step further. He heated atmospheric nitrogen in the presence of magnesium, under which conditions some of the nitrogen combines with the latter element to form nitride of magnesium.

That, it was found, made the remaining nitrogen denser still. The explanation then seemed obvious. Suppose we imagine a mixture of sawdust and iron filings: it will be heavier than an equal quant.i.ty of pure sawdust. And if we contrive to take away some of the sawdust from the mixture we shall find that what is left is heavier still, when compared with an equal bulk of pure sawdust. For it is clear that as we take away sawdust we thereby increase the proportion of the heavier iron filings and so we make the mixture heavier.

Applying a similar process of reasoning to these discoveries, the conviction grew that the nitrogen of the air was not pure, but that it had mixed with it a small proportion of some other gas of greater density. They soon succeeded in isolating this denser gas, to which they gave the name of argon. Its atomic weight was found, and, wonderful to relate, it was such that argon fell into a new column to the left of Group 1, as had been antic.i.p.ated.

The discovery of argon was announced in 1894. The next year Sir William Ramsey, investigating a gas which had been discovered locked up in the interstices of a mineral called clevite, was able to state that it was helium, the element which had been previously noticed by the spectroscope in the sun. Like argon, it was found to be extremely inactive, and its atomic weight turned out to be such that it too fell into the "Zero Group."

In 1898 Professors Ramsey and Travers found two more gases in the air, krypton and neon, and a little later still, there was found mixed with the krypton a further new gas, xenon. All of these had their atomic weights found, and fell into that new column in the periodic table.

But what has all this got to do with liquid air? The two subjects are closely related, for it is by liquid-air machines that these rare gases are now obtained, and it was from liquid air that the last three were first discovered.

For air, as we well know, is a mixture of gases, and when extreme cold and pressure are applied these gases liquefy, each behaving according to its own nature. They do not all liquefy at the same time, nor on being relieved from the pressure and heated do all evaporate again at the same temperature. Although they emerge from the liquid-air machine in the form of a single liquid, it is really a mixture of liquids, each with its own boiling-point.

In an earlier chapter we saw how petroleum can be separated into its various const.i.tuents, such as petrol, by fractional distillation, advantage being taken of the difference in the "boiling-point" of the various "fractions." The boiling-point of a liquid is, of course, the temperature at which it turns freely into vapour, and just as petroleum when heated gives off first cymogene, next rhigolene, then petrol, benzine, kerosene and so on, in the order named, so liquid air, when it is evaporated, gives off its different const.i.tuents in order. Nitrogen, oxygen, argon, helium, krypton, neon and xenon can all be separated each from the others in this way, by "fractional distillation." The heat from the surrounding objects is allowed to get at the liquid, and the gases are then given off in the order of their boiling-points.

And thus we see how the mechanical production of cold has a.s.sisted in the pursuit of pure science. The newly-found gases are not of any great use at present. They are so inactive that possibly they never will be, with one exception, and that is neon. If an electric discharge be made to pa.s.s through a tube filled with this gas, a beautiful glow is the result, and it is just possible that neon tubes may become the electric light of the future. That is only a prediction, however, and a hesitating one at that.

The inactive elements may become of value in explosives. We have seen how important nitrogen is in these dangerous substances, the chief feature of which is their instability--their readiness, that is, to change into something else--which instability is due to the reluctance with which nitrogen enters into them. Now nitrogen, though inactive, is much less so than these others, and if a way should ever be found of inducing them to enter into a compound, that compound will probably be an extremely powerful explosive.

CHAPTER VI

SCIENTIFIC INVENTIONS AT SEA

The safety of our fellow-creatures has always been a strong stimulus to our inventive faculties. The occurrence of a bad railway accident, and, roughly, its nature, can be inferred from the files of the Patent Office, for such an event brings men's thoughts to devising ways and means of preventing a recurrence, and an avalanche of such inventions descends upon the patent department in consequence. In like manner a particularly distressing accident to a lifeboat some years ago brought out many inventions for the improvement of those romantic craft. Many of the inventions which arise under these conditions are, of course, utterly worthless, but some of them "come to stay."

It is not surprising, therefore, when we think of the almost innumerable wrecks which happen, even with modern shipping, that human ingenuity has been extremely busy in devising ways for bringing more of safety and less of risk into the lives of those who go down to the sea in ships. Of these perhaps none is more fascinating than the modern lighthouse, with its tall tower, its brightly flashing light, standing undisturbed in the wildest storm, quietly and persistently sending forth its guiding rays, no matter how the elements may be buffeting it. There is something specially attractive in this perfect embodiment of quiet strength and devotion to duty.

Of course, its origin is very ancient. One of the earliest inventions, no doubt, was the bright thought of a very primitive man who lit a fire on a hill to serve as a guide to some belated friends out in their fishing canoes. From some such beginning the modern lighthouse, a magnificent product of the science of civil engineering and the science of optics, has arisen.

Of the difficulties encountered in the construction of lighthouse towers on outlying rocks much has been written. The historic Eddystone, for example, has quite a voluminous literature of its own. Of the light itself, however, much less is known.

It will be interesting first to note the different purposes for which a light may be required, and then see how the apparatus of the lighthouse is made to serve these purposes.

There is the "making" light, perched, if possible, upon some high eminence, deriving its name from the fact that the sailor sights it as he is "making" the land. Vessels approaching England from the south-west by night first see the light at the Lizard. The transatlantic vessels know they are approaching land by catching sight of the Fastnet Rock light off the coast of Ireland. Cape Race light serves in the same way for those about to enter the St Lawrence and Navesink for the entrance to New York harbour. All such as these have to be of the greatest power practicable, so that they may be visible not only at the longest possible distance, but also under unfavourable conditions, such as haze and slight fog. No light, of course, can penetrate thick fog, but in light fog and haze a powerful light can be seen at considerable distances. For the same reason these lights must be high up, or the curvature of the ocean's surface will limit their range. A light elevated 100 feet above the sea-level will be visible nearly 16 miles away, but if only 50 feet up it will be invisible at 13 miles. To be seen 40 miles away it must be as high as 1000 feet.

But then again height is in some cases a disadvantage, for sometimes fog hovers a little distance above the sea, while below it the air is clear, and the higher a light may be the more likely is it to have its lantern immersed in a floating cloud of fog. Many readers familiar with the south coast of Britain will remember that the light which used to show on the summit of Beachy Head is there no more, but has been replaced by a tower at the foot of the cliffs, the reason being that it may be below the clouds of fog which are prevalent at that point.

But the mention of Beachy Head introduces us to another cla.s.s of lights, known as "coasting" lights, since they are intended to lead the mariner on from point to point along a coast. It will be seen at once that in many cases they do not need to be visible at such great distances as the making lights. When the mariner has sighted the Lizard, for example, he knows where he is. In order that he may learn that important fact as soon as possible it is desirable that that light should have the greatest possible range, but having thus located himself, when he begins to feel his way along the English Channel he is guided by the coasting lights, and so long as they are of such range that he will never be out of sight of one or two of them that will be sufficient. Thus the Beachy Head light, in its present low position, has a sufficient range for its purpose, with the added advantage of more freedom from obscuration by fog. Thus we see how the local conditions and the purpose of each particular light have to be taken into consideration in determining its position and power.

The Eddystone, again, is an example of a further cla.s.s. It simply serves to denote the position of a group of dangerous rocks. Its function is not so much guidance, although no doubt it often serves for that, but for warning. The Lizard light beckons the on-coming ship to the safety of the English Channel; the Eddystone warns it away from danger. The latter, therefore, and similar lights are "warning" lights.

[Ill.u.s.tration: _By permission of Messrs. J. and E. Hall, Ltd._

A COLD STORE

Interior of a cold store, in which meat and poultry are kept good and fresh by the use of machine-made cold.--_See_ p. 67]

Right at the entrance to the English Channel, that greatest of all highways for shipping, there lie the Scilly Isles. This group comprises some few islands of fair size from which we draw those plentiful supplies of beautiful spring flowers, but it also includes a large number of rocky islets which have sent many a strong ship to its doom.

On one of the islets, therefore, the Bishop's Rock, there now stands a very powerful light which exemplifies many whose purpose is the double one of welcoming the mariner as he approaches our sh.o.r.es and at the same time warning him of a local danger. Such are both making and warning lights.

Of no less importance, though less impressive, are the guiding lights, which guide the ships into and out of harbours and through narrow channels. These are generally arranged in pairs, one of the pair being a little way behind and above the other. Thus when the sailor sees them both, one exactly over the other, he knows he is on the right course.

Sometimes lighthouses have subsidiary lights as well as the main light, to mark a pa.s.sage between two dangers, or to give warning of some danger. The subsidiary lights are often coloured, and they are generally "sectors" showing not all round a complete circle, or even a considerable portion of one, but just in one certain direction. They are generally shown from a window in the tower lower down below the main light.

Finally, it is important to remember that every light must be distinguishable from its neighbours. Hence every one in any given locality has a different "character" from all the others. This character is given to it by means of flashes. Instead of showing, as the primitive lights did, a steady light, the modern lighthouse exhibits a series of flashes, the duration of which, together with the intervals between, give it its distinctive character. This flashing arrangement has a further advantage over the steady light. Each flash can be made more powerful than a steady light could be. But of that more later.

The actual source of light varies with circ.u.mstances. The electric arc is, as we all know, a very powerful light, in fact it can be made the most powerful of all, but its light is decidedly bluish. Now the time when a light is most of all needed is when the weather is thick. Fogs varying from a slight haze to a thick pall of darkness are of very common occurrence, and the lighthouse light must be able as far as possible to penetrate them.

As a matter of fact clean fog, such as one gets at sea, is not by any means opaque. The black fogs of the great cities are another matter, but they are not the sort which afflict the mariner. On a foggy day in the open country or by the sea it is often particularly light; indeed the light is of a peculiarly diffuse nature which gives a nice even illumination to everything. Thus we see that fog is really transparent, but it diffuses the light. It does not stop the light rays, but simply bends them about and scatters them in all directions. Thus we can see nothing through the fog, yet a flood of light reaches us through it. In its effect it is like that "crinkled" gla.s.s which is often used for part.i.tions between rooms, which lets light through, but which cannot be _seen_ through.

We see, then, that the effect which a fog produces is mainly to refract the light rays. Each little drop of water (for it must be remembered that fog is myriads of tiny drops of liquid; it is not vapour) acts like a minute lens, and bends the rays which pa.s.s through it. And the more blue a ray is the more it is bent. On the contrary, the more red it is the less is it bent. When a beam of light is a.n.a.lysed in the spectroscope the red rays are bent least and the blue rays most, so that the red rays fall at one end of the spectrum and the blue at the other.

Now we only _see_ a thing when light rays proceeding from every part of it fall straight (or nearly so) upon our eyes. Consequently, since red rays are bent and scattered by the fog less than blue rays are, a red light will be more easily seen through a fog than a blue one. It might seem from this that a red gla.s.s put in front of a light would make it better for this purpose, but that is not the case, for the simple reason that filtering the light through red gla.s.s does not really make it any redder than it was before: it simply makes it look redder by extracting from the original light all except the red. But a source of light which is _naturally_ reddish is so because it is more plentifully endowed with red rays, while a bluish light like the electric arc is naturally deficient in red rays. Consequently we should be inclined to expect from theory that the electric arc would not be a good light for a lighthouse, since it would lack penetrating power in foggy weather. Some readers may have noticed themselves, in towns where electric lights and gas lamps are in use near each other, that the latter, though relatively feebler under normal conditions, seem to give more light in fog. And experiments show that this is really the case. So although there are some lighthouses with electric arc lights, that which is now believed to be the best is an oil lamp of special design, using a mantle of the Welsbach type.

The oil is stored in strong steel reservoirs into which air is pumped by means of a pump not unlike those used to inflate bicycle tyres. By this means a pressure is maintained upon the oil of about 65 lb. per square inch. This forces the oil up a pipe and drives it in a jet into a vaporiser, a tube heated from the outside so that in it the oil is turned into gas. This gas then rises to the burner and heats the mantle, just as the gas does in the ordinary incandescent gas light. Indeed in the case of lights on the mainland near a town the gas from the town main is often utilised. But this simple arrangement for using vaporised oil, as will readily be seen, can be employed anywhere. A little of the gas produced is led through a branch pipe and burnt to heat the vaporiser. To start the apparatus the vaporiser is heated with a little methylated spirit. Thus everything is quite self-contained and so simple that there is little to get out of order. The largest size of lamp will give 2400 candle-power, with an expenditure of 2-1/4 pints of oil per hour, just common oil, too, of the kind used with ordinary wick lamps.

Having got a source of powerful light, the next thing is to collect that light and throw it in the direction required. For the light proceeds from the lamp in all directions (practically), and much of it would be entirely wasted could it not be collected and guided in the required direction.

The earliest attempt at this was to use a reflector of bright polished metal. In the most improved form these were made to that peculiar curve known as a parabola. This is a curve obtained by cutting a cone in a certain way, wherefore it is one of the "conic sections," and its particular appropriateness for this work resides in the fact that if a light be placed at a certain point known as the "focus" all the diverging rays which fall upon the reflector will be reflected in the same direction, parallel to each other. An ordinary spherical mirror would reflect them either back to the lamp or in diverging directions.

At any distance the beam from the parabolic reflector will be more intense than that from the spherical one, since the rays will be closer together. But even with the parabolic one there is some diffusion, for the simple reason that whereas the focus is a mathematical point (position without magnitude) the most concentrated form of light known has a considerable magnitude. Hence the rays proceeding from the centre of the mantle are reflected as per the theory, but those from the outlying parts of it are somewhat diffused. This difficulty cannot possibly be overcome, and hence even in the finest examples of lighthouse architecture the flashes are not quite sharp and clear-cut.

There is a central moment, so to speak wherein the flash is almost blinding in its intensity, but it is preceded by a period of growing brightness and succeeded by one of decreasing light.

In the modern apparatus, however, metallic mirrors are entirely dispensed with, their place being taken by reflecting prisms of gla.s.s.

The metallic ones had to be continually rubbed to keep them clean, and this soon dulled their brightness, while the gla.s.s prisms need only to be wiped carefully, which operation has little effect upon their surface.

It may come as a surprise to some that reflecting prisms are possible.

The idea of refraction through a prism is quite familiar. Such forms the essential principle of the spectroscope. Refraction is explained to every school child in order to account for the rainbow. But _reflection_ by a piece of the clearest gla.s.s seems a contradiction in terms almost.

Yet it is only a question of shape. In some prisms the light is simply bent as it pa.s.ses through. In others it is bent twice, so that it leaves the prism just as if it had been reflected off a mirror. Both devices are used in the lighthouse. Let us see how they are combined so as to perform the work to be done.

Take first of all the case of a light upon an isolated rock where the warning is needed equally all round. All that is necessary here is to pick up those rays which, if left to themselves, would fall upon the water near the foot of the tower, and those which would waste themselves skywards, and then to gather all the rays into several bundles or beams.

We will suppose a simple case in which the light is supposed to give flashes at regular intervals.