Marvels of Scientific Invention Part 4

After immersion the film is wet, not with water merely, but with a mixture of a lot of spirit and a little water. Hence the speed with which it evaporates. And the non-inflammability of the mixture is due to the presence of the water.

Methylated spirit only differs from the alcohol in alcoholic beverages in that something is added to make it undrinkable. Owing to the craving for it, which is so widespread, and the doubtful effect which it has on certain citizens, most states regard it as pre-eminently a subject for taxation, thereby on the one hand bringing in a good revenue, and on the other discouraging its too free use. But those considerations apply only to drinkable alcohol. That which is to be used for industrial purposes is not in any way a legitimate object for taxation. Hence the problem arises of making a form of alcohol which shall answer all the needs of the industries which use it, and at the same time be so repulsive to the senses that no one can possibly drink it. This result is achieved by adding some of the methyl alcohol derived from the vapour given off by wood when heated. Commonly known as "wood spirit," this is so unpleasant that it renders the mixture of no use for drinking, and so it can safely be freed from taxation.

Unfortunately this spirit has less heating value than petrol. That means that a given quant.i.ty of each liquid will produce more heat in the case of petrol than in the case of alcohol. Indeed the difference is about two to one. Hence an engine to give out a certain horse-power would need to have its cylinders twice as big if it were to use alcohol instead of the other fuel. There is a certain compensation, however, in the fact that alcohol is very easily compressible. In modern internal combustion-engines much of the efficiency is due to the explosive charge which is drawn into the cylinder being compressed into a small s.p.a.ce before it is fired. It was the discovery of the value of compressing the gas which made the gas-engine so formidable a rival to the steam-engine, and the wonderful performances of the Diesel engines are due very largely to the fact that the air is compressed in the cylinder to a very high pressure. The jet of oil burns in highly compressed air. And because of the facility with which alcohol can be compressed it is said to be more effective as a source of motive power than would be expected from its comparatively feeble heat.

Thus we may sum up the possibilities of the future. Coal, petroleum and their derivatives exist in limited quant.i.ties in the world, and so far as we can see the vast drafts which we are taking from them are not being replaced, indeed at this stage of the earth's development cannot be replaced, by any more. Sooner or later we must come to an end of them. Is it not comforting, therefore, to know that there is another source of fuel at hand, inexhaustible, since it can be produced as needed. We have only to set the sun and the ground to work to produce grain, rice, potatoes, or any of the myriad substances which contain starch, and from that, by simple and well-known processes, we can obtain a cheap, safe and reliable fuel. Indeed there seems nothing but the ultimate loss of sunlight, countless millions of years hence, which can ever check the supply of this valuable commodity. What has doubtless, in many cases, been a curse in the past may turn out to be the great boon of the future.

CHAPTER IV

SOME VALUABLE ELECTRICAL PROCESSES

Students of that branch of science known as physics are coming to the conclusion that electricity plays a much more important part in the universe than was supposed. They are led to believe that electrical attraction is the cement which binds together those exceedingly minute particles out of which everything is built up. Whether electricity binds them together or not, it is certain that electrical action can in some cases _separate_ those particles, and this process of separation provides a means of carrying on some very remarkable and useful industrial processes.

Let us imagine a vessel filled with water to which has been added a little sulphuric acid, while suspended in it are two strips of platinum.

There is a s.p.a.ce between the strips, so that when their upper ends are suitably connected to a source of electric current that current flows from one strip to the other _through the liquid_.

That is an example of the apparatus for carrying out this electrical separation in its simplest form, and it will facilitate the further description if the names of various parts are enumerated.

The process itself is electrolysis; the liquid is the electrolyte, while the strips are the electrodes. The individual electrodes, again, have special names, that by which the current enters being the anode and that by which it leaves the cathode. It is not difficult to remember which is which if we bear in mind that the current traverses them in alphabetical order. Since, however, it may not be easy for the general reader to carry all these terms in his mind, we will, when it is necessary to differentiate between the two electrodes, call one the in-electrode and the other the out-electrode.

Returning now to our imaginary apparatus, let us turn on the current. At first nothing seems to be happening, although suitable instruments would show that current was flowing. Soon, however, little bubbles appear upon the electrodes, and these grow larger and larger, until they detach themselves from the platinum to which they have been adhering, float up to the surface and burst. The question which naturally arises is, What do those bubbles consist of? Are they air?

If we take means to collect the gases which formed them we get an unmistakable answer. The bubbles which arise from the in-electrode are oxygen, those from the other hydrogen. If we allow our apparatus to work for some time, and collect all the gas which arises, we shall find that there is twice as much hydrogen as oxygen. We shall also find, as the process goes on, that the quant.i.ty of water diminishes.

Perhaps I may be allowed at this point to remind my readers that water is a collection of minute ultra-microscopic particles called "molecules," each of which is formed of three smaller particles still called "atoms." Of the three atoms two are hydrogen and one oxygen.

Water therefore consists of hydrogen and oxygen, there being twice as much of the former as there is of the latter.

We see, therefore, that electrolysis gives us hydrogen and oxygen in exactly those proportions in which they occur in water, and since we also see that as these gases appear the water itself disappears, we are led to conclude that the current is splitting up the water into the gases of which it is formed.

But the strange thing is that this will not work with pure water. We have to add something to it. In the case of our imaginary experiment it was sulphuric acid. What part does that play?

This is not fully understood, but we may be able to form a mental picture of what is believed to happen as follows.

The in-electrode is surrounded by a vast a.s.semblage of these tiny molecules, most of them those of water, but a few those of the acid. The latter are more complex in their structure than the former, but they too contain hydrogen. Current flows into the electrode and instantly hydrogen atoms from the _acid_ molecules crowd round it, like boatmen at the seaside anxious to secure a pa.s.senger. Each takes on board a quant.i.ty of electricity and with it darts across the intervening s.p.a.ce to the other electrode. Arrived there, it gives up its load and, its work done, remains lying upon the electrode until enough others like unto itself have gathered there to form a bubble and so escape. These hydrogen atoms are thought to be the _craft which carry the current through the liquid_ and enable it to pose, as it were, as a conductor of electricity, which in reality it is not.

But where does the oxygen come from?

To find the answer to that we must add a second chapter to our story.

When the hydrogen "boats" took on board their load of electricity they left their former a.s.sociates, and these forthwith "set upon"

neighbouring water molecules and with the audacity of highwaymen stole from them enough hydrogen atoms to take the place of those they had lost. Thus the acid molecules became complete once more, while the scene of the conflict near the in-electrode was strewn with the remains of the water molecules from which the hydrogen atoms had been stolen. These remains, of course, would be oxygen, and they, collecting together on the electrode, would eventually be in numbers sufficient to form bubbles and so escape.

Thus it may be the acid which really does the work, yet because of its subsequent raid upon the water it is the latter which disappears, and it is the materials of the latter which are bought to the surface in the bubbles.

And there we see the mechanism whereby, so it is believed, electric current can pa.s.s through otherwise non-conducting liquids. And the important point, as far as practical utility is concerned, is that the pa.s.sage of the current is accompanied by a splitting up of something or other, either the water or something in it, the materials of which are deposited, one on one electrode and the other on the other.

And now we can proceed to those useful applications of electrolysis, the commonest of which, perhaps, is electro-plating.

We have seen how electrolysis causes hydrogen, probably out of the acid, to be deposited upon one electrode. Suppose that, instead of an acid, we put in the water one of those substances known to chemists as a "salt,"

the commonest example of which is ordinary table salt. This well-known condiment is caused by the interaction of hydrochloric acid and the metal sodium and will serve to ill.u.s.trate what all salts are.

All acids are compounds of hydrogen and something else, and their biting action is due to the readiness with which the "something else" evicts the hydrogen and takes in a metal in its place. Thus hydrochloric acid, given the opportunity, gets rid of its hydrogen and takes in sodium, thereby forming chloride of soda or common salt.

Another example is the gold chloride familiar to photographers. This is the result of the action of certain acids upon gold, wherein the acids throw out their hydrogen and take in gold instead.

To sum up, then, a salt is just the same sort of thing as an acid, like the sulphuric acid which we used in our "experiment," except that some metal has taken the place of the hydrogen.

It is not surprising, then, to find that if we put a salt in the electrolyte instead of an acid we get a similar result. In the one case hydrogen is deposited upon the out-electrode, in the other the metal. In the former case, since hydrogen is a gas, it forms bubbles and floats away, but in the latter the solid metal remains a thin, even coating upon the electrode. That is the principle of electro-plating.

The electrolyte consists of a suitable solution containing a salt of the metal to be deposited, and it is placed in an insulating vessel or vat.

The articles to be plated form the out-electrode, so that they have to be suspended in some convenient way from a metal conductor by conducting wires. Of course they are entirely immersed in the liquid. The in-electrode is sometimes a plate of platinum (the reason that expensive metal is used being that it is unaffected by the chemicals) or else a plate of the metal being deposited. In the former case, the solution becomes weaker as the work proceeds, and more salt has to be added. In the latter, however, the strength of the solution remains unchanged, for by an interesting interchange the in-electrode adds to it just what it loses by deposition upon the other one. The effect is therefore just as if the current tore off particles from the one and placed them upon the other.

This is believed to be due to the agency of the oxygen which in the case of the electrolysis of water becomes free, but which in this case forms with the metal electrode a layer of oxide upon its surface, this oxide being then dissolved away by the liquid. Thus as fast as the metal is deposited upon the out-electrode its place is taken by more metal from the in-electrode.

In some processes it is desired to deposit metal upon a non-conducting surface, and it is evident that such cannot be used as an electrode. Nor is it any use to attempt to deposit upon anything except an electrode.

The only thing to do, then, is to make the object a conductor by some means. Models in clay, wax and plaster, once-living objects like small animals, fruit, flowers or insects, can, however, have a perfect replica made of them by electrical deposition, by the simple method of coating the surface to be plated with a thin layer of plumbago. This skin, although extremely thin, is a sufficiently good conductor to make the process possible. Process blocks for printing are copied in this way, so that a particularly delicate example of the blockmaker's art need not be worn down by much pressing, copies or "electros" being made off it for actual use in the press.

The original block is a plate of copper on which the picture is represented by minute depressions and prominences. On this a layer of soft wax is pressed, so as to obtain a perfect but reversed copy. Having been coated with plumbago, this is then put into a vat containing a solution of copper salts and is used as the out-electrode, the other being a plate of copper. When the current is turned on the copper is thus deposited on the wax until a thin sheet of copper is formed which is an exact but reversed copy of the wax, a direct copy, that is, of the original block.

The back of this thin sheet is then covered with molten lead or type metal to fill up any depressions and to give it sufficient strength.

Anyone who has seen one of these "half-tone" blocks covered with minute depressions so slight that they can scarcely be seen, yet so perfect that a beautiful print can be obtained from them, will realise the wonderful power of this electrolytic process, the marvellous accuracy with which the original is copied, and the unerring way in which the electric current carries the particles of copper into every one of the myriad recesses in the wax.

Another specimen of the marvellous work of this system is the wax cylinder of the phonograph. The sound is produced by a needle trailing along a groove of varying depth cut in the surface of the cylinder. This groove forms a spiral, pa.s.sing round and round like the thread of a screw, and it encircles the cylinder one hundred times in every inch of its length. Consequently, at any point one may take, there is but one one-hundredth of an inch from the centre of one turn to the centre of the turn on either side of it. And at its deepest the groove is less than one-thousandth of an inch deep. The phonograph itself cuts the first "master" record, as it is termed, and the problem is to take a number of casts off this model of such delicacy and accuracy that every variation in that exceedingly fine groove shall be faithfully reproduced. Such a task might well be given up as hopeless, but with the help of electrolysis it is accomplished easily and cheaply.

To attempt to press anything upon the surface of the "master" would but smooth out the soft wax and obliterate the groove altogether. To apply anything softened by heating would be to melt it. But electrolysis, without tending in any way to distort or damage the delicately cut surface, deposits upon it a surface of metal from which thousands of casts can be made. The gentle fingers of the electricity overlay the soft wax with the hard, strong metal with a minute perfection almost beyond belief.

To commence with, the master record is placed upon a sort of turntable in a vacuum and turned round in the neighbourhood of two strips of gold-leaf strongly electrified. By this means the gold is vaporised and a perfect coating of gold is laid upon the wax. This is far too thin to be of any use, except to render the cylinder a conductor, for the coating is so fragile that it is no stronger than the wax itself. It enables the cylinder, however, to be electro-plated with copper until it is surrounded by a strong metallic sh.e.l.l a sixteenth of an inch thick.

It takes about four days to deposit this thickness. The copper sh.e.l.l is then turned smooth in a lathe and fitted tightly into a bra.s.s jacket. A little cooling causes the wax record to shrink sufficiently to free it from the copper sh.e.l.l and allow it to be lifted out. A copper mould is thus formed in which any number of additional records can be cast. The molten wax is simply introduced into the inside, and allowed to set; the inside is bored out in a lathe, and then with a little cooling it shrinks and can be withdrawn, a completely finished record, every tiny depression or swelling in the original master being reproduced with an accuracy almost incredible.

Another valuable use to which this process is put is the purification of metals. The electro-chemical action works with unerring precision: it never mistakes an atom of iron for an atom of copper, for example.

Pa.s.sing through a solution of copper salt, the current deposits only copper.

For modern electrical machinery and apparatus copper is required of the utmost possible purity, for every impurity adds to its electrical resistance, in other words, diminishes its value as a conductor.

Consequently thousands of tons of "electrolytic" copper, as it is termed, are produced every year. The electrodes used are plates of ordinary copper. A coating of pure metal is deposited by electrolysis upon the out-electrode from the other one. When the deposit is thick enough the out-electrode is taken out and the deposit torn off it, the union between the two being sufficiently imperfect for this to be done without difficulty. The metal of which the in-electrode is made has already been purified by other processes, until it contains but one per cent. of foreign matter, and by this means even that small percentage is entirely got rid of. The impurities fall to the bottom of the vessel in the form of "slime," which is periodically removed.

And not only is electrolysis thus unerring in picking out certain atoms from among a mixture, but there is an exact relation between the work done and the quant.i.ty of current used. Consequently it forms a very exact method of measuring currents. The method of measuring current by the strength of the magnetic field which it produces has been mentioned already, and such measurements can be checked by electrolysis. Thus the practical definition of the ampere is "that current which when pa.s.sed through a solution of silver nitrate in water will deposit silver at the rate of 001118 gramme per second."

The electric acc.u.mulator or secondary battery, one of the most useful appliances, is the result of electrolysis reversed. Many large electric-lighting plants have in addition to their generating machinery a large battery of secondary cells, which, being kept charged, are able to help the machinery in times of heavy demand, or even to supply the whole current needed for, say, half-an-hour, so that the whole of the machinery could, in the event of an accident, be shut down for that time and the supply maintained from the batteries. This would be sufficient in many cases for fresh machinery to be brought into action or emergency arrangements to be made.

It may be that this book is being read by someone seated serenely in his arm-chair while engineers and workmen at the generating station are working in frantic haste to set right some sudden breakdown before the batteries are run down. The batteries may have saved the town half-an-hour's darkness.

Large telegraph offices are fitted with secondary batteries. Many motorists owe the ignition which keeps their engines at work to secondary batteries. It is secondary batteries which keep the wireless apparatus at work on a wrecked vessel after the engines have stopped.

Indeed secondary batteries are one of the most beneficent inventions.

And if only they could be made in a lighter form than is possible at present their value would be infinitely increased.

We have seen how the pa.s.sage of current through acidulated water produces hydrogen and oxygen. If those gases be collected in closed vessels over the water, so that they remain in contact with the water, as soon as the current is stopped a reverse action sets in. The gases tend to recombine with the electrolyte and in so doing to give back a current equal to that which formed them. Fig. 4 shows the construction of what is called a voltameter, in which the gases arising from the electrodes are collected in little gla.s.s vessels placed just above them.