The foregoing illustration shows how much practical rules deviate from the results of theoretical calculations; it shows also, however, that the latter are the true starting-points the foundations to build on with practical experiments. The same principle should be followed throughout the whole field of machine-designing. There are even many cases where a calculation beforehand is out of the question, where one must almost lay out the whole machine by intuition and then go over it with the magnifying-glass of theory, and add where we find too little and take off where we find too much.

ANCIENT philosophers regarded fire and

water as two elements which were in direct opposition to each other. But modern science has demonstrated that water can be decomposed into two gases, one of which will burn; and again, that water is one of the products of combustion. For experimental purposes, the decomposition of water into hydrogen and oxygen is effected by electrolysis-a method fully described in HOME STUDY MAGAZINE for December, 1896. We will now describe the commercial process employed in the manufacture of hydrogen- or water-gas-explaining how it is made to furnish light and heat for public use.

The first operation is carried on in the "generator." This is practically a large receptacle or fire-pot, so constructed that it can be hermetically sealed at the bottom, in order to prevent the entrance of air. It is filled with a mass of anthracite coaltechnically called a "charge." This is fired and brought to a state of incandescence through the aid of a blast of air from a blower. This is the only draft that the fire receives, since the generator is sealed. The heated gases-the product of this vigorous combustion-pass upwards through the "superheater" or "regenerator." Here they meet with a surface of hot brickwork, arranged in a checkered fashion in order to expose the ascending gases to a large heating area. The air-blast is continued until, on looking through the peephole at the side of the regenerator, these bricks are seen to be very highly heated. The airvalve is then closed and a jet of steam is forced upward through the glowing coal. The heated carbon, searching for oxygen, cannot find it in a free state now that the air-blast is shut off, so it takes it from the steam, uniting with it to form carbon dioxide (CO) and liberating the hydrogen. But in passing through the remainder of the glowing coal, the greater part of the CO2 loses one more atom of oxygen and becomes carbon monoxide (CO). This is the gas

which burns with a pale blue flame on the top of a freshly-coaled fire. It is the result of carbon burning in an insufficient supply of oxygen.

The gases, then, which pass upwards into the regenerator are hydrogen, carbon monoxide and carbon dioxide; and, in addition to these, there are sulphur gases formed from the impurities in the coal. As all these gases enter the lower end of the regenerator, they meet with a spray of petroleum, which is forced in by a small jet of steam. The purpose of this petroleum is to furnish carbon to the gas, petroleum itself being what is called a hydrocarbon, that is, composed largely of hydrogen and carbon. Now, hydrogen alone is not an illuminant; it

After the steam-blast has been in operation for about 20 minutes, it is necessary to reheat the apparatus, so the steam-valve is closed and the air-valve opened. One charge of coal lasts through two complete charges of steam and air, or about 80 minutes. These figures vary considerably under different circumstances. The quantity of gas liberated at each coal charge is from about 12 to 15 thousand cubic feet. In the 24 hours of a winter's day, about 10 charges of coal are used.

The chemical action which takes place in the regenerator is called "fixing the gas." This hot fixed gas contains some tarry products which are condensable. These must be removed, or the gas-pipes would soon become choked. So the gas is passed through a sort of trap, filled with water. This cools it somewhat and also prevents any backward rush. It then enters what is called a "scrubber. " There are various forms of scrubbers, but the main principle of them all is to bring the gas in contact with a constantly renewed supply of cold water. The cooling condenses the heavy tars and they flow out at the bottom of the scrubber. The gas is then passed through other condensers, similar in construction to the ordinary upright tubular boiler. The hot gas passes through the tubes, entering them at the top and passing out at the bottom. These tubes are surrounded by cold water, which enters at the bottom, under pressure, and flows out at the top. From one condenser the gas passes on to a second and sometimes to a third, and when it makes its final exit, it is quite cool.

The gas next passes into a system of pipeworks, through which it is diverted by means of valves into the purifiers. These purifiers consist of large flat iron boxes. They are arranged in sets of four. Three of them are always in use, the fourth being kept for a change-off during the cleaning of any one of the other three. Each box contains a mixture of oxide of iron, lime, and sawdust. The sawdust has no chemical effect, being added merely for the sake of keeping the lime and oxide of iron in a state of mechanical separation. The lime unites with the carbon dioxide and forms calcium carbonate. It also takes up some of the sulphur, forming sulphide of calcium. The remaining sulphur compounds are taken up by the iron oxide, and form sulphide of iron. The gas is now ready for use and passes into the station meter.

This consists of a large cylindrical vessel about 10 feet in diameter and 15 feet long.

An exterior view is shown in Fig. 1. The interior is represented in Fig. 2. The vessel is a little more than half-full of water. The gas enters at P above the surface of the water, and passes into one of the four partitions of the drum, as shown. Its exit is cut off at C by the water, so it raises the partition; in other words, it causes the drum to rotate in the same direction as the hands of a clock. As the outer end of the partition emerges from the water at D, the gas escapes into the space between the drum and the external cylinder and passes out through a pipe not shown in the figure. This instrument is known as a wet meter and is an accurate index of the output of the

works, for it is operated entirely by the pressure of the gas; and since each partition holds a known amount, the total volume which passes through the meter may be measured by recording the number of revolutions. This is done by a mechanical counting device attached to the cylinder, the dials in front recording the number.

After escaping from the meter, the gas flows through pipes to the gas-holder shown in Fig. 3. This consists of an inverted vessel of sheet iron, which floats in a tank of water, and rises and sinks as the volume of gas in it varies. The pipe through which the gas enters, opens above the surface of the water, and another pipe placed on the opposite side provides for its exit. As the holder rises and falls, it is kept in position by the rollers or pulleys shown in the figure. The modern gas-holder is a triumph of mechanical skill. Much ingenuity has been expended in constructing a holder which would have the necessary strength, and still

that is manufactured during the part of the day when there is little demand, and, when the gas is in use, forces it out with a nearly constant pressure by reason of its own weight. It also mixes the gas from different charges so that the product as sent through the mains will be of uniform illuminating power.

The amount of pressure in the mains is regulated by the "station governor." This instrument is so delicately constructed that the opening of 20 burners on the mains will cause it to automatically increase the supply. Near the governor is a U tube containing water. One arm is open to the air, and the other is subject to the pressure of the gas. The difference in height of the two columns indicates the pressure. In the office, an automatic register and pressure-gauge are in sight of the superintendent. These read the same as the U tube in front of the governor, and a breakdown is immediately discovered. From the governor the gas passes out through the mains. Sometimes, when the mains extend a great distance from the works, an auxiliary holder is constructed, which regulates the pressure for that district. At the consumer's the gas is again measured by a small dry meter.

Let us now examine the flame of the burning gas. Just above the tip is the blue space marked H in Fig. 4. If we insert the end of a glass tube into this space, we may observe a deposit of water near the upper end. On the approach of a match the issuing gas will ignite. This blue area of

intense heat and low illuminating power is burning hydrogen. It unites with the oxygen of the air and forms water, which condenses in the upper part of the tube. Along with the hydrogen come the hydrocarbons. They are decomposed in the dark space just above H-marked C-and the carbon is free. During its passage through the flame, it becomes incandescent, and then unites with the oxygen of the air to form carbon dioxide. When there is an excess of carbon or a deficiency of oxygen, all the carbon is not consumed, and it escapes in the form of soot. The dark fringe around the flame is caused by the condensation of the carbon as it meets with the cool air. If we raise the tube from its first position, until the end lies in the area of illumination, the little flame will go out, and in its place we will have smoke.

If a cold glass plate is brought near the top of the flame, a band of soot will be deposited upon its surface. If now we take a tube and blow through the flame horizontally at H, the entire flame will become blue. If we now hold the cold plate near this flame, no soot will be deposited, showing that the carbon has all been consumed. If a wire is held in this blue flame, it will become white hot in a few seconds, owing

motion of the gas, and furnishes to the flame an abundant supply of oxygen. The new gasburner of the Welsbach type consists of an incombustible ash mantel which is heated to incandescence by a small Bunsen burner. The rapid introduction of electricity as an illuminant has stimulated the gas manu

facturer to the production of a better quality of gas, and the provision of means for its economical use, both as an illuminant and as a heating agent, and the time is possibly not far distant when gas will be used instead of coal or wood, for cooking our food and warming our homes.

OF

F ALL our surroundings, we are probably most familiar with that invisible something which we call the atmosphere, and which we are industriously engaged in pumping into our lungs "for dear life," every moment of our existence.

The atmosphere is the aerial envelope which surrounds the earth, and constitutes the ocean of air at the bottom of which we are living. The exact height of the atmosphere is unknown; it is generally given as from 30 to 35 miles; observations, however, upon the zodiacal light and meteoric showers lead us to believe that it may be from 50 to 60 miles.

Thanks to certain self-acting arrangements in the nervous system of the animal organism, the respiratory organs work mechanically and perform their unceasing labors so quietly and regularly that, as a matter of fact, we scarcely give a thought to them, unless, in some way or other, they get out of order. It hardly ever occurs to us that we are living and breathing at the bottom of an immensely deep aerial ocean, deeper and wider to an enormous extent than the watery oceans we are familiar with, and agitated by tides and currents and furious whirlpools, compared to which the disturbances of the watery oceans are the merest pigmies.

The first question which occurs to the thinking mind is, of what is this immense aerial ocean composed; what are the gases and what their proportions?

The exact composition of the atmosphere has repeatedly been made the subject of experimental research. It has been found that the air is a mixture of oxygen and nitrogen in the proportion, according to

Dumas and Boussingault, of 23.13 of oxygen

to 76.87 of nitrogen. It also contains a little less than 1 per cent. by volume of a gas, the existence of which has been recently discovered by Lord Rayleigh and Professor Ramsey, and named by them argon; traces of carbonic acid gas, and a variable proportion of vapor of water; in addition to these it contains traces of ammonia, and, under certain conditions, a little hydrogen disulphide.

Air has been brought from the lofty Alpine heights, and from the dry, overheated plains of Egypt; it has been brought from an elevation of 21,000 feet by the aid of a balloon; it has been examined in large cities, such as London and New York, and in small, out-of-the-way country places; still, astonishing as it may seem, the proportion of oxygen and nitrogen remains pretty nearly always constant. The presence of carbonic acid gas, however, being caused by local influences, varies somewhat.

His

That the atmosphere is a purely mechanical mixture of gases, and not a chemical compound, has and can be ascertained as well by analysis as by synthesis. The former method is the one by which Lavoisier first established the composition of air. experiment, now a classic one in chemistry, was performed in the following way: A glass balloon with a long neck, as shown at a in Fig. 1, was partially filled with mercury. This was heated. The neck passed down under the surface of the mercury in an adjoining trough b and then up into a bellglass c-also full of air-whose mouth was sealed by the mercury. On raising the temperature of the mercury in a to near the boiling-point, a red powder began to