Acetylene dissociates at 780 deg. Cent, into carbon and hydrogen; under pressure of two atmospheres or more, the gas is tricky and is liable to explode. But acetylene is readily soluble in a number of liquids, among them acetone. Acetone is fairly cheap, inert, and incombustible - very essential properties. It boils at 56 deg. Cent., has a strong affinity for acetylene, and is not decomposed by it. At atmospheric pressure and 15 deg. Cent, acetone dissolves 24 times its volume of acetylene. At 12 atmospheres, which is the pressure given the storage cylinders, it dissolves about 300 volumes of the gas and increases in volume 50 per cent. The pressure of such a tank is doubled with every rise of 30 deg. Cent., while undissolved acetylene triples its pressure for each rise of 8 deg. Cent.

Berthelot and Vielle1 experimented with the solution of acetylene in acetone and found that it could not be exploded with an electric spark though under high pressure. Hutton2 says that, "In practice 1,000 liters of acetylene carry off the vapor of 0.06 liter of liquid acetone" - not an appreciable amount.

All of the above characteristics of the solution recommend acetone as a solvent or body for the storage of acetylene for commerce. The French government was the first to officially recognize acetone storage tanks as safe. The railroads of this country now accept the cylinders for carriage as non-explosive.

Acetone storage was worked out by the Belgian chemists Claude and Hesse, and patented by them in 1897. The principal difficulty to overcome was the factor of expansion of the solution with increased acetylene content, and the corresponding shrinking as the acetylene was drawn out of the tank. The cylinder would be full at 12 atmospheres and only two-thirds full under normal pressure. This meant that a considerable part of the tank would contain the gas alone, subject to the danger of explosion. To overcome this, the cylinder was filled with a porous or absorbent body, which was saturated with the acetone-acetylene solution. Porous brick or stoneware of four-fifths porosity was used; also charcoal cake, bound together with soluble glass; in this country asbestos fiber with soluble glass binder is used. These absorbents will all carry from 50 to 80 per cent, of the solution per volume. When the acetylene is all drawn off, the tank is still perfectly safe and can be recharged simply by passing in acetylene under pressure.

The tanks themselves are pressed-steel cylinders, such as are used for soda-water, and are fitted with cocks and a pressure-regulating valve. They are delivered under 10 atmospheres' pressure, and contain about 100 volumes of the gas - considerably below the saturation content of acetone at that pressure. They should be kept in a cool place, out of the sunlight, because the pressure doubles with 30 deg. rise of temperature. If exposed to too great heat, the pressure might rise to the danger point, and an explosion take place.

1 Elec. and Metal. Industry, March, 1903.

2 R. S. Hutton, Elec. and Metal. Industry, April, 1903.

Acetylene storage tanks are of the following size and capacity:

Acetylene Storage Tanks1

Carbid will produce about 4 cubic feet of acetylene per pound; the present price is below four cents a pound. This brings the material cost to about one cent per cubic foot of gas. Stored acetylene costs about twice as much, but its adaptability is much greater and in many cases much more than nullifies the difference. It is claimed to be the purest form of the gas, being practically free from sulphur and phosphorus, because it receives four to six washings.

Practice

The directions for the use of the oxy-acetylene flame are few and the process is simple, but it takes a skilled workman to get results. Six months' practice is none too long before efficiency can be looked for. It is one thing to melt metals together and quite another to make a weld of homogeneous metal in which strains are at a minimum. The companies print and issue directions, which will be here abstracted and added to. Beyond that it is a question of individual gray matter.