Oxygen can be bought in steel cylinders. There are two industrial processes at present for making oxygen. It can be drawn from the atmosphere by the liquid-air method or it can be produced by the electrolytic decomposition of water. This latter method to which there are many variations is largely used in Europe.2 The patents for the former method are owned and operated under, in this country, by The Linde Air Products Company.

By the Linde process the atmosphere is compressed by a double-stage pump up to 1800 pounds. This eompression raises the temperature 1 deg. Fahr. for every 2 atmospheres pressure. The compressed air is cooled by ice and salt (for small plants) or by ammonia (for large plants). If this air, then, at 1800 pounds and about 15 deg. Cent., be expanded, the temperature will drop considerably. If some is expanded on the outside of a cylinder under the same pressure, the air in the cylinder will be lowered in temperature and will liquefy because it is under pressure. Liquid air consists of approximately 80 per cent, nitrogen, critical temperature, - 149 deg. Cent.; and 20 per cent, oxygen, critical temperature, - 119 deg. Cent. The oxygen is then separated by fractional distillation similar to the rectification of spirits.

1 June, 1904. 2 "Electrolysis of Water," Victor Engelhardt, 1904.

Storage oxygen can be obtained in cylinders of sizes and weights shown in the following table:

Oxygen In Cylinders (The Linde Air Products Co.)

The oxygen is under pressure of 120 atmospheres. It is guaranteed 95 per cent, pure, the residue being nitrogen. As nitrogen is inert and not in sufficient quantity to absorb heat or retard action, its presence is negligible.

These tanks may be rented and the oxygen bought or both gas and tank bought outright. In either case the company will recharge them, subject to certain conditions. On account of the very high pressure, the cylinders are annealed at least once in four years, and are carefully inspected and labeled on charging. Each tank must have reducing valves and, when in use, pressure gauges. The railroads receive them charged as second-class merchandise, and discharged as fourth class, and also on account of the pressure.

1This size of cylinder cannot be hired.

The factor of safety is large with these tanks, but nevertheless they must be kept in a cool place. In the heat of the direct sun's rays the pressure will greatly increase.

The present price of this stored oxygen is from two and one-half to four cents, depending on the quantity bought. Rent is charged on the tanks after the first month. This is exclusive of transportation. The advantage of having a large volume of oxygen, in small bulk, when a generator is unhandy or expensive, is very apparent. The principal disadvantage is the possibility of leakage, which is a variable factor. Datum is not at hand, but it is common opinion that tanked oxygen is at least five per cent, purer than that produced by the chlorate process.