As the industry has grown and the use of various gases has developed, the transportation of these gases has become an important factor. When gases first were employed commercially they were transported in large metal containers under relatively small pressure. As the demand for gas increased, however, it was found that the old type of container did not permit of transportation in sufficient quantities because of the limitations placed upon the size of con tainers by the necessity of convenient handling. To overcome these difficulties the containers were made stronger and smaller and the quan tity of gas in an individual container, or cylinder . as it is termed in trade circles, was increased by subjecting the gas to a great increase of pressure. In this connection one must under stand that as the pressure on the gas increases its volume decreases in certain proportions which vary with different gases. By increasing the pressure on the gas to a certain point and at the same time reducing the temperature the gas may be changed to liquid form and many gases are shipped in the form of liquids but on being released from the presure in the con tainer they again assume the gaseous form desirable for utilization. It is because of this fact that these gases are termed commercially liquefied or compressed gases as distinguished from other gases like illuminating, gas which are employed generally at approximately at mosph:ric pressure.
With the change in the character of con tainers there grew up the hazard from breakage of a container and the sudden expansion of the compressed gas by its release from pressure. In consequence the Federal and State authori ties found it necessary for the safety of the public in devise rules and regulations governing the character of containers which would be permitted for certain gases and rules for their handling while in transit. This necessity has
resulted in legislation governing the transporta tion, storage and employment of gases under pressure. The chief agency having charge of these matters and the one which has shaped in large measure the legislation in this field is the Bureau of Explosives.
With the development of the industry there came also the need for co-ordination between manufacturers of compressed and liquefied gases and of equipment used in connection there with. In consequence there have been organized several trade organizations having for their object the promotion of uniform and proper legislation governing compressed and liquefied gases, as well as the commercial application of the gases generally. In most works of reference the treatises on gases have been separate, arranged alphabetically in differ ent parts of the work. However, it has been deemed advisable in this article to deal under one title with all gases which in a compressed or liquefied state have a commercial use be cause this industry represents to-day a separate and distinct field, and because there is much about the employment, transportation and use of every compressed gas which is common to all. The foregoing short history of the commercial use of gases is followed immediately by separate treatises, alphabetically arranged, which deal at length with all the more important liquefied or compressed gases.