In an automatic apparatus the fundamental idea is that the generating chamber, or one at least of several generating chambers, shall always contain a considerable quantity of undecomposed carbide, and some receptacle always containing a store of water ready to attack that carbide, so that whenever a demand for gas shall arise everything may be ready to meet it.

Inasmuch as acetylene is an inflammable gas, it possesses all the properties characteristic of inflammable gases in general; one of which is that it is always liable to take fire in presence of a spark or naked light, and another of which is that it is always liable to become highly explosive in presence of a naked light or spark if, accidentally or otherwise, it becomes mixed with more than a certain proportion of air. On the contrary, in the complete absence of liquid or vaporised water, calcium carbide is almost as inert a body as it is possible to imagine, for it will not take fire, and cannot in any circumstances be made to explode. Hence it may be urged that a non-automatic generator, with its gasometer always containing a large volume of the actually inflammable and potentially explosive acetylene, must invariably be more dangerous than an automatic apparatus, which has less or practically no ready-made gas in it, and which simply contains water in one chamber and unaltered calcium carbide in another. But when the generating vessels and the gasometer of a non-automatic apparatus are properly designed, the gas in the

• latter is acetylene practically free from air, and therefore, while being inflammable it is devoid of explosive properties, always assuming that the temperature of the gas is below 280° C. and that the pressure under which the gas is stored remains less than two atmospheres.

It may be well to remember that not only must calcium carbide and water be kept out of premature contact, but that moisture, or vapour of water, must not be allowed to reach the carbide; or alternatively, that, if water vapour reaches the carbide too soon, the undesired reaction shall not determine overheating, and the liberated gas be not wasted or permitted to become a source of danger.

The evolution of the gas must be slow and regulated so that an apparatus, for instance, with a storage of 17 kilogrammes of carbide, representing a capacity of 17 X 300 = 5,100 litres of gas, can be used during ten hours with a consumption of 500 litres of gas per hour ; a similar apparatus with a storage of 50 kilogrammes of carbide with a consumption of 1,500 litres of gas per hour, and an apparatus with 100 kilogrammes storage of carbide with a consumption of 3,000 litres of gas per hour.

For a consumption larger than 3,000 litres of gas per hour, the non-automatic apparatus must be employed.

It is difficult, and it may not even be advisable, to give a direct answer to the question as to which is the best type of acetylene generators. Experience has, however, proved, so far as welding is concerned, that the apparatus in which granulated carbide is used, the generated gas is of too impure a quality to be used for welding, producing, as it will, a brittle weld.

The type in which the water is allowed to drip on to the carbide should on no conditions be used, as being the most unsuitable of all.

The carbide-to-water generator, by reason of its many advantages from a theoretical point of view, has, also for practical purposes, proved to be the best, producing, as it will, a perfect weld.

When deciding the size of the apparatus to be selected, it should be borne in mind that as little as, for instance, a dynamo can produce an unlimited quantity of energy without causing harm to the machine, as little can an acetylene apparatus generate an unlimited quantity of gas. The quicker the evolution, the more impure is the gas and the more unsatisfactory will the weld be.

A preliminary estimate should therefore be carefully made of the quantity of gas that would probably be required.

Assuming, for instance, that 100 metres of plate, with a thickness of 8 m.m. shall be welded per day, four welders would be required, each with a blowpipe consuming 650 litres of gas per hour, making for the four blowpipes 2,700 litres of gas per hour. It would be necessary, therefore, to provide an apparatus capable of generating 3,000 litres of acetyleneper hour.

The weight of calcium carbide required to produce a certain quantity of gas may be estimated from the formula : Carbide in kilogrammes =consumption per hour in litres X 10 / production gas per kilogramme carbide , or

3,000 X 16 / 300 = 100 kilogrammes of carbide would be required.

As each kilogramme of carbide requires at least 7.5 litres of water, the gas generator should be provided with a space large enough to hold at least 750 litres of water.

It is indifferent, however, how the generated gas is consumed, if by ten welders, each with 150 litres, or by one welder with 1,200, and two welders with 150 litres each per hour, but the consumption per hour must be provided by acetylene generated in a normal manner.

The decomposition of the carbide in the water requires ample time. It may, however, be possible to provide additional quantities of carbide, but then the temperature would also increase accordingly, even up to 675° C. In addition to this the acetylene requires considerable time to cool, and only a certain quantity can pass through the chemical purifier to enable a normal and effective work to be done. The gas produced at too high a temperature would not only increase the danger of explosion but the products of condensation would tend to make the gas impure. The higher the temperature the greater is the quantity of sulphur and organic sulphur compounds. The more rapid the generation, the greater is the impurity of the gas, and the more unsatisfactory will the weld be.

Some automatic apparatus are being offered in the foreign market for a production of 4,000 and even 12,000 litres of acetylene per hour; they contain, however, only 8 and 25 kilogrammes of carbide, and should therefore be used for a consumption of 250 and 750 litres per hour respectively.

Another apparatus with an indicated generating power of 25,000 litres of gas per hour is likewise being offered. This apparatus should therefore have a storage capacity for 800 kilogrammes of carbide and 6,000 litres of water. In reality it contains 40 kilogrammes of carbide.

The effective size and not the indicated production should constitute the regulator of price of the apparatus.

It is easily understood the harm that may be done by offering acetylene generators of too small a size in order to facilitate a sale. The inevitable results will appear in increased danger of explosion and an imperfect weld, followed by suspicion and consequent delay in the development of this new, but so important, industry.