There are various purifying substances described by Messrs. Leeds and Butterfield, analytical chemists, as follows: -

There are three principal chemical reagents in regular use. These are chromic acid, cuprous chloride (sub- or proto-chloride of copper) and bleaching powder.

Chromic acid is employed in the form of a solution acidified with acetic or hydrochloric acid, which, in order to obtain the advantages attendant upon the use of a solid purifying material, is absorbed in the highly porous and inert silica or kieselguhr.

This substance was first recommended by Ullmann, and is termed commercially Heratol. As sold it contains about 136 grammes of chromic acid per kilogramme.

Cuprous chloride is used as a solution in strong hydrochloric acid, mixed with ferric chloride, and similarly absorbed in kieselguhr. From the name of its proposer this composition is called Frankoline.

As a special acetylene purifier bleaching powder exists in at least two chief modifications. In one, known as Acogine, it is mixed with 15 per cent. of lead chromate, and sometimes with about the same quantity of barium sulphate, the function of the latter being simply that of a diluent, while to the lead chromate is ascribed by its inventor, Wolff, the power of retaining any chlorine that may be set free from the bleaching powder by the reduction of the chromic acid.

Puratylene, as the second modification of bleaching powder, contains calcium chloride and quick or slaked lime.

It will be observed that both Heratol and Frankoline are powerfully acid, whence it follows they are capable of extracting any ammonia that may enter the purifier, but for the same reason they are liable to act corrosively upon any metallic vessel in which they are placed, and they therefore require to be kept in earthenware or enamelled receivers. But since they are not liquid the casing of the purifier is immaterial.

Puratylene also removes ammonia by virtue of the calcium chloride in it.

Acogine would probably pass the ammonia, but this is no objection, as the latter can be extracted by a preliminary washing in water.

Of all these materials Heratol is the most complete purifier of acetylene, removing phosphorus and sulphur most rapidly and thoroughly, and not appreciably diminishing in speed or efficiency until its chromic acid is practically quite used up.

On the other hand, Heratol does act upon pure acetylene to some extent, so that purifiers containing it should be small in size and frequently exchanged.

Frankoline is very efficacious as regards the phosphorus, but it does not wholly extract the sulphur. It does not attack acetylene itself.

Acogine and Puratylene, both being bleaching powders more or less, are alike in removing phosphorus to a satisfactory degree, but they leave some sulphur behind. Acogine evidently attacks acetylene to a slight extent.

Although some of these materials attack acetylene slightly and some leave sulphur in the purified gas, they may be all considered reasonably efficient from the practical point of view, for the loss of true acetylene is too small to be noticeable and the quantity of sulphur not extracted too trifling to be harmful or inconvenient.

For employment in acetylene installations a solid purifying material is generally preferable to a liquid one.

Acogine and Puratylene, although they may bo excellent for lighting installations, are, however, unsuitable for a welding plant by reason of the severe suction of the acetylene by means of the oxygen, which may cause small particles of lime to be drawn into the burner.

For practical purposes about 1 kilogramme of purifying material is sufficient for 15 to 20 cubic metres of gas, i.e., for about 50 kilogrammes of carbide.

Acetylene Generators for Welding. A. Automatic Low-pressure Oxy-acetylene Welding Plant.

A low-pressure welding plant consists of an automatic or a non-automatic acetylene generator, oxygen cylinders with regulators, one hydraulic back-pressure valve, rubber tubing for oxygen, rubber tubing for acetylene, spectacles, cylinder key, and one or more low-pressure blowpipes. The ordinary sizes of automatic acetylene plants have a container for 15, 30 and 60 lbs. of carbide, representing a supply of respectively 15, 80 and 60 cubic feet of gas.

B. Non-automatic Loiv-pressure Acetylene Plant.

This consists of one or more cast-iron generators, hydraulic main and washer, counter weighted gasholder of suitable size, and purifier.

The generation of acetylene by merely bringing calcium carbide and water into mutual contact within a suitable and closed vessel, and the great facilities offered to consumers to obtain the carbide in a condition ready for use and decomposition, is, at least from a theoretical point of view, characterised by extreme simplicity.

When, however, the question comes to select an apparatus for welding purposes great difficulties appear at once. The generator should be of the best make and principally of the right type, able to produce the gas in sufficient quantity and of the high degree of purity which is absolutely essential in producing a perfect weld. Moreover, it must not be forgotten that every maker does not possess that technical knowledge which is required to give proper advice, so often the case in a new industry, although it must freely be admitted that there are no generators made by responsible firms at the present time, which are not safe ; from this, however, it must not be deduced that every type of generators is suitable for welding purposes.

The relative advantages of automatic and non-automatic apparatus, irrespective of type, from the consumer's point of view, may be stated to be as follows : -

The fundamental idea underlying the employment of a non-automatic generator is that the whole of the calcium carbide put into the apparatus shall be decomposed into acetylene as soon after the charge is inserted as is natural in the circumstances ; so that after a very brief interval of time the generating chambers shall contain nothing but spent lime and water, and the gasometer be as full of gas as is ever desirable.