shaped blocks of paraffin. The author has made use of this device, and can express a favourable opinion of it.

High-flashing distillates also find wide application as an insulating medium in transformer oils.

In 1892 Chenhall and Chenhall patented a process for the "solidification of petroleum by means of resin soap. By heating together a mixture of a suitable description of petroleum with resin and caustic soda, a solid compound was obtained which contained about 75 per cent. of petroleum and burned without liquefying.

The process patented by J. W. Leadbeater (1897) consisted in mixing together, with as little heat as possible, and in approximately the following proportions, which have to be varied according to the consistency required in the product :-petroleum, crude or otherwise (including ostatki), 128 parts; animal and vegetable stearin, 1 part; resin, 12 parts; unslaked lime, 8 parts; and sawdust, 8 parts. The water needed for the chemical reaction, which is accompanied by considerable rise of temperature when the process is carried out on a sufficiently large scale, is apparently derived from the sawdust. For the manufacture of briquettes, 5 to 10 per cent. of the petroleum-product, prepared as already described, is mixed with coal-dust or other combustible material, with the addition of an agglutinant, such as pitch.

The "solidifying" action on petroleum of a decoction of the bark of Quillaia saponaria was pointed out in 1875 by Morgan-Brown, who, in that year, patented a process based upon it. The similar employment of an extract of a lichen, such as Japanese, Chinese, or Corsican moss, was patented in 1879, and among other solidifying agents since then proposed are fish-glue and casein.

A process of "solidifying " petroleum, which was patented in this country by Dr. Thomas van der Heyden (No. 10,797 of 1904), consists in forming an emulsion of the oil with a warm solution of glue, allowing the mixture to cool in moulds, when it forms solid blocks, and then hardening the glue by means of formaldehyde.

During the year 1911 considerable publicity was given in the press to the invention by Mr. J. Tarbotton Armstrong of a process for the conversion of petroleum into a solid, suitable for use as fuel and for other purposes. In this process glue was employed with a solution of iron sulphate as a hardening agent, and five patents (Nos. 868, 869, 10,119, 18,300, and 21,059 of 1911) were granted to J. T. Armstrong and J. Morgan.

In 1894 Weygang patented in this country a process for obtaining a petroleum product miscible with, or soluble in, water, and subsequently obtained patents for his invention in Germany and the United States. The process substantially consisted in the production of a clear oleaginous liquid by the partial saponification of resin previously dissolved in petroleum, only about one-half of the resin acid present being combined with the alkali employed, and the resin soap thus formed being rendered soluble in the petroleum, in which it would not otherwise dissolve, by the presence of the unsaponified resin. From this primary product the inventor obtained (1) a "siccative oil " by treatment with a drier, (2) a turpentine substitute, (3) a "soluble solid oil " suitable for use in soap-making, (4) a "soluble semi-solid, or liquid oil" for employment as an insecticide, or as a sheep-wash, or in the dressing of wool and fibres generally, (5) "lubricating oils and creams," (6) "refined miscible oil, more or less inodorous, from which various medicinal and toilet preparations are made," and (7) " residues, some of which form the basis of colours applicable in the manufacture of printing inks." Weygang also patented the use of the soluble oil" in admixture with water as liquid fuel.

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In carrying out one form of the process patented by Boleg in 1899 for rendering mineral oils soluble in, or readily emulsifiable with, water, air was blown through a mixture of resin oil and caustic-soda solution, heated to 75° or 80° C., until the product on cooling became a jelly-like mass, and this product, together with an aqueous solution of a fatty-oil soap, was mixed with mineral lubricating oil. The product thus obtained is intended to be employed, in admixture with water, as a lubricant for machinery, and its lubricating properties were reported to have been tested in the Mechanico-Technical Laboratory at Charlottenburg, and elsewhere, with satisfactory results. Boleg's "soluble oils" are also claimed to be applicable with great advantage in the lubrication of the tools used in boring and cutting metals, in the polishing of metallic surfaces, in the oiling of wool and other fibres, in the colouring and glazing of tiles and bricks, in the oiling of leather, and in the manufacture of soaps, perfumes, disinfectants, and various pharmaceutical preparations.

In 1903 Westrum patented the use of a soluble oil," such as that of Boleg, in admixture with a large quantity of water, for the sprinkling of roads with the object of "laying "dust. Many stretches of road were treated with "Westrumite," as the product was termed, especially in connection with motor-car races, and the "laying " of the dust was found to be thus effected with greater permanence than by the employment of water, the presence of the oil apparently arresting evaporation.

Petroleum has for many years been largely introduced into soaps for household use on account of its detergent properties. It is also employed in association with soap in the manufacture of lubricating greases and insecticides.

A substitute for oil of turpentine for mixing with varnishes and paints is obtained from petroleum spirit by fractionation, and it has been found that other, less volatile, products of petroleum can also be used successfully in the preparation of paints. Petroleum distillates are sold under the names of "Patent Turpentine," (Patent 1885, No. 12,239), “ Turpenteen,” and “ English Turpentine," for use in mixing paints. Samples of these products, examined by the author, were found to resemble oil of turpentine in boiling-point, and to have flashing-points above 73° F. (Abel test).

Certain descriptions of solid bitumen, occurring in Kansas and elsewhere in the United States, dissolve completely in a suitable solvent, and the varnish thus formed, when applied to wood or metal, gives a hard, and yet sufficiently elastic, protective coating with a brilliant surface.

Petroleum spirit is largely used in the process of " dry-cleaning," and in the extraction of grease from leather. It is also employed by floor-cloth manufacturers, and as a solvent in other industries.

The domestic use of petrol as a detergent, except on a very small scale, should be discouraged. The facility with which petrol, intended only to be employed in motor-cars, can now be obtained, has led to the dangerous practice of cleaning garments in the dwelling-house with this highly volatile and inflammable liquid. Apart from the risk of fire, cases have occurred in which death by asphyxiation caused by inhaling the vapour has been narrowly escaped, and the lives of those who work in the sewers have been endangered by explosions ascribed to petrol, which had been used for the purpose referred to, having been got rid of by pouring it into the house-drains. Another dangerous and sometimes fatal practice is the use of petroleum spirit for cleansing the hair.

A process for the purification of alcohol by means of hydrocarbons was described by Dr. Squire in a paper read before the Society of Chemical Industry.1 1 Journ. Soc. Chem. Ind., viii, 441 (1889).

Oils and fats possess the property of absorbing the more or less volatile bodies which impart a characteristic odour to substances, and this property has been taken advantage of in the present process. Parsons, of New York, in 1869, took out a patent for the purification of alcohol-90 per cent. at least-with paraffin, the mixture being heated by means of a steam-coil with continued agitation. Warm water was then introduced so as to bring the mixture down to about 50 per cent. of alcohol, and the whole allowed to cool, with continued agitation. The paraffin, which separated in flakes, was then filtered off from the purified spirit. In 1884 the subject was taken up by a Danish chemist named Bang, who became associated with M. Ruffin, of Paris. He pointed out that the fusel oil could be more effectively separated if dilute spirit were employed, and proposed the use of a volatile liquid product of petroleum as the purifying agent. Ultimately, however, a petroleum product of higher boiling-point was found preferable. The details of the apparatus employed in carrying out the process on a commercial scale, and of a modified form devised in 1887 by Bowick, are given in the paper above referred to. The process was investigated in 1889 by Messrs. Newlands Brothers, to whom the author is indebted for the information that ordinary petroleum having a boiling-point of 140° C. was used, and that the quality of the crude spirit (manufactured from molasses) was found to be very much improved by the treatment.

SWITCH AND TRANSFORMER OILS.1

The operation of any modern power-station of even moderate size is, apart from the lubrication of machinery, dependent upon the utilisation of mineral oil of high quality, primarily as an electrical insulator and conductor of heat in static transformers, and secondarily for the purpose of quenching arcs in high-tension switch-gear.

While it is entirely feasible to design air-cooled transformers for working pressures up to, say, 10,000 volts, consideration of space and the need to improve and maintain a high-insulation value have led to the supersession of the air-cooled transformer. At the time of writing, the best modern practice is to use oil-cooled transformers even for such small transformers as those of 5 kilowatts capacity on 440- and 500-volt alternating current circuits.

Oil-Cooling. The practice of different designers varies greatly as to the volume of oil used in any transformer of any given size, and is dependent upon (1) the heat-radiating surface from the core plates and coils to the oil; (2) the heat-radiating surface of the transformer to the surrounding air, and (3) the efficiency of natural circulation of the oil within the transformer case.

Both British and American makers have often supplemented the cooling due to heat radiation from the transformer case by using cooling coils placed at the top of the oil through which water is pumped. A better practice was that more largely used by Continental makers of cooling the oil externally by pumping the oil through pipes immersed in water. The circulation within the transformer can be improved, temperature can be closely controlled and the volume of oil in use reduced.

1 Contributed by Mr. W. Pollard Digby.

The following articles and papers should be consulted :-" Physical Properties of Switch and Transformer Oils," W. Pollard Digby and D. B. Mellis (Journ. Inst. Elec. Engs., 1910, xlv, 165); "The Formation of Deposits in Oil-Cooled Transformers," A. C. Michie (loc. cit., 1913, li, 222); "Reports International Congress of Applied Chemistry, 1913" (see Journ. Soc. Chem. Ind., 1913, xxxii, 62–72); “Research Committee Reports, Institution of Electrical Engineers" (Journ. Inst. Elec. Engs., 1915, liii, 146, and 1916, liv, 497).

Initially in using oil as a cooling and insulating medium, the only factors receiving attention were (1) flash-point, (2) viscosity, (3) dielectric strength. Experience has shown the need of specifying other properties, and standard specifications with the general adoption of standardised methods of testing may be looked for as the result of the work of the Insulating Oils Research Committee of the Institution of Electrical Engineers. As instancing the need for such a research, mention may be made of the need to define the rate of thermal transference in unit quantity in unit time. The rate of thermal transference is not in practice the equivalent of thermal conductivity on account of the variation in the viscosity with the temperature of the oil and also of the effect of convection currents within the oil when used in a transformer.

Flash-Point.—It is very rare, indeed, for it to be necessary to seek an oil having a higher flash-point than 175° to 180° C. as measured in the PenskyMarten apparatus.

Colourless mineral oils having a flash-point as low as 125° to 130° C. have been extensively used in North America, but are not suitable for use in tropical countries.

As a general rule, the flash-point should be from 60° to 70° C. above the working temperature of the transformer room plus the rise in temperature of the oil when the transformer is at full load and the rate of radiation of heat exactly balances the heat given to the oil.

Temperature measurements in oil-cooled transformers are very often misleading except where thermo-couples are used intelligently. Measurements calculated from the increase of electrical resistance of the windings give a mean value only part of the windings are in cool oil at the bottom of the tanks, part in hotter oil at the top of the tanks. What is of vital consequence is the temperature which may be reached at certain hot spots where the natural circulation of the oil is either impeded or stopped altogether. The alternative workshop conventional practice of measuring the temperature of the oil at the top of the transformer and describing that as the maximum temperature rise under any given conditions of load is scientifically unsound, as obviously thin films of the oil have been in contact with surfaces hotter than the oil at the surface where it is mixed with oil from cooler portions.

Viscosity and Specific Gravity.-Except as indicating the source (when stated in conjunction with the other physical properties) it is not necessary, although usual, to specify these. Given equality in other respects, oils having low viscosity are preferable for transformers, but in considering this factor regard should be given to the viscosity-temperature curves. Oils differing widely at atmospheric temperatures have closely similar values at ordinary working temperatures.

Sludge. The expression "sludging" is used to describe the tendency of certain insulating oils to form solid deposits due to the combined action of heat and air. Such deposits adhere to the windings and retard the circulation, thus leading to overheating. Dr. A. C. Michie has shown that with any given volume of air (or oxygen) passed through, the tendency to "sludge " is very largely increased by the presence of bare copper acting catalytically.

In general, the darker the oil the greater the tendency to form sludge; relative change of colour on heating is a rough indication of the value of an oil in this respect.

Dryness of Oil.-An oil containing traces of moisture is fatal to the operation of any high-tension transformer as occasioning short circuits between the windings. Chemical tests with desiccated copper sulphate are qualitative rather than quantitative. Some makers are content with a high dielectric

test figure. This only discriminates to a small degree, and closer indications, although only inferentially quantitative, can be obtained by measurements of specific resistance. When an oil is found to be wet, drying at 90° to 95° C. in vacuo is the only safe remedy. Drying at higher temperatures in air, as is sometimes done, is to be strongly deprecated as tending to cause alterations to specific gravity and viscosity through fractional evaporation.

Dielectric Strength and Specific Resistance. The breakdown voltage of a dielectric strength test is regarded as an indication of the amount of moisture present in the oil. The methods of conducting this test are being standardised, but the results need study in connection with quantitative determinations of the amount of moisture present. The relation between specific resistance and dielectric strength requires study. It seems probable that the specific resistance tests will be a better indication of the dryness of the oil than dielectric strength test.

The leading properties of a number of transformer oils in common use prior to and since 1914 are set out below:

A sub-committee of the Institution of Electrical Engineers is dealing with the whole subject of the properties of switch and transformer oils, and is engaged on the standardisation of methods of testing, both physical and chemical. References to the preliminary report and recommendations of the sub-committee are given on p. 887.

MINERAL-OIL LAMPS.

The various forms of lamps in which mineral oils are burned generally resemble those previously in use with fixed oils, but certain modifications have been rendered necessary by the comparative volatility and inflammability of mineral oils, and the high percentage of carbon which they contain. Thus it is necessary to guard against any considerable heating of the bulk of the oil, and especially against the ignition of any inflammable or explosive mixture of petroleum-vapour and air which may be formed in the upper part of the lamp reservoir, whilst at the same time the illuminant must be continuously supplied in proper quantities to the flame, and the air admitted in such a manner as to produce a smokeless and odourless flame of high illuminating power. Special appliances are also usually provided to facilitate the lighting

1 At 15° C. compared with rape oil as 100.