If a lift pump set in the top of the car is used, it should be a 3 or 4-inch size. With it one man can fill a 600-gallon distributor in twenty minutes. In many cases a hose or pipe is connected to the bottom of the car and run to an oil pump, operated by a steam or gasoline engine, which forces the oil into the distributor. A 1 or 2-inch power-driven rotary pump will deliver 600 gallons in ten to fifteen minutes. These pumps work with either hot or cold oil. A water tank pump can be used with cold oil but hot oil will ruin the valves speedily. A 2-inch suction tank pump will fill a 600gallon tank in thirty to forty minutes. The hose used in the connections between the pump and the bottom of the tank car should be as short as possible because the oil often destroys it rapidly. It is desirable to have a cut-off valve in the connection pipe. When everything is coupled ready for use, the discharge valve in the bottom of the car is raised by means of a vertical stem running up to the dome of the car, and the flow of oil is controlled by the cut-off valve, for the manipulation of the tank valve is quite troublesome. Heating Road Oil.-As the fixed and operating charges at an oil storage plant are about the same irrespective of the amount of oil delivered into distributors, it is desirable to load as many carts daily as practicable, in order to reduce the unit cost of such work. In California, where large amounts of oil are used in surfacing concrete roads, oil stations have been designed with particular attention to effecting such economics. It is considered desirable to have the oil at a temperature of 300° F. when it is applied, so it is heated to 325° for delivery within 10 miles and 350° for longer deliveries. The oil is discharged from the cars into storage tanks or pits holding 10,000 to 25,000 gallons. These contain steam coils to warm the oil sufficiently to enable it to be pumped into a circulating tank holding 2000 to 3000 gallons, where it is heated further by steam coils. The oil is then pumped through a heater and back into the circulating tank until its temperature is about 200°, after which the temperature of the heater is raised and the oil pumped through it into the distributor. The whole operation takes one and one-half hours. The heater resembles a return tubular boiler. The furnace has a fire brick arch and walls and is heated by oil burners. The heated gases pass over the furnace arch in a chamber formed of ordinary brick masonry, and finally escape through a steel stack. The oil is pumped through a multiple grid of 3-inch pipes. The design is made on the assumption that with furnace temperatures of 1800° to 2000°, 1 square foot of heating surface will transmit 3 British thermal units per hour per degree of change in temperature. Volume of Oil at 60°F. Equivalent to Unit Volume at Stated Temperatures in Fahrenheit Degrees 100 0.984 0.980 0.977 0.973 0.969 0.965 0.962 0.958 0.954 0.951 200 0.947 0.943 0.940 0.936 0.933 0.929 0.926 0.922 0.919 0.916 300 0.912 0.909 0.906 0.903 0.899 0.896 0.893 0.890 0.886 0.883 400 0.881 0.877 0.875 0.871 0.868 0.865 0.862 0.859 0.856 0.853 NOTE: This table is based on the assumption that the volume of oil increases 0.4 per cent for every increase of 10°F. above 60°. This rule is exactly applicable only to some oils. In Los Angeles County, Cal., the rate of increase in volume is taken at 0.3 per cent in the specifications of the county road department. Purchasing Oil.-Oil increases in volume from 0.3 to 0.4 per cent for each 10°F. rise in its temperature. The oil is bought on the basis of its volume at 60°F. and if measured at any other temperature its volume must be computed, or, in the case of an oil having an increase in volume of 0.4 per cent per 10°F., the accompanying table will give the volume at 60° with a minimum amount of figuring. To use it, multiply the tabular number for the temperature at which the measurement was made by the measured quantity of oil. For example 1,100 gallons of oil at 375°F. multiplied by 0.888 gives 978 gallons as the volume at 60°F. If the rate of increase per 10°F. was 0.3 per cent, the volume at 60°F. would be 995 gallons. TAR AND TAR PRODUCTS1 The tar used in roadbuilding is obtained by refining the crude tar produced in the destructive distillation of coal, in making enriched water gas and in certain classes of coke ovens. It is a complex mixture of many hydrocarbons and is not a simple chemical substance. In a city gashouse, gas is produced by heating coal in retorts usually about 8 feet long, 15 inches high and 18 inches wide. The tar is driven off with the gas and is collected for the most part in "hydraulic mains" which act as water seals for the gas. The gas is further cooled in a condenser, where more tar is deposited, and the remaining tar is removed in a tar extractor and scrubbers. The tar obtained at each stage in the process is different from that obtained at the other stages, but all of it is usually run into large wells, where the accompanying ammoniacal water rises and is drawn off. The character of the tar varies greatly. It is much affected by the temperature at which the coking is conducted, as well as by the character of the coal used. High temperatures result in an increase in the amount of free carbon in the tar, and this increase in free carbon is accompanied by an increase in specific gravity. The presence of ammoniacal water with oils distilling below 110°C. is stated by Prevost Hubbard to be the distinguishing features of all crude coal tars. Another class of tar is obtained from by-product coke ovens. The retorts in this case are much larger but are operated in much the same way as the retorts of illuminating gas plants, except that the main endeavor is to produce the maximum amount of coke instead of gas. For this reason the temperatures are lower than those usually employed in coal-gas works and the tar is likely to have a comparatively low amount of free carbon and a comparatively high amount of oils. There are several types of by-product coke ovens, and some produce tars better suited for road work than other types. Water gas is made by passing steam over hot coal, in which process no tar is produced. This gas is a mixture of hydrogen and carbon monoxide, and burns with a flame of no value for 1 Revised by Prevost Hubbard, chief of road materials tests and research, U. S. Office of Public Roads. illumination. It must therefore be mixed with hydrocarbons, which are usually obtained by cracking a grade of petroleum distillate called gas oil. In the purification of this enriched or "carburetted" gas, tar is obtained which is called water-gas tar. It is lighter than coal tar and the water it contains is practically free from ammonia, which is an identifying characteristic of this material. It has a comparatively high amount of heavy oil and a low amount of pitch. In some gas works both coal gas and water gas are made and the tar from both processes are collected together, resulting in mixtures which may vary greatly in composition. The crude tar is stored in tanks at the refineries, each class by itself. As much water is removed by settling as is possible, since this is the cheapest method of getting rid of it. After settling, the tar is pumped into a still. Sometimes the tars from several sources are mixed so that a product with certain characteristics can be obtained which are unattainable by refining tar from one source. The stills are set in brick like horizontal boiler shells and are heated very carefully at first to prevent the water in the tar from causing foaming. The vapors from the still are liquified in condensers. Water and light oils are first driven off, then intermediate oils and finally heavy oils. The road materials are obtained from the residuum. The distillation must be stopped early if a light road tar is desired, while the process is carried much further if a binder is desired. In the final stages, the contents of the still are agitated by jets of air to prevent coking. The composition of several crude tars and of the heavy pitches made by refining them is given in the accompanying table. The figures must not be considered more than representative of general characteristics, for individual tars in the same class vary greatly. Tar products for road purposes are called "straight-run" when they are the residuums left after refining crude tars to the degree which will furnish a material of suitable composition, and "cutback" when they are made by fluxing a hard pitch with a lighter distillate. The effect of free carbon in tar upon its utility for road purposes has been a subject of protracted controversy. Philip P. Sharples makes this comment: Experience has seemed to settle that a moderate amount of free carbon is beneficial in a road tar, thus bearing out the practical experience gained in the use of coal tar materials in other directions. At the same time, an excess of free carbon is not desirable, since it tends to make the material difficult to work and also reduces to a considerable degree the amount of true bitumen available. On the other hand, a certain percentage of free carbon seems to enhance the binding power of the refined tar. The upper 2 Semet-Solvay ovens. Concerning this type Prevost Hubbard makes the following comment: "The low percen- |