Petroleum belongs to the so-called bitumen fam ily, which is made up of compounds of carbon and hydrogen, occurring either as gases, fluids, or sol ids, and all the result of much the same process of formation. Natural gas is the only important gaseous member of this group. Petroleum is, of course, the most important of the fluid bitumens, but the viscous earth tar, mineral tar, or maltha, as it is variously called, is also a valuable product for certain uses. A clear, waterlike, liquid form, passing under such names as rock oil and naphtha, is found occasionally in small quantities, the ordi nary petroleum being always more or less colored. Among the solid bitumens, asphalt is at once the most familiar and most valuable, while the mineral pitch and mineral wax, or ozocerite, are of fairly frequent occurrence.
The fluid petroleum may be regarded as the pivot in the group, since, under varying condi tions, it will give practically every one of the different forms named. Natural gas in varying amounts is always found with petroleum, and is undoubtedly a product of its formation in under ground processes. The relation of petroleum to the semisolid and solid varieties, on the other hand, depends largely on the presence of air. When petroleum is exposed to the atmosphere, evapora tion and combination with oxygen gradually pro duce a concentration of the solid substances, with some chemical changes, resulting in the formation of tar, pitch, wax, or asphalt, as the case may be. Thus, many surface deposits of these solid bitu mens which occur in different parts of the world are regarded as the result of slow exudation of petroleum from near-by subterranean accumula tions.
Petroleum, the most abundant and also most valuable of the bitumens, varies greatly in char acter, not only from one country to another, but also in deposits relatively close together. It is al ways an oily liquid, sometimes thin and at others thick and viscous. The color ranges from water white, in a few rare deposits, through straw color, amber, and yellow, to a dirty brown or almost black, the darker colors being the more common.
The odor is occasionally not especially unpleasant, but more often it is highly offensive, decidedly penetrating and persistent, and, in many respects, the odor has been the worst obstacle to overcome in introducing petroleum products. The pioneer oil business, in New York City, for example, orig inally established at 184 Water Street, in 1857, was soon forced to move because the neighboring mer chants complained so bitterly about the intoler able odor. To anyone who has ever encountered any of the so-called " stinking oils," such a condi tion of affairs is not surprising.
The most important physical character of petro leum, however, is its volatility, or its habit of pass ing partly into a vapor or gas when heated or when exposed to the air. It is this property which
causes petroleum to become gradually denser and denser until it reaches the viscous or solid state after continued exposure. On the volatility de pends the whole success of refining, in which the crude petroleum is broken up or separated into the well-known products. Refining is the real key to the modern importance of petroleum, for with out such treatment the ordinary crude oil has no great commercial value except as fuel. This same property also makes petroleum a dangerous com pound, since the gases derived from the crude oil are very inflammable even by themselves, and when mixed with air form an extremely explosive combination. It is this character which makes it necessary to store oil in tight vessels to prevent loss by evaporation, while, at the same time, great precaution must be taken in guarding against fire near the wells and storage tanks.
Whatever their outward appearances, petrole ums are always compounds of carbon and hydro gen, often with minor impurities giving an individ ual character to some particular deposit. The amount of carbon varies, as a rule, between 80 and 90 per cent, while the hydrogen makes up only about 10 to 15 per cent of the total. The most com mon impurities are sulphur and nitrogen, while compounds of sulphur with either carbon or hydro gen, as well as oxygen, arsenic, and phosphorous are occasionally found. Fortunately none of these impurities are ever present in very large amounts —sulphur, the most common, rarely exceeding 3 per cent; yet even this small amount of sulphur, for example, seriously impairs the value of the crude oil, by making necessary rather expensive purifying processes.
On the basis of chemical composition and char acter, oil men speak of petroleum as paraffin or asphalt oil, from the nature of the solids obtained in refining, or as light or heavy oil, the relative weights being expressed in degrees of the so-called Baume hydrometer scale. Thus, a heavy oil would be described as 20° Baume, or simply 20° B., while a light oil would range around 60° B., or higher. On the same scale water, the standard, ranks as 10° B., so that the oils standing high on the Baume scale are very much lighter than water, while the heavier oils in some cases quite closely approach the density of water. As a general rule the lighter oils belong to the paraffin class and are more val uable, since they yield a larger proportion and a better quality of the higher-priced products in dig filiation. For special purposes, such as for fuel, however, or for the manufacture of lubricating oils, petroleum of lower or intermediate densities may be superior.