42 ACIDS.
part of the body with comparative impunity. In one instance, a man employed at a carbolic acid works, wbo often had his entire arms covered with the acid, died from the effects of some of the same acid spilt on his leg.
Associated with the real carbolic acid in the lower grades of the commercial article, there is always more or less of another body known as cresol or cresylic acid, which has the composition and is consequently the second member of the homologous series of which carbolic acid is the first.
Cresylio acid has a density of about 1.044. It closely resembles carbolic acid, but is liquid at ordinary temperatures, fax leas soluble in water than is carbolic acid, and boils at a higher tempera ture (198 to 203°). Several isomeric varieties of cresylic acid are known, having slightly different boiling points. One variety is solid at ordinary temperatures.
The presence of cresylic acid in samples of commercial carbolic acid greatly reduces their solu bility in water and alkaline solutions, and lowers the melting point. Hence, none except the very best commercial samples of carbolic acid correspond exactly in properties with the pure substance. The lower grades of "carbolic acid," from which much of the pure substance bas been separated, contain a fax larger percentage of cresylic acid than of real carbolic acid. In taste and smell they are far more offensive than the pure article, but their antiseptic value is in no way diminished. In fact, cresylic acid is said to have an antiseptic power considerably above that of carbolic.
Carbolic acid is a constituent of the pharmaceutical preparation called castoreum, and is formed in many chemical reactions. It has also been observed in the tar obtained by the distillation of pine-wood. In practice, however, the whole of the carbolic acid of commerce is obtained from tho tar produced by the distillation of coal for the manufacture of illuminating gas.
Coal—or gas—tar is of a very complex composition, and yields a variety of useful products. The carbolic and cresylic acids constitute a not inconsiderable proportion of the whole tar. It is obtained by a process of fractional distillation, supplemented by purification by solution in alkali, reprecipitation by acid, &c.
The quantity of tar operated on at ono time, and the exact method of procedure, are conditions which vary considerably, but the following description gives a fair idea of the operation of fractional distillation as usually carried out at large works.
The first thing in the morning the still is charged with 1200 gallons of the tar, and the fire is lighted; in about an hour the tar begins to rise in the still, and the fire requires easeful watching, until the actual distillation commences, which is usually about an hour later.
The first distillate consists of gases, ammonia water, and "first light oils," or naphtha ; when 60 or 70 gallons have condensed, consisting of about naphtha and I water, the distillation almost stops for a time, generally about an hour, or an honr and a half, during which time little but water passes over. The stoppage of the distillation is known as the " break," and during the subsequent period the still is said to be " on the rattles," owing to the peculiar noise with which the generation of steam is accompanied. When the temperature has risen somewhat higher, the still "comes off the rattles" and yields about 20 gallons of oil lighter than water (" secoad light oils "). The point at which the distillate begins to be heavier than water is found by observing whether in a small sample the oily or aqueous portion floats on the surface. As soon as the oil becomes heavier than water, the shoots are changed and the still is said to be "on the oil." In the course of the next three hours about 300 gallons are distilled. The first portion of this consists chiefly of naphthalene, which is very apt to block up the worm. After about 150 gallons have passed over, the nature of the product changes, the naphthalene being readily soluble in the oils then running. The still is now said to be "on the soft oils," of which about 100 gallons are obtained. After this, the oil begins to thicken on cooling, the approach to this point being ascertained by catching a few drops of the oil on a piece of cold iron. The product now running is known as "anthracene oils," and measures somo 50 gallons. When the oil sets to the consistence of butter on cooling, the process is arrested, and the remaining 4 tons of pitch run out into a tank.