Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/799

Rh I L I L 739 gradients, but still an assumption, viz., that, all else being the same, the flux of heat is strictly proportional to the gradient of temperature. An exactly similar assumption is made in the statement of Ohm s law, i.e., that, other things being alike, the strength of the current is at each point proportional to the gradient of potential. It happens, however, that with our modern methods it is much more easy to test the accuracy of the assumption in the case of electricity than in that of heat ; and it has accordingly been shown by Maxwell and Chrystal that Ohm s law is true, within the limits of experimental error, even when the currents are so powerful as almost to fuse the conduct ing wire. The value of Ohm s work was but imperfectly recognized until it was stamped by the award of the Cop ley medal of the Royal Society in 1841. OIL-CAKE. The solid compressed mass remaining after the expression of oil from the many oil-yielding seeds, nuts, &c., forms a material of considerable commercial importance. It retains after the most perfect treatment by pressure no inconsiderable portion of oil, with practically the whole of the albuminous matter, sugar, mucilage, and the starchy components of the seeds, &c., thus forming in most cases a concentrated nutritious food for cattle and sheep, specially valuable for fattening store animals for the market. The only commercial cakes which are unfit for animal food are such as contain purgative or other active principles, as, for example, the cakes of castor-oil seed, croton seed, purging- nut seed, and mustard seed. The most valuable and at the same time the most abundantly produced feeding-cake is that obtained from ordinary LINSEED (q.v.). Next in value is the cake yielded by rape seed, but its wholesomeness is frequently marred by the presence of a large proportion of acrid mustard seed ; the best is that yielded by the German green rape seed (Brassica rapa oleifera). Cotton-seed cake is also a feeding stuff of considerable importance. It is prepared in two forms, either as decorticated cake, in which the husks of the cotton seed are removed previous to the expression of the oil, or as undecorticated cake, which may contain as much as 40 to 50 per cent, of indigestible woody husk. Among other cakes useful for feeding purposes may be enumerated ground-nut cake from Arachis hypogsea, palm -kernel cake from the seeds of the oil palm, sesame or til cake from the seed of Sesa- mum orientate, and hemp cake from the seed of Cannabis sativa. The following table indicates the average composi tion of a few of the principal commercial cakes. English Lin seed Cake. German Green Rape Cake. Decorticated Cotton Cake. Undecorticated Cotton Cake. Sesame Cake. Hemp Cake. Palm-Kernel Cake. Water 12-41 10-82 9-28 11-46 8-06 10-00 9-50 Oil. 15-64 8-72 16-05 6-07 11-34 8-26 8-43 Albuminous bodies * Mucilage, sugar, and digest ible fibre 27-87 23-79 33-81 28-05 41-25 16-45 22-94 32-52 36-87 25-05 21-50 48-00 ) 30-40 14-85 11-49 8-92 20-99 8-14 .. r Mineral matter 5-44 7-10 8-05 6-02 10-54 12-24 10-72 &quot;Containing nitrogen 4 -40 5-41 6 -58 3-67 5-90 3-30 4-50 Oil-cake should be thoroughly dried by exposure before being packed or stored, otherwise it is apt to heat, and turn sour and mouldy, in which condition it becomes injurious to cattle. The cake also made from rancid seed is fre quently deleterious, and so disagreeable in taste as to be refused by animals. Oil-cakes of high value are subject to adulteration ; and compounds of varying composition are prepared and sold under commercial brands simply as feeding cake or mixed cake with no indication of their component materials. These are usually largely intermixed with bran, husks, ground rice, and maize sif tings, &c. OIL CITY, a city of the United States, in Venango county, Pennsylvania, at the junction of Oil Creek with the Allegheny river, lies 132 miles north-north-east of Pitts- burg by the Allegheny Valley Railroad, in the heart of the Venango petroleum district, and possesses the principal oil exchange of the world, the transactions in 1883 amounting to 1,129,199,500 barrels, valued at $1,209,654,064. The business part of the city is on the low ground north of the river, the residences occupying the south side and the high bluffs on the north side. Besides manufactories con nected with the oil industry, the city has machine-shops, foundries, flour-mills, and breweries. Founded in 1860, and partially destroyed by flood in 1865 and by flood and fire in 1866, Oil City was incorporated as a city in 1871. Its population was 7315 in 1880, and now (1884) is esti mated at 10,000. OILS. The term oil is a generic expression under which are included several extensive series of bodies of diverse chemical character and physical properties. In its most comprehensive ordinary acceptation the word embraces the hard solid odourless waxes, tallows, and fats, the viscid fluid fixed oils, the odorous essential oils, and the solid, fluid, and volatile hydrocarbons obtained in nature or by destructive distillation. Further, in former days, when substances were principally classified by obvious physical characteristics, the word applied to various substances which, beyond an oily consistency, possess no other pro perties in common with ordinary oils. Thus we have still in common use for sulphuric acid the term &quot;oil of vitriol,&quot; a substance which, it need hardly be said, is widely different from any oil. Leaving out of account bodies of this nature, the remaining diverse bodies have in common the char acters that they are compounds consisting principally, in some cases exclusively, of carbon and hydrogen, that they are mostly insoluble in water, and that they are all readily inflammable. The mineral hydrocarbons obtained either in nature or by destructive distillation do not come within the range of this article (see NAPHTHA, PARAFFIN, PETRO LEUM), which is restricted to the series of neutral bodies formed naturally within animal or vegetable organisms. These bodies are divided into two well-defined groups the fixed oils and fats, and the essential or volatile oils. FIXED OILS. The fixed or fatty oils, although varying considerably in external appearance, form in reality a well-defined and homogeneous group of substances having great similarity of chemical composition. They appear to be essential con stituents of the most highly -organized forms of animal and vegetable life, being found in plants chiefly in the seed, and in animals chiefly enclosed in the cellular tissue and in special body cavities, but some proportion of fatty matter is found in almost all tissues and organs. Although oils and fats are universally distributed and perform most important functions in animal and vegetable life, those used for technical purposes are not drawn from any very great number of sources ; and many bodies might be uti lized for the production of oil which at present are not so employed. As found in commerce, oils possess a faint characteristic taste, a slight odour, and some amount of colour, generally brownish yellow. These characteristics, however, are due to certain impurities ; in a really pure condition most oils have scarcely any characteristic taste, odour, colour, or physiological influence. In a few cases only they have special properties which appear to be inseparable char acteristics, such as the purgative principle of castor oil, croton oil, and some others. At the ordinary tempera tures most vegetable oils are fluid, but a few, produced especially by tropical plants, such as palm oil, cocoa butter,