Dr. Ure, in his valuable dictionary of arts, &c., gives the following table of plants yielding fat oils of commerce :— The fat oils are contained in that part of the seed which gives birth to the coty ledons ; they are not found in the plum ula and radicle. Of all the families of plants, the cruciform is the richest in oleiferous seeds ; and next to that are the drupacem, amentacese, and solanese. The seeds of the graminem and leguminosse contain rarely more than a trace of fat oil. One root alone, that of the cyperus esculenta, contains a fat oil. The quan tity of oil furnished by seeds varies not only with the species, but in the same seed, with culture and climate. Nuts contain about half their weight of oil • tl e seeds of the brassica oleracea and canipestrie, one third; the variety called colza in France, two fifths; hempseed, one fourth ; and linseed from one fourth to one fifth. Unverdorben states that a last or ten quarters of linseed yields 40 ahms=120 gallons of oil ; which is about 1 cwt. of on per quarter.
The flit oils, when first expressed with out much heat, taste merely unctuous on the tongue, and exhale the odor of their respective plants. They appear quite neutral by litmus paper. Their fluidity is very various, some being solid at ordi nary temperatures, and others remaining fluid at the freezing point of water. Lin seed oil indeed does not congeal till cool ed from 4° to 18° below 0° F. The same kind of seed usually affords oils of diffe rent degrees of fusibility ; so that in the progress of refrigeration one portion con cretes before another. Chevreul, who was the first to observe this fact, consid ers all the oils to be composed of two species, one of which resembles suet, and was thence styled by him stearine; and another which is liquid at ordinary tem peratures, and was called elaine, or oleine. By refrigeration and pressure between the folds of blotting paper, or in linen bags, the fluid part is separated, and the solid remains. By heating the paper in water, the liquid oil may be obtained separate. When alcohol is boiled with the natural oil, the greater part of the stearine remains undissolved.
Oleine may also be procured by digest ing the oil with a quantity of caustic soda, equal to one half of what is requi site to saponify the whole ; the stearine is first transformed into soap, then a por tion of the oleine undergoes the same change, but a great part of it remains in a pure state. This process succeeds only with recently expressed or very fresh oils. The properties of these two prin ciples of the fat oils vary with the nature of the respective oils, so that the sole dif ference does not consist, as many sup pose, in tne different proportions of these two bodies, but also in peculiarities of the several stearines and oleines, which, as extracted from different seeds, solidify at very different temperatures.
In close vessels, oils may be preserved fresh for a very long time, hut with con tact of air they undergo progressive changes. Certain oils thicken and even tually dry into a transparent, yellowish, flexible substance ; which forms a skin upon the surface of the oil, and retards its further alteration. Such oils i re said to be drying or eiccative, and are used on this account in the preparation of var nishes and painter's colors. Other oils do not grow dry, though they turn thick, become less combustible, and assume an offensive smell. They are then called rancid. In this state they exhibit an acid reaction, and irritate the fa uces when swallowed, in consequence of the pre sence of a peculiar acid, which may be removed in a great measure by boiling the oil along with water and a little com mon magnesia for a quarter of an hour, or till it has lost the property of redden ing litmus. While oils undergo the above changes, they absorb a quantity of oxy gen equal to several times their volume. Saussure found that a layer of nut oil, one quarter of an inch thick, inclosed along with oxygen gas over the surface of quicksilver in the shade, absorbed only three times its bulk of that gas irJ the course of eight months; but when exposed to the sun in August, it absorb ed 60 volumes additional in the course of ten days. This absorption of oxygen diminished progressively, and stopped altogether at the end of three months, when it had amounted to 145 times the bulk of the oil. No water was generated, but 211 volumes of carbonic acid were disengaged, while the oil was transform ed in an anomalous manner into a gelati nous mass, which did not stain paper. To a like absorption we may ascribe the elevation of temperature which happens when wool or hemp, besmeared with olive or rapeseed oil, is left in a heap ; circumstances under which it has fre quently taken fire, and caused the de struction of both cloth-mills and dock yards.
In illustration of these accidents, if paper, linen, tow, wool, cotton, mats, straw, wood shavings, moss, or soot, be imbued slightly with linseed or hemp seed oil, and placed in contact with the sun and air, especially when wrapped or piled in a heap, they very soon become spontaneously hot, emit smoke, and fin ally burst into flames. If linseed oil and ground manganese be triturated together, the soft lump so formed will speedily be come firm, and ere long take fire.