45. Bile appears as a perfectly homogeneous fluid, of a yellowish green, or sometimes of a brown colour; in consistence, it is viscid and unctuous; its taste is bitter and pungent; and its odour peculiarly faint and nauseous. It is well known that the older chemists considered the bile as an animal soap, composed of soda and a resin; and this opinion received no small degree of support from the appearance of lather, which is produced by its agitation in a phial, and from that detergent quality so well known to every scourer of cloth. But in the present advanced state of science, the chemist is little disposed to infer the composition of a body from its external characters; he submits it to the ordeal of experiment, and tortures it by the most refined operations: the happiest results have followed this rigorous process of inquiry. To the labours of Fourcroy, Thenard, and Berzelius, we are principally indebted for our knowledge of the composition of bile; and as the subject is one of deep interest to the physiologist and physician, I shall here give some account of their researches.

46. Boerhaave, by an extravagant error, regarded the bile as one of the most putrescible fluids; and hence originated many hypothetical and absurd theories on diseases and their treatment. Dr. Saunders has shewn, by a comparative experiment, that the bile of a healthy animal is far less disposed to putrefy than its blood. The bile of the ox, from the greater quantity which may be procured, has been usually the subject of experiment. The odour, colour, and taste of bile appear to reside in a resinous matter, which is solid, very bitter, and, when pure, green; but when melted, it passes to yellow. It is soluble in alcohol and in pure alkalies; and is precipitable from the former by water, and from the latter by acids. The uncombined soda in bile does not exceed 1/200 its weight; and, as this very minute quantity of alkali must be quite incapable of dissolving the large pro-10 portion of resin which exists in bile, Thenard was induced to turn his attention to the discovery of some other solvent of resin, existing as a component part in that fluid. Acetate of lead (the common sugar of lead of commerce) precipitates, he found, not only the resin, but the peculiar substance of which he was in search, in union with oxide of lead.

But an acetate, with a larger proportion of base, (formed from eight parts of sugar of lead and one of litharge,) produced a different effect, and precipitated only the albumen and the resin. When the remaining liquid was filtered, and the lead separated by means of sulphuretted hydrogen gas, it gave, on evaporation, a residue having less bitterness and considerably more sweetness. In this state the solvent of the resin could not be considered as pure, since it retained in solution a quantity of acetate of soda, arising from the decomposition, by the acetate of lead, of the salts of soda existing in the bile. He again, therefore, precipitated the solution by the acetate of lead saturated with oxide, and obtained an insoluble compound of the peculiar matter and oxide of lead. This was dissolved in vinegar, the oxide of lead separated by sulphuretted hydrogen, and the acid driven off by evaporation.

47. This substance, to which Thenard has given the name of picromel, possesses the property of rendering the resin of bile easily soluble in water. Three parts are sufficient to one of the resin. The characters of picromel are, that it is insoluble in water and alcohol, and incapable of being crystallized; that it precipitates nitrate of mercury, and acetate of lead with excess of oxide; and that it forms, with resin and a minute quantity of soda, a triple compound, which is not decomposable by acids, nor by alkaline or earthy salts. Chevallier has shown that it exists in human bile taken from the gall-bladder after death, but he could not detect it in bile discharged by vomiting 1. It has been analyzed by Dr. Thomson, who obtained products indicating five atoms of carbon + three of oxygen + one of hydrogen2.

48. Besides picromel, there exists a peculiar principle in bile, to which the name of yellow matter has been given. It is precipitated by acids. It appears to be the source of those concretions which form in the gall-bladders of oxen, and which are valuable as a pigment, on account of the extreme beauty of their colour.

49. The composition of ox-bile has been determined as follows: -

50. In analyzing the bile of other animals, a simikrity of composition was discovered, which little accords with the known diversity of their aliments; thus, that of the dog, the sheep, the cat, and the calf, was found to be precisely similar. The bile of the pig, on the contrary, contained neither yellow matter nor picro-mel. It consisted merely of resin in great quantity, of soda, and of salts, the nature of which has not been ascertained. It was entirely decomposed by acids, and even by the weakest - the acetic.

1 Ann. de Chim. et Phys. ix. 400. 2 Ann. of Phil. xiv. 70.