On the Milk of the Cow Tree, and on vegetable Milk in General. By M. Humboldt. Abridged from an essay in Ann. de Chim. for Feb. 1818, which is an extract from a memoir read to the Academy of Sciences. M. Humboldt and his companions, in the course of their travels, heard an account of a tree which grows in the valleys of Aragua, the juice of which is a nourishing milk, and which, from that circumstance, has received the name of the cow tree. As the milky juices of plants are in general acrid, bitter, and even poisonous, M. Humboldt was at first scarcely disposed to credit the account, but experience proved it to be correct. The tree in its general aspect resembles the chrysophyllum cainito; its leaves are oblong, pointed, leathery, and alternate, marked with lateral veins, projecting downwards, they are parallel, and are ten inches long. M. Humboldt had no opportunity of seeing the flower; the fruit is somewhat fleshy, and contains one or sometimes two nuts. When incisions are made into the trunk, it discharges abundantly a glutinous milk, moderately thick, without any acridness, and exhaling an agreeable balsamic odour. The travellers drank considerable quantities of it without experiencing any injurious effects; its viscidity only rendering it rather unpleasant. The superintendent of the plantation assured them that the negroes acquire flesh during the season in which the cow-tree yields the greatest quantity of milk. When this fluid is exposed to the air, perhaps, in consequence of the absorption of the oxygen of the atmosphere, its surface becomes covered with membranes of a substance that appears to be of a decided animal nature, yellowish, thready, and of a cheesy consistence. These membranes, when separated from the more aqueous part of the fluid, are almost as elastic as caoutchouc; but at the same time they are as much disposed to become putrid as gelatine. The natives give the name of cheese to the coagulum, which is separated by the contact of the air; in the course of five or six days it becomes sour. The milk, kept for some time in a corked phial, had deposited a little coagulum, and still exhaled its balsamic odour. If the recent juice be mixed with cold water, the coagulum is formed in small quantity only; but the separation of the viscid membranes occurs when it is placed in contact with nitric acid. This remarkable tree seems to be peculiar to the Cordilliere du Littoral, especially from Barbula to the lake of Maracaybo. There are likewise some traces of it near the village of San Mateo; and, according to the account of M. Bredmeyer, in the valley of Caucagua, three days' journey to the east of the Caraccas. This naturalist has likewise described the vegetable milk of the cow tree as possessing an agreeable flavour and an aromatic odour: the natives of Caucagua call it the milk tree. M. Humboldt offers some general observations upon the milky juices of plants, and concludes with some particular observations upon the fluid which is procured from the carica papaya; this has been analyzed by M. Vauquelin; but the specimen which he examined had had its properties altered by having been conveyed to a great distance, and kept for a long time. Ann. de Chim. xliii. 267. The younger is the fruit of the papaw, the more milk does it yield; in proportion as the fruit ripens, the milk, which is less abundant, becomes more watery: there is then less of that animal matter which is coagulable by acids and by the absorption of oxygen. When nitric acid is poured drop by drop into the milky juice procured from a very young fruit, a very extraordinary phenomenon is observed. In the centre of each drop there is formed a gelatinous pellicule, divided by greyish striae; these striae are merely the juice which is rendered more watery, because the contact of the acid has caused it to lose its albumen. At the same time the centre of the pellicule becomes opaque, and of the colour of the yolk of the egg; while it increases in bulk by the prolongation of the diverging fibres. The whole fluid at first has the appearance of an agate with milky clouds; and it appears as if organic membranes were produced under the eye. When the coagulum is moved, it becomes granulated like soft cheese; the yellow colour is reproduced by adding a few more drops of nitric acid. The acid in this case acts in the same manner with the oxygen of the atmosphere, at the temperature of from 80·5 to 95° (Far.); for the white coagulum becomes yellow in two or three minutes by exposure to the sun. After some hours, the yellow colour turns brown, undoubtedly because the carbon is more liberated in proportion as the hydrogen, with which it was combined, is burned. The coagulum formed by the acid becomes viscid, and acquires the waxy odour, which is perceived when the muscular fibre or fungi are treated with nitric acid. From the interesting experiments of Mr. Hatchett, it may be supposed that in this case the albumen is partially converted to the state of gelatine. When the coagulum of the papaw is thrown into water, it softens, becomes partially dissolved, and gives the water a yellowish tinge; the milk, when placed in contact with water, also forms membranes; a tremulous jelly, similar to starch, is immediately precipitated, and the appearance is more remarkable if we employ water at the temperature of from about 100° to 140° (Far.). If carbonate of soda be added to the fluid, the coagulum is not formed; but it is immediately produced by the addition of an acid. If we compare together the milky juices of the papaw, the cow tree, and the haevea caoutchouc, we find a striking resemblance between the juices which abound in caseous matter, and those in which the caoutchouc predominates. According to the opinion of M. Gay-Lussac, we may consider the caoutchouc as analogous to the oily part or the butter of the vegetable milk; in the vegetable milk we find caseum and caoutchouc; in animal milk, caseum and butter. The albuminous and the oily principles exist in different proportions in the different species of animals and milky plants; and in the last, they are frequently united to other substances which render them injurious as articles of food.