On the Milk extracted from the Cow Tree (l' Arbre de la Vache), and on Vegetable Milk in general.-- By Humboldt. Annales de Chimie.--" We have given (say the editors of the Annales) the extract of a memoir read by M. Humboldt in one of the last sittings of the Academy of Sciences. The readers who may desire a more extended detail on a subject so interesting in vegetable chemistry, will find it in the sixth volume of the Relation Historique, by the author, which will be very shortly published. "We had for several weeks past heard mention made of a certain tree in the valleys of Aragua, the juice of which was said to be a nourishing milk; the tree, indeed, was called the cow tree, and we learnt that the negroes of the place, who drank it abundantly, looked upon it as a very wholesome species of aliment. This account excited surprise in our minds, since the general characters attached to the milky juices of plants, are those of being acrid, bitter, and more or less poisonous. But since our residence in Barbula (province of the Caraccas), we have found that in this account of the Palo de vaca there was no exaggeration. The tree thus named, is one of a very beautiful appearance. Its flowers we had not an opportunity of seeing; the fruit of it is rather pulpy, and encloses sometimes one and sometimes two kernels. When incisions are made into the trunk of this tree, which appears to belong to the sapota tribe of plants, it gives out an abundance of a glutinous thick kind of milk, void of all acrimony, and exhaling an odour by no means unpleasant. We drank considerable quantities of it, both in the evening before we retired to rest and early in the morning, without experiencing the slightest inconvenience: it is only the glutinous nature of the fluid in question that occasions its taste to be at all unpleasant. Both the slaves and others employed upon the plantations drink it freely, and mix it with the maize and capada plant. The master of the plantation assured us that the slaves always thrived and gained flesh during the season in which the Palo de vaca furnished them with milk. When this milk is exposed to the air, its surface becomes covered with a strongly animalized substance of a yellowish hue, and fibrous stringy appearance, resembling a cheesy matter. This change in the juice is probably produced by an absorption of oxygen from the air. When the membranous substance of which we speak is separated from the more fluid part of the milk, it proves to be nearly as elastic as caoutchouc; but it undergoes in the course of time a like putrefaction with gelatine. The people of the place call this matter cheese. It becomes sour in about five or six days, according to the observation which I made on some portions of it which I took with me to Orenoque. The milk, enclosed in a well stopped vial, had thrown down a small quantity of coagulum, which, far from being fetid, constantly exhaled a sort of balsamic odour. The fresh milk scarcely coagulated at all upon being mixed with cold water; but a separation of this viscous matter took place when I heated it with nitric acid. We sent two bottles of this milk to M. Fourcroy, at Paris. The one bottle contained the juice pure, in the other it was mixed with a quantity of carbonate of soda. This remarkable production, the Palo de vaca, appears to be confined to the Cordilleras (la Cordilliere du Littoral), especially near the lake of Marakabo. It is found also near the town of San Mateo; and, according to M. Bredmeyer (whose voyages have so much contributed to enrich the beautiful greenhouses of Schonbrun and Vienna), it is to be seen in the vale of Caucagua, three days' journey east of the Caraccas. This naturalist, like ourselves, found the vegetable juice in question of an agreeable taste and aromatic odour. The natives of Caucagua call the tree which furnishes it, the milk tree arbol de loche. The inhabitants of the Andes had been in the practice of fabricating wax lights from the wax which is found on the trunk of the palm tree long before the chemists of Europe had discovered quantities of wax in the pollen of flowers, varnish in leaves, and farina in fruits; in like manner the caseum, the basis of cheese, has but recently been detected in the emulsions of almonds; whereas we find that ages ago, in the mountains of Venezuela, the milk of a tree, and the cheese which separates itself from such milk, were used as aliment. How are we to account for these singularities in regard to the developement of our knowledge of nature's productions? How can we explain the fact, that the people of another hemisphere have discovered and applied properties which had for so long a time escaped the penetration of men whose very occupation it is to search Nature's laws, and penetrate her mysterious operations? It would appear that the fact is to be explained partly from the circumstance of the elements and principles of plants being distributed among so many orders and families of the vegetable creation, partly from that difference of quantity, with respect to their essential principles, which is observed in the vegetable world, according as the particular plant is a native of equatorial, or of cold and temperate latitudes, and partly by an ingenuity derived from necessity, which impels uncultivated man to seek for his sustenance in the natural productions by which he is surrounded. Thus the juices, the bark, the roots, and the fruits of trees, become the subjects naturally of instinctive investigation; and when poisonous productions are combined with those that are wholesome and nutritive, man is taught by the same necessity to separate the one principle from the other, as in the arum, tacca, &c. The American savage, as well as the inhabitants of the South Sea islands, have thus learnt the art to prepare the fecula of plants, by compressing it and separating its juice. In the milk of plants, as also in the milky emulsions, materials considerably nutrient, such as albumen, the caseous and the saccharine principles are intermixed with caoutchouc, and deleterious ingredients, such as morphine and hydrocyanic acid. These combinations vary not only in the different tribes of plants, but also in the respective species of the same genera. Sometimes it is the morphine or narcotic principle, which predominates in vegetable milk; in other instances the caoutchouc is the characterizing ingredient; and lastly, as we have seen in the juice of the papaya, and cow tree, the caseous is the main principle. The lactiferous plants belong principally to the three families of the Euphorbia, the Urticaria, and the Apocyna; and as upon investigating the different distribution of vegetable growth in the several parts of the world, we find that the species of these orders of plants are most numerous in the tropical regions, we infer that a very high temperature is necessary for the proper elaboration of the milky juices, as well at to the complete formation of the caoutchouc of albumen, and of the caseous principle. The juice of the Palo de vaca certainly presents one of the most striking examples of a vegetable milk in which the acrid and deleterious principles are not united to albumen, caseum, or caoutchouc; but the euphorbium and asclepias genera, so generally known by their caustic properties, had before furnished us with some species of which the juice is bland and innocent, as in the instance of the tabayba dulce, (euphorbia balsamifera) of the Canary Isles, and the asclepias lactifera of Ceylon. Bruman has told us, that the inhabitants of Ceylon make use of this last in lieu of milk, and that they mix its leaves in cookery with those articles of food that are generally prepared with milk. It is to be wished that Mr. John Davy may pay attention to this particular during his stay at Ceylon; for it appears probable, as has been suggested by M. Decandolle, that it is only the juice which exudes from the young plant, which is used for the purposes in question, viz. that which flows from the vegetable before the development of the acrid principle. Indeed, in some countries, the first shoots of even the apocyna are eaten. I have thus endeavoured to institute a general resemblance between the juices which circulate in plants, and the lactiferous emulsions which are yielded by the almond and palm tribe; and I may here add a few remarks respecting some experiments upon the carica papaya, which I made during my stay in the valleys of Aragua; not, of course, of so satisfactory a nature as they might have been, had I been furnished with the several reacteives necessary to conduct investigations of this kind. I ascertained that the younger the fruit of the plant, the greater was the quantity of milk which it furnished: this juice, indeed, was found even in the germ of the fruit; and as it proceeded to maturity, its milk became not merely less abundant, but also more aqueous. In it was found less of the animal coagulable matter (matiere animale coagulable), and it is conceivable that as the plant advances in growth, this matter, abstracted from the fruit, comes to constitute its pulpy or fleshy parts. The coagulum from the very young fruit, heated with nitric acid, becomes viscous, and exhales a waxy odour, similar to that which I have observed in muscular flesh and in small mushrooms (morilles), when the acid is added to them in the same manner. Following the hints of Fourcroy and Vauquelin, I mixed the milk of the papaya with a solution of carbonate of soda. The mixture did not run into coagulated lumps, even when water was added to it. The membranous substance did not show itself, until, by the addition of an acid to the solution, the soda became neutralized. The coagulum formed by the nitric acid, lemon juice, and hot water, I caused to disappear, by adding to the mixture the carbonate of soda. By this addition the juice resumed its milky and liquid appearance; but the experiments only answered when the addition was made to recently formed coagula. In conclusion, it may be remarked, that a comparison of the milky juices of the papaya with those of the cow-tree and the hevea, offers a striking analogy between the juices which abound in the caseous, and those that contain the caoutchouc principle. All the productions in which the latter is predominant, as likewise the impenetrable garments (manteaux impermeables) that are fabricated in Spanish America, are made to exhale a like animal and disagreeable odour, by placing the milk of the hevea between two layers of them; a circumstance which would seem to argue that the caoutchouc in coagulating, attracts to itself the caseum, which is no other, perhaps, than albumen in a different form. The fruit of the bread-tree is not any more bread than are the bananas before their maturity, or the tuberous and amilaceous roots of the cassada, the convolvulus battatas, or the potatoe. On the contrary, the milk of the Palo de vaca contains the caseous principle in the same manner as does the milk from the mammalia. In pursuing these analogies between the animal and vegetable world, we may, with M. Gay-Lussac, consider the caoutchouc as the oily part, the butter as it were of the vegetable milk. In a word, we may find in vegetable milk, caseum and caoutchouc; in animal milk, caseum and butter. The two principles vary in their proportions in different animals, as they do in the various species of lactiferous plants. In these last, they are more generally mingled with other materials which are not alimentary; but from such materials they may, perhaps, be always separated by chemical processes. Vegetable milk becomes wholesome and nourishing when it is deprived of its acrid and narcotic principle, and when it abounds more in caseous matter than in caoutchouc.