of it in water, but only a reabsorption by it of that quantity of fluid with which it is naturally associated. Another peculiar phenomenon characteristic of organic substances is their coagulation. Those which are naturally fluid suddenly assume, under certain conditions, a solid or semi-solid consistency. They are then said to be coagulated; and after coagulation they cannot be made to reassume their original condition. Thus fibrin coagulates on being withdrawn from the blood vessels, albumen on being subjected to the temperature of boiling water, casein on being placed in contact with an acid. When an organic substance thus coagulates, the change which takes place is a peculiar one, and has no resemblance to the precipitation of a solid substance from a watery solution. On the contrary, the organic substance merely assumes a special condition; and in passing into the solid form it retains all the water with which it was previously united. Albumen, for example, after coagulation, retains the same quantity of water in union with it, which it held before. After coagulation, accordingly, this water may be driven off by evaporation, in the same manner as previously; and on being again exposed to moisture, the organic matter will again absorb the same quantity, though it will not resume the fluid form. By coagulation, an organic substance is permanently altered; and though it may be afterwards dissolved by certain chemical reagents, as, for example, the caustic alkalies, it is not thereby restored to its original condition, but only suffers a still further alteration. In many instances we are obliged to resort to coagulation in order to separate an organic substance from the other proximate principles with which it is associated. This is the case, for example, with the fibrin of the blood, which is obtained in the form of flocculi, by beating freshly drawn blood with a bundle of rods. But when separated in this way, it is already in an unnatural condition, and no longer represents exactly the original fluid fibrin, as it existed in the circulating blood. Nevertheless, this is the only mode in which it can be examined, as there are no means of bringing it back to its previous condition. Another important property of the organic substances is that they readily excite, in other proximate principles and in each other, those peculiar indirect chemical changes which are termed catalyses or catalytic transformations. That is to say, they produce the changes referred to, not directly, by combining with the substance which suffers alteration, or with any of its ingredients; but simply by their presence, which induces the chemical change in an indirect manner. Thus, the organic substances of the intestinal fluids induce a catalytic action by which starch is converted into sugar. The albumen of the blood, by contact with the organic substance of the muscular fibre, is transformed into a substance similar to it. The entire process of nutrition, so far as the organic matters are concerned, consists of such catalytic transformations. Many crystallizable substances, which when pure remain unaltered in the air, become changed if mingled with organic substances, even in small quantity. Thus the casein of milk, after being exposed for a short time to a warm atmosphere, becomes a catalytic body, and converts the sugar of the milk into lactic acid. In this change there is no loss nor addition of any chemical element, since lactic acid has precisely the same ultimate composition with sugar of milk. It is simply a transformation induced by the presence of the casein. Oily matters, which are entirely unalterable when pure, readily become rancid at warm temperatures, if mingled with an organic impurity. Fourthly, The organic substances, when beginning to undergo decay, induce in certain other substances the phenomenon of fermentation. Thus, the mucus of the urinary bladder, after a short exposure to the atmosphere, causes the urea of the urine to be converted into carbonate of ammonia, with the development of gaseous bubbles. The organic matters of grape juice, under similar circumstances, give rise to fermentation of the sugar, by which it is converted into alcohol and carbonic acid. Fifthly, The organic substances are the only ones capable of undergoing the process of putrefaction. This process is a compli cated one, and is characterized by a gradual liquefaction of the animal substance, by many mutual decompositions of the saline matters which are associated with it, and by the development of peculiarly fetid and unwholesome gases, among which are carbonic acid, nitrogen, sulphuretted, phosphoretted, and carburetted hydrogen, and ammoniacal vapors. Putrefaction takes place constantly after death, if the organic tissue be exposed to a moist atmosphere at a moderately warm temperature. It is much hastened by the presence of other organic substances, in which decomposition has already commenced. The organic substances are readily distinguished, by the above general characters, from all other kinds of proximate principles. They are quite numerous; nearly every animal fluid and tissue containing at least one which is peculiar to itself. They have not as yet been all accurately described. The following list, however, comprises the most important of them, and those with which we are at present most thoroughly acquainted. The first seven are fluid, or nearly so, and either colorless or of a faint yellowish tinge. 1. FIBRIN.-Fibrin is found in the blood; where it exists, in the human subject, in the proportion of two to three parts per thousand. It is fluid, and mingled intimately with the other ingredients of the blood. It occurs also, but in much smaller quantity, in the lymph. It is distinguished by what is called its "spontaneous" coagulation; that is, it coagulates on being withdrawn from the vessels, or on the occurrence of any stoppage to the circulation. It is rather more abundant in the blood of some of the lower animals than in that of the human subject. In general, it is found in larger quantity in the blood of the herbivora than in that of the carnivora. 2. ALBUMEN. Albumen occurs in the blood, the lymph, the fluid of the pericardium, and in that of the serous cavities generally. It is also present in the fluid which may be extracted by pressure from the muscular tissue. In the blood it occurs in the proportion of about seventy-five parts per thousand. The white of egg, which usually goes by the same name, is not identical with the albumen of the blood, though it resembles it in some respects; it is properly a secretion from the mucous membrane of the fowl's oviduct, and should be considered as a distinct organic substance. Albumen coagulates on being raised to the temperature of 160° F.; and the coagulum, like that of all the other proximate principles, is soluble in caustic potass. It coagulates also by contact with alcohol, the mineral acids, ferrocyanide of potassium in an acidulated solution, tannin, and the metallic salts. The alcoholic coagulum, if separated from the alcohol by washing, does not redissolve in water. A very small quantity of albumen has been sometimes found in the saliva. 3. CASEIN. This substance exists in milk, in the proportion of about forty parts per thousand. It coagulates by contact with all the acids, mineral and organic; but is not affected by a boiling temperature. It is coagulated also by the juices of the stomach. It is important as an article of food, being the principal organic ingredient in all the preparations of milk. In a coagulated form, it constitutes the different varieties of cheese, which are more or less highly flavored with various oily matters remaining entangled in the coagulated casein. What is called vegetable casein or "legumine," is different from the casein of milk, and constitutes the organic substance present in various kinds of peas and beans. 4. GLOBULINE.-This is the organic substance forming the prin cipal mass of the red globules of the blood. It is nearly fluid in its natural condition, and readily dissolves in water. It does not dissolve, however, in the serum of the blood; and the globules, therefore, retain their natural form and consistency, unless the serum be diluted with an excess of water. Globuline resembles albumen in coagulating at the temperature of boiling water. It is said to differ from it, however, in not being coagulated by contact with alcohol. 5. PEPSINE. This substance occurs as an ingredient in the gastric juice. It is not the same with the substance which Schwann extracted by maceration from the mucous membrane of the stomach, and which is regarded by Robin, Bernard, &c., as only an artificial product of the alteration of the gastric tissues. There seems no good reason, furthermore, why we should not designate by this name the organic substance which really does exist in the gastric juice. It occurs in this fluid in very small quantity, not over fifteen parts per thousand. It is coagulable by heat, and also by contact with alcohol. But if the alcoholic coagulum be well washed, it is again soluble in a watery acidulated fluid. 6. PANCREATINE.-This is the organic substance of the pancreatic juice, where it occurs in great abundance. It coagulates by heat, and by contact with sulphate of magnesia in excess. In its natural condition it is fluid, but has a considerable degree of viscidity. 7. MUCOSINE is the organic substance which is found in the dif ferent varieties of mucus, and which imparts to them their viscidity and other physical characters. Some of these mucous secretions are so mixed with other fluids, that their consistency is more or less diminished; others which remain pure, like that secreted by the mucous follicles of the cervix uteri, have nearly a semi-solid consistency. But little is known with regard to their other specific characters. The next three organic substances are solid or semi-solid in consistency. 8. OSTEINE is the organic substance of the bones, in which it is associated with a large proportion of phosphate of lime. It exists, in those bones which have been examined, in the proportion of about two hundred parts per thousand. It is this substance which by long boiling of the bones is transformed into gelatine or glue. In its natural condition, however, it is insoluble in water, even at the boiling temperature, and becomes soluble only after it has been permanently altered by ebullition. 9. CARTILAGINE.-This forms the organic ingredient of cartilage. Like that of the bones, it is altered by long boiling, and is converted into a peculiar kind of gelatine termed "chondrine." Chondrine differs from the gelatine of bones principally in being precipitated by acids and certain metallic salts which have no effect on the latter. Cartilagine, in its natural condition, is very solid, and is closely united with the calcareous salts. 10. MUSCULINE.-This substance forms the principal mass of the muscular fibre. It is semi-solid, and insoluble in water, but soluble in dilute muriatic acid, from which it may be again precipitated by neutralizing with an alkali. It closely resembles albumen in its chemical composition, and like it, contains, according to Scherer, two equivalents of sulphur. The four remaining organic substances form a somewhat peculiar group. They are the coloring matters of the body. They exist always in small quantity, compared with the other ingredients, but communicate to the tissues and fluids a very distinct coloration. They all contain iron as one of their ultimate elements. 11. HÆMATINE is the coloring matter of the red globules of the blood. It is nearly fluid like the globuline, and is united with it in a kind of mutual solution. It is much less abundant than the globuline, and exists in the proportion of about one part of hæmatine to seventeen parts of globuline. The following is the formula. for its composition which is adopted by Lehmann: When the blood-globules from any cause become disintegrated, the hæmatine is readily imbibed after death by the walls of the blood vessels and the neighboring parts, staining them of a deep red color. This coloration has sometimes been mistaken for an evidence |