the kind and amount of other materials which it includes. The alveoli may contain a watery fluid, fat globules, yolk spheres, and various solid products of metabolism, such as starch grains, glycogen, pigment granules, and crystals. Some of these are substances which may be utilized as food, while others represent products of the breaking-down processes of living matter. There are many kinds of substances found in protoplasm, but those which are of the greatest importance are the following:

The Proteins. These are complex compounds containing carbon, oxygen, hydrogen, nitrogen, and frequently also phosphorus, sulphur, iron, calcium, sodium, potassium, magnesium, and more rarely other elements. Examples of proteins are furnished by albumen, which occurs in the white of egg; the casein of cheese; and the protein of muscle fiber. Many of the proteins are soluble in water, from which they may be precipitated by the proper reagents. Most of them can be coagulated by heat and various chemicals. Their molecules are relatively large and complex, and they may be broken down by enzyme action into simpler and simpler compounds. The simpler compounds which are formed by this process afford many clues as to the way in which the protein molecule is constituted. Among the simplest products of protein decomposition are the amino acids, of which there are many kinds. Starting with the amino acids, chemists have succeeded in synthesizing compounds having many of the properties of the simpler proteins. On account of the number of different groups of substances composing protein, there is the possibility, through making different permutations and combinations of these components, of producing an almost infinite variety of protein molecules. In the hemoglobins alone, as Reichert and Brown have shown, there is a special type of compound characteristic of the different species of vertebrate animals. Hemoglobin is an iron-containing protein which forms the chief oxygen carrier of our blood, and which gives to the blood its red color. It forms crystals whose shape depends upon the kind of animal from which it is derived, and it is probable that these peculiarities in crystalline form are indicative of differences in chemical

composition. In fact, there are many other indications that such chemical differences occur. There seems to be an almost endless variety of possible hemoglobins. And there is evidence that many other classes of proteins include an equally extensive series.

The carbohydrates are composed of carbon, hydrogen, and oxygen, the two latter elements being in the proportion in which they occur in water, H2O. They have the general formula C(H2O)y. Common examples of carbohydrates are starch, sugar, and cellulose. Carbohydrates are less complex and more limited in number than the proteins, but, like the proteins, they may be split up by enzyme action into simpler compounds.

The fats, like the carbohydrates, contain the three elements, oxygen, carbon, and hydrogen, but these elements have a quite different arrangement, and hence cause the fats to differ markedly from the carbohydrates in their properties. Chemically fats are compounds of glycerin, C3H5(OH)3, with some fatty acid. We are all familiar with fats as they occur in butter, meat, and the oils of plants. Fats, like carbohydrates, are important food materials and are utilized especially for the production of energy.

The Vitamines. In addition to the classes of foods just mentioned, there are other substances known as vitamines which have been found essential for the life and growth of many kinds of organisms. Until a few years ago, the existence of these important substances was entirely unknown. One of these, the fat-soluble vitamine A, is found in animal fats and in the tissues of green vegetables. If young rats are kept upon a diet lacking this vitamine, their eyes undergo a peculiar degeneration which may entirely destroy the power of vision, and the animals also fail to grow normally and finally die. Eye defects due to malnutrition occasionally develop in children, probably as a result of the lack of vitamine A.

Deficiency in another vitamine (water-soluble vitamine B) is now known to be the cause of a disease known as beriberi which is quite prevalent in countries whose people subsist largely upon a diet of polished rice. The milling methods, which remove the

outer parts of the seed, get rid of most of this vitamine. A condition resembling beriberi has been produced in pigeons fed on a diet of polished rice, but the administration of a very minute amount of vitamine B suffices to cure the abnormal condition.

The discovery of a third vitamine C has finally given the clue to the cause of scurvy. This disease was formerly much more prevalent than it is now, especially among sailors who were forced to subsist for long periods on a diet containing no fresh vegetables. It has long been known that scurvy can be prevented or cured by a diet of fresh vegetables or acid fruits, but the reason for this fact was discovered only after carefully controlled experiments on animals showed that it was due to a vitamine. Owing to the fact that vitamine C is destroyed by heat, it is not present in most canned materials.

Absence of a fourth vitamine D, formerly confused with vitamine A, produces a condition known as rickets. This disease is especially prone to attack badly nourished children, but treatment with substances rich in vitamine D, such as cod-liver oil, has been found to produce very salutary results.

Knowledge of the causes of the diseases mentioned has already been the means of saving thousands of lives and untold suffering. One cannot over-emphasize the general principle that an understanding of the normal mechanism of life processes is essential for an understanding of why the mechanism occasionally goes wrong. And if we do not understand why the mechanism goes wrong, it is only by fortunate accident that we stumble upon the means of making it go right. We shall encounter further illustrations of this fact when we consider the causes of contagious diseases.

A fifth vitamine E, discovered by Dr. H. M. Evans, has a marked influence upon the fertility of rats. Absence of this vitamine has little apparent effect upon the vitality of the animals, but it renders both the males and the females sterile. Fertility may be restored if the animals are given a small amount of green vegetables in their diet.

Little is known of the chemical composition of vitamines, or

just how they act in maintaining life. They are derived from plants, and they are required only in exceedingly minute amounts in order to perform their proper rôle.

The Inorganic Constituents. The most abundant of the inorganic ingredients of protoplasm is water. It commonly forms between fifty and seventy-five per cent of living matter. We do not look upon water as living substance, but it forms the matrix in which most of the soluble materials of living tissue are dissolved, and therefore affords a condition for most of the chemical reactions which are involved in vital activity. Both plants and animals are able to absorb water, and they frequently exhibit devices for preventing the undue loss of water through evaporation. While living tissue imbibes most of its water from the outside, water may be formed through the oxidation of the products of metabolism. Professor Babcock has shown that metabolic water, as he terms it, may be formed by many animals that are fed upon thoroughly desiccated food.

While mineral salts have long been known to occur in living tissue, it is only recently that their importance has been adequately recognized. Dr. Loeb has shown that perfectly pure water is a protoplasmic poison, since it robs the protoplasm of mineral salts which diffuse from it. He has shown that absolutely pure solutions of simple substances, such as ordinary salt (NaCl), may likewise be poisonous, but that their action may be counteracted by a small quantity of some other salt. The salt which exercises this life preservative function may itself be poisonous. The eggs of the marine fish Fundulus, put into pure sodium chloride solution of the same density as sea water, fail to develop; but if a small quantity of a salt of calcium, strontium, or even copper or lead, be added, the poisonous effect of the pure-salt solution is neutralized. These salts have an antagonistic action. A certain balance in their influence must be maintained as an essential condition of life.

Protoplasm commonly hovers on the verge between coagulation and solution, passing from the one state to the other, or

rather toward the one state or the other, with relative ease. It is a protean kind of stuff belonging to the group of substances known as colloids. The English chemist, Professor Graham, first clearly distinguished colloids and crystalloids. Both are soluble, but the crystalloids readily diffuse through membranes (or dialyze) while the colloids do not, or do so but slightly. There are all gradations between crystalloids and colloids, and some substances ordinarily crystalloid may occur in the colloidal state. Colloidal particles are relatively massive, and their solution has some of the properties exhibited by suspensions of very fine particles. The colloids of protoplasm readily coagulate somewhat, or pass from the sol to the gel state, when they come into contact with water, forming in this way a sort of membrane which is permeable to many substances, but which prevents the protoplasm from going into solution. Through this plasma membrane water, salts, and dissolved food materials may diffuse into the protoplasm, while other materials, the products of destructive metabolism, may diffuse out. The properties of semipermeable membranes are of the utmost importance in biological processes. Many dead membranes are semi-permeable, allowing salt solutions to diffuse through, while they retain such substances as colloidal egg albumen. But living membranes are found to possess several properties which are different from those of dead membranes. They frequently allow substances to pass through them in one direction only; they may exhibit an electrical polarity, and their properties change under different conditions of external stimulation and with different states of the protoplasm within. It is commonly held that the outer surface of protoplasm is covered by an exceedingly thin film of a lipoid, or fatlike substance such as lecithin. The semi-permeability of living membranes has been explained as due to the presence of minute pores or spaces which would allow small molecules to pass through, but would keep out larger ones. It has been found, however, that penetrability is by no means dependent on the size of the molecules of a dissolved substance. There seems to be more probability, however, in the view of Nernst that