substances to be absorbed must be soluble in the wall or surface layer of the living cell, from which they then pass into the interior.

The films of protoplasm exhibit a considerable degree of tension, or tendency to contract. This property of surface tension is a general characteristic of physical objects, and is well illustrated in the tendency of drops of water, or of other liquids, to assume a spherical form. It is doubtless due to surface tension that fluids within protoplasm commonly take the form of spherical drops. Protoplasm appears much like an emulsion of one fluid within another. If we examine an emulsion of oil and water, or a more permanent emulsion such as milk, we observe a lot of discrete drops scattered about in a nearly homogeneous medium. If to such an emulsion a third substance is added, which tends to occupy the surface film of the liquid in which it is dissolved, the drops become surrounded by a sort of specialized membrane which tends to prevent their coalescence. This is probably what occurs in protoplasm, and gives it the character of an emulsion that is relatively permanent.

Protoplasm, as we have said, passes easily from the state of a sol to that of a gel, and vice versa, but it can coagulate only a little bit if it is able to recover its fluidity. If it passes a certain point it cannot get back, and that means death. If one makes a microscopic examination of living protoplasm which is subjected to gradually increasing temperature, the transparent material may be seen to show a whitish opacity as the thermal death point is approached. A similar change is familiar in the coagulation of the white of an egg. The assumption of a state of rigidity, or rigor mortis, which commonly occurs soon after death in the higher animals, is due to a coagulation of the protein of the muscles. This condition is temporary, and is followed by a stage in which the protoplasm again becomes more fluid. Very commonly death is brought about by agencies which cause protoplasm to liquefy. Normally it is prevented from going into solution by a protective, semi-permeable membrane, but if this coating is injured, the living substance may disintegrate. Micro

scopic organisms, when injured, may frequently be seen to melt away, or dissolve, in the surrounding water, leaving only a few granules, or other contents of their living substance. In some kinds of marine flatworms the whole animal may melt away by a process of disintegration which starts in at a given point of injury and quickly spreads until it involves the destruction of the whole body.

The destructive processes which lead to death are not foreign activities imposed upon the organism. As Osterhout remarks, we should "regard the death process as a normal part of the life process, producing no disturbance unless unduly accelerated by an injurious agent which upsets the normal balance and causes injury, so that the life process comes to a standstill." Destruction and death of living tissue is a necessary element of vital activity, or, in Claude Bernard's paradoxical phrase, “La vie c'est la mort "—"Life is death." Life is maintained through a balance of constructive and destructive activities; when the latter begin to predominate, life is threatened, but there may be recovery under certain conditions which check the disintegrating agents that act on the organism. The transition between life and death is commonly a gradual one. Osterhout has found that, as death is approached, living tissue shows less and less resistance to the electric current, but after the death point is reached the resistance remains constant. He used in his experiments pieces of seaweed whose resistance to the electric current was measured by a galvanometer. When the seaweed was placed in a pure solution of sodium chloride, it was injured, and its resistance to currents and along with it the resistance to penetration by reagents decreased. After a certain amount of injury, the seaweed would more or less completely recover its normal conductivity if replaced in sea water. If the injury was slight, recovery might be complete, but if it was severe, recovery was only partial. Injury and recovery could both be subjected to definite quantitative measurements, and the influences of various substances on the vital activities of the living tissue ascertained with much precision. An accurate knowledge of the causes and

conditions of death, and how they may be controlled, is obviously of the greatest importance, for although death will overtake the protoplasm of all of us in due time, and may never be postponed beyond a certain period characteristic of our species, we are naturally desirous of circumventing it as long as we can.

Death is essentially a chemical phenomenon. The protoplasm of many forms may undergo marked physical changes, but so long as these do not entail too great a change in the chemical balance of the organism, they may not destroy life. Protoplasm is always killed by a temperature not many degrees above the boiling point of water, and usually at a much lower temperature. When it contains little water, protoplasm is more resistant to heat. The seeds of a few species of plants and the spores of some kinds of bacteria may withstand several hours of boiling and a degree of dry heat thirty to fifty degrees above the boiling point. The imbibition of water facilitates chemical changes, and it is by inducing such changes that heat produces its lethal effects.

The lower limits of temperature at which life resists destruction are exceedingly variable in different forms. Warm-blooded animals cannot stand, as a rule, a very great reduction of temperature. On the other hand, a frog may be frozen until its legs become brittle, and then recover. Fishes frozen in solid ice have often been observed to revive and swim about after the ice is melted; and hosts of aquatic insects, worms, and other small creatures, may not only be frozen, but kept for months far below the freezing point without losing the capacity to survive after they are gradually thawed out. Bacteria have been cooled down to the temperature of liquid air, and even to that of liquid hydrogen, without destroying their power of growth and multiplication. In fact, for some forms there seems to be no lower limit of temperature beyond which their life is destroyed.

Life processes can hardly be said to go on in a completely frozen organism. It is much the same with those organisms which are capable of withstanding prolonged drying. The protoplasm of beans, grains of wheat or corn, and many other seeds may become very dry and hard without impairing its capacity for

living. While the stories of the germination of seeds that were taken from the cases of Egyptian mummies have been shown to be devoid of foundation, some seeds are known to retain their vitality for many years. Among animals there are several kinds of rotifers, or wheel animalcules, and the so-called bear animalcules, or Tardigrada, which may be thoroughly dried into a mere shriveled-up mass of hardened tissue, and nevertheless regain

F10. 3A bear animalcule (Tardigrade): A, in the normal active state; B, in a driedup condition; I-IV, appendages. (From Verworn. Courtesy Macmillan Co.)

their activity when placed again in water. This revival is all the more remarkable since these creatures have a complex organization of digestive, nervous, excretory, and muscular systems, which apparently need only to soak up water in order to function in a thoroughly efficient manner.

The life of these frozen or desiccated organisms is often spoken of as latent life. Metabolism probably does not go on at all at the temperature of liquid air, and it goes on very slowly in a desiccated seed or animalcule, for even dried seeds have been

shown to give off a very slight amount of carbon dioxide. With cooling and drying, chemical changes gradually cease. Whether we say that life in these forms was destroyed and later resumed, or whether we call their life latent, or suspended, is largely dependent on how we define our terms-a question of words rather than one of fact.

REFERENCES

BAYLISS, W., The Colloidal State. London, Frowde, Hodder, and Stoughton, 1923.

BULLOWA, J. G. M., Colloids in Biology and Medicine. N. Y., Van Nostrand, 1919.

BÜTSCHLI, O., Protoplasm and Microscopic Foams. London, A. and C. Black, 1894.

FUNK, C., The Vitamines. Baltimore, Williams and Wilkins, 1922. HUXLEY, T. H., The Physical Easis of Life. N. Y., Appleton, 1898. LILLIE, R. S., Protoplasmic Action and Nervous Action. University of Chicago Press, 1923.

MCCOLLUM, E. V., The Newer Knowledge of Nutrition (3rd ed.). N. Y., Macmillan, 1925.

MATHEWS, A. P., Physiological Chemistry. N. Y., W. Wood, 1923. MENDEL, L. B., Nutrition: The Chemistry of Life. New Haven, Yale University Press, 1923.

MOORE, B., The Origin and Nature of Life. N. Y., Holt, 1912.

OSTERHOUT, W. J. V., Injury, Recovery, and Death. Philadelphia, Lippincott, 1922.

WILSON, E. B., The Physical Basis of Life. New Haven, Yale University Press, 1923.