oxidization of carbon and hydrogen, and the formation of such new compounds as starch, cellulose, and sugar.

Its energy is moreover applied to the rearrangement of particles and the formation of structure. Plants grow most rapidly when the sun is shining; when they grow during the night they derive the forces required for molecular rearrangements from the potential energy stored up in their hydrocarbonaceous material by its reoxidization. Neither carbon nor hydrogen can be separated from oxygen without energy being expended or rendered potential or imperceptible; and such energy is always again rendered sensible by the reoxidization of carbon-compounds.

Certain silver salts are readily decomposed by the action of light; but the process requires, or is at least much accelerated by, the presence of colloids, such as collodion or albumen. This is especially seen in photography. The deoxidization of carbon by light in the presence of protoplasm is probably a closely similar phenomenon.

Although comparatively few bodies have their molecules sensibly affected by light-vibrations, or undergo change of structure under the action of this form of energy, all substances are more or less influenced by the less rapid vibrations of heat; and just as protoplasm undergoes change, or

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RELATION OF HEAT TO LIGHT.

dies, when deprived of the peculiar forms of energy to which its atoms are susceptible, so all substances would become entirely changed if they were deprived of the vibrations of heat which they continually receive from without.

No line can be drawn between heat- and lightvibrations, except by the peculiar physiological action of the latter upon the human retina: the one passes insensibly into the other. A continuous gradation of similar vibrations exists in the spectrum of an incandescent body, beginning with the most rapid beyond the extreme violet, and ending with the slowest and most intense far beyond the extreme red. Certain substances, as silver and gold salts, are most affected by the most rapid vibrations (the so-called actinic or chemical rays); vegetable protoplasm is chiefly sensitive to the intermediate vibrations of the visible spectrum, whilst most bodies are maintained in their present physical condition by the vibrations of heat beyond the extreme red. The action of energy on protoplasm is therefore peculiar in degree rather than in kind.

It has already been observed that the sun's energy may be stored up by becoming potential or latent in solid carbon and its compounds with hydrogen, and that it may be set free again by their recombination with oxygen. As Meyer, long

ACTION OF POTENTIAL ENERGY.

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ago, and Tyndall, more recently, have shown*, the combustion of coal gives back the energy rendered latent or potential by the plants of the coal era. Protoplasm has the power of converting potential energy derived originally from the sun's light, and freed by the oxidization of carbon-compounds, into molecular forces, which give rise to changes in its form and condition. The changes, however, which take place in a bar of iron during its oxidization, or rusting, are of a precisely similar nature: force is evolved or set free by the rusting of the iron; this force warms the metal and gives rise to molecular movements of the bar.

The capacity of assimilating suitable material to itself has long been considered a peculiarity of living matter. Its increase of mass, however, is entirely analogous to that which a piece of brass undergoes when immersed in a suitable solution (an alkaline solution of zinc and copper) and supplied with energy by means of a voltaic current. The alkaline salts of the two metals are decomposed, and the metals themselves are united into an alloy. Protoplasm, under the influence of energy, gives rise to similar decompositions and recompositions of certain suitable kinds of matter.

The fact that the nutrition of protoplasm is interstitial, whilst the accretion of new matter to in* Tyndall, 'Heat a mode of Motion.'

organic masses takes place on the surface alone, is often spoken of as a distinction of fundamental importance. In both cases, however, the accretion of new material takes place at the point of contact between the increasing mass and the food-material. In the case of protoplasm the food-matter is disseminated throughout the mass, whilst in the case of the brass medal the two are in contact at the surface only. The cases are quite analogous to the phenomena of combustion. In the ordinary flame this takes place on the surface only, at the points of contact between the supporter of combustion and the combustible gas; in the oxyhydrogen flame of the lime light it is interstitial, because the supporter of combustion and the combustible gases are intimately mixed.

The fact that protoplasm undergoes change, or dies, when it is either deprived of food-material or energy, cannot be used as a means of distinguishing it from other kinds of matter, since this alteration is clearly due to its chemical instability; and numerous inorganic bodies are prone to undergo similar change.

It is now well established that fish and amphibians may be frozen and kept in this condition for long periods, perhaps indefinitely; and yet, when thawed with sufficient care, they are found to be alive and to have suffered in no way by the process.

PECTOUS AND GLACIAL DEATH.

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The ova of salmon have been transported in this way to the antipodes, and have retained their vitality perfectly.

To all appearance, the albumen of an egg solidified by boiling and that of one consolidated by freezing are identical; yet the two differ in this, that the former cannot be restored to fluidity without undergoing chemical change, whilst the latter readily returns to the fluid state without its composition being altered. No less an authority than Dr. Richardson* has proposed the terms "Pectous and Glacial Death" to designate these two conditions, thus recognizing their close affinity.

These and similar facts go far to show that the difference between dead and living organic matter is a chemical difference, or a difference of molecular arrangement giving rise to a change in the action of energy upon it. Exact parallels are found in the magnectic properties of iron and some of its compounds, as an alteration of chemical constitution renders these insusceptible of the influence of the magnetic force—and in the electric conductivity of copper wire, which is materially affected by its hardness and ductility.

Hence it appears that, although living matter or protoplasm is an extremely remarkable body, it is

* "A Lecture on Experimental Medicine," published in the 'Medical Times and Gazette,' Feb. 1871, p. 181.

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