the port, s', will be uncovered by the inner edge of the valve, so that the steam in the right end of the cylinder will escape through the port, s', into the interior hollow of the valve, and thence into the exhaust-passage, e, 5 which leads either into the open air or into a condenser. This arrangement of the valve is shown in figure 9. If, when the piston has reached the end of its forward stroke, the valve be moved back to the corresponding

position on the other side, the steam from the boiler 10 can enter the cylinder through sand force the piston back from right to left, while the steam on the left of the piston will escape through s into the exhaust-passage. The sliding-valve is thrown backward and forward by the action of an eccentric moving on the same 15 axle as the crank-pin. This piece of mechanism, how ever, will be explained in another chapter.

II.

The Physical Basis of Life.

THOMAS HENRY HUXLEY, 1825-.

The substance of this paper was contained in a discourse delivered in Edinburgh Sunday evening, November 8, 1868, the first of a series of Sunday evening addresses upon secular topics, instituted by the Rev. J. Cranbrook. The paper was printed in 1871 under the title, "On the Physical Basis of Life," in Huxley's "Lay Sermons, Addresses and Reviews," and again in 1894 in the first volume of Huxley's collected writings, entitled "Methods and Results."

This piece of scientific exposition has three notable excellences. For one thing, the words are so simple, that, though not perhaps intelligible to a child, they may be easily understood by a man of fair education. Secondly, the divisions of the subject are distinct: in order to show that "a unity of power or faculty, a unity of form, and a unity of substantial composition " pervade "the whole living world," Huxley discusses each of these three unities separately. Lastly, he makes general statements vivid by the use of specific examples.

In order to make the title of this discourse generally intelligible, I have translated the term "Protoplasm," which is the scientific name of the substance of which I am about to speak, by the words "the physical basis of life." I suppose that, to many, the idea that there 5 is such a thing as a physical basis, or matter, of life

may be novel-so widely spread is the conception of life as a something which works through matter, but is independent of it; and even those who are aware that matter and life are inseparably connected, may not be 5 prepared for the conclusion plainly suggested by the phrase, "the physical basis or matter of life," that there is some one kind of matter which is common to all living beings, and that their endless diversities are bound together by a physical, as well as an ideal, unity. In 10 fact, when first apprehended, such a doctrine as this appears almost shocking to common sense.

What, truly, can seem to be more obviously different from one another, in faculty, in form, and in substance, than the various kinds of living beings? What com15 munity of faculty can there be between the brightlycolored lichen, which so nearly resembles a mere mineral incrustation of the bare rock on which it grows, and the painter, to whom it is instinct with beauty, or the botanist, whom it feeds with knowledge?

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Again, think of the microscopic fungus-a mere infinitesimal ovoid particle, which finds space and duration enough to multiply into countless millions in the body of a living fly; and then of the wealth of foliage, the luxuriance of flower and fruit, which lies between 25 this bald sketch of a plant and the giant pine of California, towering to the dimensions of a cathedral spire, or the Indian fig, which covers acres with its profound shadow, and endures while nations and empires come and go around its vast circumference. Or, turning to 30 the other half of the world of life, picture to yourselves the great Finner whale, hugest of beasts that live, or have lived, disporting his eighty or ninety feet of bone,

muscle, and blubber, with easy roll, among waves in which the stoutest ship that ever left dockyard would flounder hopelessly; and contrast him with the invisible animalcules-mere gelatinous specks, multitudes of which could, in fact, dance upon the point of a needle 5 with the same ease as the angels of the Schoolmen could, in imagination. With these images before your minds, you may well ask, what community of form, or structure, is there between the animalcule and the whale; or between the fungus and the fig-tree? And, 10 à fortiori, between all four?

Finally, if we regard substance, or material composition, what hidden bond can connect the flower which a girl wears in her hair and the blood which courses through her youthful veins; or, what is there in com- 15 mon between the dense and resisting mass of the oak, or the strong fabric of the tortoise, and those broad disks of glassy jelly which may be seen pulsating through the waters of a calm sea, but which drain away to mere films in the hand which raises them out of 20 their element?

Such objections as these must, I think, arise in the mind of every one who ponders, for the first time, upon the conception of a single physical basis of life underlying all the diversities of vital existence; but I pro- 25 pose to demonstrate to you that, notwithstanding these apparent difficulties, a threefold unity—namely, a unity of power or faculty, a unity of form, and a unity of substantial composition-does pervade the whole living world.

No very abstruse argumentation is needed, in the first place to prove that the powers, or faculties, of all kinds

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of living matter, diverse as they may be in degree, are substantially similar in kind.

Goethe has condensed a survey of all powers of mankind into the well-known epigram :—

"Warum treibt sich das Volk so und schreit? Es will sich ernähren

Kinder zeugen, und die nähren so gut es vermag.

Weiter bringt es kein Mensch, stell' er sich wie er auch will."

5 In physiological language this means, that all the multifarious and complicated activities of man are comprehensible under three categories. Either they are immediately directed towards the maintenance and development of the body, or they effect transitory changes 10 in the relative positions of parts of the body, or they tend towards the continuance of the species. Even those manifestations of intellect, of feeling, and of will, which we rightly name the higher faculties, are not excluded from this classification, inasmuch as to every 15 one but the subject of them, they are known only as transitory changes in the relative positions of parts of the body. Speech, gesture, and every other form of human action are, in the long run, resolvable into muscular contraction, and muscular contraction is but a 20 transitory change in the relative positions of the parts of a muscle. But the scheme which is large enough to embrace the activities of the highest form of life, covers all those of the lower creatures. The lowest plant, or animalcule, feeds, grows, and reproduces its kind. In 25 addition, all animals manifest those transitory changes of form which we class under irritability and contrac