force; that as organic matter is so much matter taken from the common fund of mineral matter, and for a brief space raised into a higher sphere, only to be again returned, so also life force is only so much force taken from the common fund of natural forces, raised for a little while into a higher sphere, only to be returned again at death into the common fund of force from which it was taken. Thus force as well as matter circulates eternally in ever recurring cycles. (b) Respiration. Decay, as we have seen, gives rise to a circulation between the organic kingdom as a whole and the atmosphere. But a portion of the matter embodied in the organic kingdom does not wait for decay, but finds its way back to the atmosphere by a quicker route through animal excretions; certain of the elements, carbon and hydrogen, through respiration, and certain others, as nitrogen, through other excretions. It is the circulation of carbonic acid produced by respiration which alone we propose here to consider. This animal function gives rise to a still more beautiful and complex circulation between the three kingdoms, animal, vegetable and mineral. In order to make this circulation perfectly clear, I must briefly explain two points in physi ology, viz: first, the absolute dependence of the animal upon the vegetable kingdom; and, second, the necessity of respiration. 1. As already several times stated, the food of plants is mineral matter in the form of carbonic acid, water and ammonia. From these ingredients plants create organic matter and form their tissues. Animals cannot thus make organic from mineral matter for themselves, but feed upon organic matter already made to hand by plants, and with this organic matter they form their tissues. Thus plants feed upon animal matter and change it into organic matter; animals feed upon vegetable organic matter and change it but slightly to form animal organic matter. Thus all matter may be said to exist on four planes raised one above the other. The first and lowest is the plane of chemical elements; the second, the plane of chemical compounds or mineral kingdom; the third, the plane of vegetable life, and the fourth, the plane of animal life. Now, it is a curious fact that there is a special force whose function it is to raise matter from each plane to the plane next above, and to preside over the changes which occur on the latter plane. Thus it is the special function of chemical force (affinity) to raise matter from number one to number two (combine elements to form compounds), and to preside over all the shiftings, the changes and reactions occurring on number two, and which, together, constitute the science of chemistry. It is the special prerogative of vegetable life force to lift matter from number two to number three, and to preside over all the changes on number three, which, together, constitute the beautiful science of vegetable physiology. Finally, it is the special prerogative of the force of animal life to lift matter from number three to number four, and to preside over all the changes occurring on this last and highest plane, which, together, constitute the complex science of animal physiology. But there is no force in nature which is able to raise matter at one step from number one to number three, or from number two to number four, without resting on the intermediate plane and there receiving a fresh accession of force of another kind. It is impossible for plants to feed upon the elements, carbon, hydrogen, oxygen and nitrogen, but only on compounds of these, carbonic acid, water and ammonia. It is equally impossible for See my paper "On correlation of physical, chemical and vital forces."-Am. Jour. Science and Art, vol. 28, p. 305; and Lond., Edin. and Dub. Phil. Mag., vol. 19, p. 133. animals to feed upon mineral matter, but only on organic matter. Thus each plane feeds upon the plane below. There is a profound reason for this, based upon the principle of conservation of force, but it would carry me too far into the higher regions of science to explain farther. I am only anxious to impress upon you the absolute dependence of animals upon plants, and of plants upon the small quantity of the compounds, carbonic acid and ammonia, in the atmosphere. 2. Again, the animal body is distinguished from that of plants by a state of constant and rapid internal waste. Decomposition is not put off until death, but is going on incessantly during the whole life. All the functions and activities of the animal body are performed at the expense of the tissues, or at least by the decomposition of organic matter of some kind. Every movement we make, every thought we think, every emotion we feel, destroys a portion of our body. In proportion to the energy of our activity is also the rapidity of our internal decay. Now, this internal waste creates two necessities. The first and most obvious necessity is food to repair the waste. Of course the amount of food must be in proportion to the amount of waste, and, therefore, to the energy of the life. Thus the mature animal body may be compared to a pool, into which and out of which runs an equal stream. The water changes but the pool remains. The larger the stream, the more rapid the change and the fresher the water. Or it may be compared to a temple of beautiful proportions and complex structure, upon which two adverse forces, the force of life and the force of death, are engaged in deadly struggle, the one tearing down, the other building up, the struggle ceasing only with life and therefore with the final triumph of death. Now, no such internal waste has been proved to exist as a necessary condition of vegetation. Plants take food to grow, and they grow therefore without limit. Animals, in a mature state, take food, not for growth but for repair and animal activity. The second and far more urgent necessity created by this waste is respiration. The waste matters of the body are very poisonous, and if not removed would quickly destroy life. The two principal waste pipes of the body are the kidneys and the lungs; through the one, the waste matters are carried out in the form of solution, through the other in the form of gas-in the one case they are washed out, in the other they are burned up. Now, of the CHO and N of the decomposing organic matter of the body, only the Cand H are removed through the lungs. These, as already stated, are burned up. The oxygen taken in by inspiration unites with the carbon and hydrogen of the waste and form carbonic acid (CO2) and water (HO). Thus, respiration is a true process of combustion of carbon and hydrogen, and animal heat is the result of this combustion. Life has been often compared to a burning candle; see the truth of the comparison -in both cases carbon and hydrogen unite with oxygen of the air and form carbonic acid and water, and produce heat. Con See, then, the opposite relations of plants and animals to organic and mineral matter, and the beautiful circulation which results. Plants create organic matter, animals consume it; plants decompose carbonic acid, water and ammonia, animals recompose the same substances. fining ourselves to carbonic acid, plants take carbonic acid from the air, decompose it and retaining the carbon as a portion of its tissues, returns the useless oxygen to the air; animals take carbon from the plant in *See paper on correlation of physical, chemical and vital forces, already referred to. food, and oxygen from the air in respiration, unite these together and thus re-form carbonic acid and return it to the air. See, again, the beautiful relation of mutual dependence which exists between these two kingdoms. Carbonic acid is the necessary food of plants but a deadly poison to animals, while oxygen is the vital air of animals but useless to plants. Plants, therefore, are constantly purify. ing the atmosphere for animals, by removing the carbonic acid and returning oxygen in its place, while animals are as constantly replenishing the air with carbonic acid, the necessary food of plants. Thus plants are dependet, at least to a great extent, on animals, for the supply of carbonic acid, a portion of their food; while animals are dependent upon plants for the withdrawal of carbonic acid produced in respiration and the replacing of it by oxygen, and still more completely for the whole of their food. Thus plants might live, though not so well, without animals, since carbonic acid and ammonia are produced also by decay; but animals cannot live without plants. Both of these circulations (a and b) are beautifully illustrated by an aquarium. By this term I mean a reservoir containing both water and air, and the whole hermetically sealed, with a glass cover, admitting light and heat but not air-allowing ingress and egress of force but not of matter. In such a sealed reservoir, properly constructed, a due proportion of vegetable and animal life may continue in successive generations without limit; the plants supplying both food and oxygen for the animals, and the animals carbonic acid and ammonia for the plants, thus producing perpetually the circulation referred to. Again, the plants and animals of one generation take the carbonic acid and ammonia of the air inclosed, embody it in living forms, and again return it in death and decay. The next generation again takes the same, embodies it and again returns it. Thus the same extremely limited amount of carbonic acid and ammonia is worked over and over again, in successive generations, in a perpetual circulation. The earth itself, with its atmosphere and living organisms, is a great aquarium, sealed hermetically by vacuous abyss of interplanetary space, through which force may enter and return, but not matter. Here also, therefore, and within these narrow limits, the same small quantity of matter circulates endlessly, although the force of this circulation is derived from without Two very brief reflections and I pass on to other illustrations of the law of circulation. It is obvious, and therefore universally recognized, that the law of death necessitates the law of generation; but it is equally certain, though not so generally perceived, that, conversely, the law of generation necessitates the law of death. The quantity of matter to be embodied in living forms is limited; if, therefore, the embodiment goes on by continued generation, the supply must go on pari passu by death. All the forces conspicuously modifying the surface of the earth as great geological agents are divisible, as to their source, into two general kinds, viz: earth force and sun force. Earth force shows itself in volcanoes, earthquakes, upheaval of continents and subsidence of sea bottoms, and all other phenomena connected with the interior heat of the earth, and classed by geologists as igneous agencies. Sun force shows itself on all the circulations of matter occurring on the surface of our earth, and classed by geologists as aqueous and organic agencies. Sun force lifts water from the ocean to the clouds, whence it again runs downward back to the ocean. Sun force rarifies and lifts air from the heated surface of the earth, and thus determines its continual circulation Rivers, ocean currents and winds are all driven by the sun, and in our water-mills and wind-mills we utilize sun force. Finally, though less obviously, not less certainly, does this same force determine that higher and more subtle circulation which produces in its course the phenomena of life. As sun heat lifts matter from the sea to the clouds, which thence by a lower force (gravity) runs back again to the sea, so also sun light, transformed into vital force in the green leaves of plants, lifts matter from the mineral into the organic kingdom-from plane number two to plane number three-whence, then, by a lower force (chemical) it runs downward, again seeking its natural level. As sun heat disappears-is consumed-in doing the work of converting water into vapor, so also sun light disappears-is consumed-in doing the work of converting mineral into organic matter. As the clouds represent not only so much matter taken from the sea, but so much force taken from the sun, so also the organic kingdom represents not only so much embodied atmosphere, but so must embodied sun force. In our steam engines and all machinery in which force is developed by the combustion of wood or coal, we are again only utilizing sun force. Thus all the circulation on our earth, and all the machinery used by man, is driven by the sun. It is a curious and suggestive fact that as astronomy, the science of the stars, is older and more perfect than geology, the science of the earth, so also in geology the phenomena produced by sun force is much better understood than the phenomena determined by earth force. 4. Planetary and Stellar Motions. If, now, leaving the surface of the earth, we rise into the regions of astronomy, we find there a continuous series of illustrations of the law of circulation; nothing but cycle rising above cycle in continued succession. In every case, circulation of matter determines a cycle of events or phenomena. The rotation of the earth on its axis determines the daily cycle-morning, noon, evening and night, to be again succeeded by morning in endless succession. The revolution of the earth about the sun determines the annual cycles: spring, with its buds and blossoms; summer, with its leafy boughs; autumn, with its clustering fruits and waving corn, and winter, with its deathsleep, to be again followed by the awakening of spring in endless round. Above and beyond these, astronomers tell us of still higher cycles: the magnetic cycle of eleven years; the lunar cycle of nineteen years; cycles of planetary perturbations of greater and still greater period, rising one above the other, wheel within wheel, until we can no longer follow them. Beyond and above these are the stellar motions, producing cycles whose periods the mind of man cannot even conceive. 5. Society. Man, proud man, thinks himself an exception to this universal law; not, indeed, as an individual, for, alas, the short cycle of the individual life is too evident; but as a race. Man is endowed with a spiritual nature, and therefore he imagines, in his collective capacity as a race, he moves straight onward and upward forever. But, no; the spirit of man is entangled in matter, and, therefore, conditioned by its laws. The movement of society, therefore, is also cyclical. Standing on the swift car of progress and feeling the backward rush of air against his face, man is gloriously conscious of the onward motion, but perceives not the wide sweeping curve of the track which finally brings him back to his former position. Only in proportion as the higher spiritual nature of man is freed from slavish bondage to his lower, sensuous and material nature, can we hope to approximate steady, onward, rectilinear progress. We all fondly hope that the race will eventually realize this condition that social organization and social movement will be reduced to law, and sociology become a science; but thus far, at least, history reveals nothing but cyclical movement. Liberty degenerates into license; license passes into anarchy; anarchy necessitates despotism, and despotism results finally in revolution and the re-establishment of liberty; and so the cycle goes round. The phases of civilization embodied in great nations-Egyptian civilization, Persian civilization, Greek civilization, Roman civilization-successively rise, grow, culminate, decline and perish; and so the cycle goes round. A great thinker founds a system of philosophy; ardent followers join in the eager search after truth, but gradually, under the influence of criticism, the system disintegrates and dies. Again, another great thinker; another new system; again eager followers; again eureka! eureka! is heard on all sides; but, again, unsparing criticism, disintegration and death; and so the cycle goes round. German philosophy is now passing through phases similar to those of Greek philosophy, and thus has only repeated the phases of the still more remote Hindoo philosophy. In all this "there is nothing new under the sun." Is there, then, no exception among purely human works to this law of cyclical movement? Are we, then, in all our human strivings, like the -"dog which turns the spit, His own weight brings him down again?" Is there no exception to this melancholy law? Yes; there is one, viz: science. Amid the cyclical movement of society, the rise and fall of nations and civilizations, the flux and reflux of opinions, the revolutions of all kinds which agitate like a seething cauldron the popular mind, science alone-because it is the simplest and purest embodiment of the human intellect, unaffected by the passions which mingle with all other pursuits-science alone, among all human works, moves steadily onward and upward, ever increasing in grandeur and beauty. Like a magnificent temple, grandly and steadily it rises under the busy hands of thousands of eager workers, the greatest monument of human genius; the only temple ever erected by man worthy to be dedicated to the Great Author of nature. Thus stands science, ever rising, ever increasing, the pledge of human progress and its type. WORK. We have thus briefly traced the law of circulation or cyclical movement through all the departments of nature. But in circulation, do all things return precisely to the former condition? Besides the mere work of circulation itself, is there no permanent work accumulating from cycle to cycle? There is in all cases such work accumulating. Water and Air. In the circulation of water and air, besides the immediate work of circulation itself-besides watering our meadows and turning our mills and tempering the air and blessing all, there are certain enduring results: 1. Saltness of the Sea. Every drop of water which falls on the land surface bears with it to the sea certain saline matters dissolved from the rocks and soils. It rises again in the form of vapor, as pure distilled water, to again fall and bear back its tribute of salt. Thus, throughout all geological times, circulating waters have slowly and steadily leached out soluble matters from the land and accumulated them in the sea. The salting of the sea has therefore been one appointed work of circu |