« PreviousContinue »
have just erected to him (1854) a bronze statue, in the new Piazza of his romantic little metropolis Corte. It is not illimagined. The General stands in the sort of half-military costume which he wore, with broad-skirted coat, and bottes à revers, in act as if about to address an assembly. The execution, we must confess, falls somewhat short of the conception, and the hero has rather the air of a bourgeois endimanché.' But it is the pride of that secluded district, far and near; and among the many sturdy, wild-looking mountain figures which you may observe around it, indolently gazing for hours, you will find scarce a man who will not recount to you, with more or less of detail, the main outlines of the life of the great Generale e Gobernatore della Corsica, who expelled the Genoese and the French, raised his country to independence, led its sons to battle as a chosen champion, governed and judged it as a chosen sovereign, and left the savings of his poverty to educate its children.
2 vols. post
Art. VII. - The Chemistry of Common Life. By JAMES F. W.
JOHNSTON, M.D., F. R. S. L. & E., Reader in Chemistry and Mineralogy in the University of Durham.
8vo. Blackwood : 1855. The common life of man is full of wonders, chemical and
physiological. The manner and means of our existence, every necessary we consume, -every material comfort we enjoy, -all the parts and functions of the bodily organs through which we enjoy them, - everything, in short, which concerns our daily individual life,-abounds in admirable marvels, which chemistry and chemical physiology disclose. Dr. Johnston has described and discussed these subjects, at once so familiar and so obscure, — so universally felt and so imperfectly understood, in one of the most agreeable and instructive publications of the present day. We shall follow him rapidly through the general divisions of his subjects, and terminate our observations by some of the examples which the Doctor draws from the habits and wants of our daily lives.
If we begin, for example, with that universal air which floats around us, — wbich expands our lungs and permeates every tissue of our bodies-modern chemistry informs us that, though considered simple and elementary by the ancients, this air is a mixture of at least three elastic fluids, equally subtle and invisible, and equally essential to the purposes which the atmosphere is intended to serve. These are the now well-known gases nitrogen, oxygen, and carbonic acid. In the first, flame dies and no life can persist; in the second, bodies burn and animals live with great intensity; in the third, both life and flame are extinguished. Though so different in their properties when taken singly, the admixture of them, which forms our atmosphere, is adjusted — in kind and in the relative proportions of each — to the condition of things both living and dead, which now obtains on the surface of the earth.
Did the air consist of nitrogen only, the sun's rays would be the sole source of heat wherever the atmosphere extended, and no existing plant or animal could flourish on the globe. Were it formed of oxygen only, fire, once kindled, would refuse to be extinguished, and conflagration would spread, till everything combustible in the earth was consumed. Did it consist of carbonic acid only, death and comparative stillness would reign everywhere, and the production of light and heat such as we can now command, would be utterly impossible. But the happy mixture of the three gases which now prevails, renders everything possible. Under their united inAuence the rocks crumble to form a fertile soil, plants flourish to cover it with verdure, animals live to adorn and enjoy it, and light and heat are awakened or extinguished at will. The inactive nitrogen dilutes the too energetic oxygen, so as to make animal life longer, and to subject living fire to human control; while the poisonous carbonic acid is rendered harmless to animal life by the very small proportion in which it is mixed with the other airs.
One of the most admirable, indeed, of Nature's wonders in the material world, is the purpose served by this carbonic acid gas. Itself poisonous in a high degree, it can be breathed by man with impunity only in very minute quantity; that is, in an extreme state of dilution. Hence, the atmosphere in which man lives contains only one gallon of this gas in every 2,500. And so small is this quantity, that the weight of carbon in this form which the whole atmosphere contains, amounts only to thirty-three grains out of the fifteen pounds of air which press upon every square inch. Yet by this comparatively minute quantity all vegetable life is nourished and sustained.
Look out in the coming spring-time at the bursting bud. Watch how beneath the mid-day sun, the tiny Jeaflet spreads out its yellow surface to the favouring rays. See how from day to day its hue becomes greener, and its several parts increase in size. This growth will continue till closing summer finds the little bud changed into a magnificent plant, clad with copious leaves, and successively blooming with gay flowers, or borne
down by a burden of tempting fruit. Autumn will succeed, to stop the growth and give a new colour to its leaves; and chill winter will strip it of all its leafy pride, and leave it naked as when spring time began.
Such is the yearly plant-life, as seen by the ordinary cultivator, or watched with daily care by the lover of vegetable nature. But, beneath this outer open life, there is an inner secret life which the common eye does not see. A constant invisible intercourse has all the time been taking place between the external air and the most hidden parts of the internal plant. No sooner does the little leaf burst the swelling bud, than a thousand unseen mouths open on its surface to suck in the airy food which now for the first time comes within their reach. These minute mouths (stomata) are scattered in millions over the leaf, now on its upper, now on its under side, and now on both, - according to the circumstances in which the plant is destined to live. Beginning with the first dawn of sunlight, they perpetually suck in carbonic acid from the atmosphere, and give off oxygen gas in nearly equal volume, till the sun goes down. Then, with a view to other chemical ends, and, obedient to the retiring sun, they change the nature of their work. While darkness lasts, they take back carbonic acid from the air, and give out again pure oxygen gas. And thus, day after day, the leafy labour proceeds, and by the aid of the raw materials which the working mouths thus incessantly carry out and in, other vital parts within the plant produce the varied forms of matter of which the vegetable substance consists. The solid stem is formed, as it were, of compressed and hardened air; and vast forests on a thousand hills thus steal from the atmosphere the carbonaceous matter of which they mainly consist.
But a marvel of wondrous forethought discloses itself as we interrogate more nearly this mutual relation between terrestrial plant-life and the air which surrounds it. The quantity of carbon in the air, as we have seen, is small; ---some thirty odd grains over every square inch. The active growth of vegetable matter over the entire surface of the globe, is able to convert the whole of this carbon into the substance of solid wood within the lifetime of a single generation of men. But hundreds of generations of men have already lived on the earth, and thousands of generations of other animals before him, yet carbon is as abundant in the atmosphere as ever, and vegetable growth, in similar circumstances, quite as luxuriant. There must, therefore, be some natural sources of supply from which carbonic acid gas flows into the air, as fast as the leafy mouths
withdraw it. These sources, also, must be watched and regulated, that they may not pour it in so fast as to increase unduly the natural proportion of this poisonous gas in an atmosphere which man and countless other animals perpetually breathe. These several conditions are beautifully fulfilled by a series of compensating natural operations, which, like the growth of plants, form a part of the existing system of things; and, like it, never cease to proceed at a duly measured pace.
Thus, plants die, and the carbon of their stems and leaves is gradually resolved again into carbonic acid by the gradual progress of decay, or by the quicker agency of fire. Or the plant is eaten by the living animal, and after many chemical changes within the animal's body, its carbon is breathed forth again from the lungs and skin in the form of carbonic acid. In these several ways the very same carbon which the plant-leaf has taken from the air, is again, in a great measure, returned to it. A certain small and indefinite proportion of their carbon is indeed yearly buried in the soil, or covered up in the depths of the sea, or accumulated in bogs and dismal swamps. But to make up for this, the earth itself, from bubbling springs, from myriads of unseen fissures, and from the open mouths of many volcanoes pours forth a ceaseless contribution of carbonic acid gas,- ceaseless, yet in such wise limited, that so long as vegetation lasts, it cannot render the atmosphere unwholesome to animal life. To the knowledge of these and many similar adjustments, the study of the chemistry of the air we breathe has gradually led us.
Turn now to the water we drink. In this admirable Auid, so clear, so bright, so grateful to the system, so healthful to the temperate, so necessary to all, — the delight of Grecian song, the charm of the Eastern paradise, — of this fluid, lauded with justice by the physiologist, and worshipped, not unduly, by the total abstainer, --chemistry tells us that three-fourths of our apparently solid bodies consist, and that it forms nearly as large a proportion of all living vegetables during the height and vigour of their growth. In this fluid, looked upon as elementary till nearly our own times, modern research has taught us to see the result of a subtle union between the oxygen we have spoken of, and another gas, to which the name of hydrogen (water-former) has been given. Kindle this latter gas in the air, and it burns with a pale flame. Hold a cold bell glass over the flame, and its under surface will become bedewed with moisture, and drops of water will trickle down its sides. Collect this water and submit it to a current of electricity; the liquid will disappear, and in its stead the two gases oxygen and hydrogen will remain. These experiments prove, first, that while burning in the air, the hydrogen unites with the oxygen of the atmosphere and forms water; and, second, that the water thus formed consists of these two gaseous constituents only, compressed and bound together by some incomprehensible connexion which it makes us no wiser to call chemical combination.
It is, indeed, incomprehensible how water, the enemy of fire, should itself consist of two gases, the one of which burns most readily, while the other is the great natural supporter of living fire. And it is equally strange that oxygen, so indispensable to animal life, should form eight-ninths by weight of a liquid in which few terrestrial animals can live for more than two or three seconds of time. By no known theory of physical or mechanical union can we satisfactorily explain how properties so new should be the result of such chemical combinations.
The chemical study of this water in its relations to animal and vegetable life presents new points of interest. The most important of its chemical properties are so familiar to us that we rarely think of them, and certainly do not sufficiently prize them. Pure water has neither taste, nor smell, nor pungency. It is neither sour like vinegar, nor sweet like sugar, nor alkaline like soda. It irritates no nerve of sensation, even the most delicate, nor is the tenderest part of the animal frame disturbed by contact with this universal fluid. It is thus fitted to penetrate unfelt into the subtlest tissues, and without causing the slightest jar to flow along the finest, most sensitive, and most hair-like vessels. It soothes and assuages wherever it comes, lessening inflammation, lulling pain, diluting unhealthy fluids within the body, and washing morbid humours and waste materials from the sickly and changing frame.
Again, as a cooling agent water is equally invaluable. In a dry and thirsty land we feel and acknowledge the pleasure of bathing our heated bodies in the sea or the running stream. But we are less sensible how it watches over us, as it were, every passing moment, dispelling each rising heat, and removing from the body every excess of warmth which might disturb the equable working of its many parts. Do we eat inflammatory food, or drink over-stimulating fluids, the excess of bodily warmth produced converts a portion of water into vapour, and the lungs throw it off into the air. Do we by hard labour, or other unusual exertion, exalt the temperature of the body, the same water again takes up the superfluous heat, and bathing with perspiration both skin and lungs restrains within due bounds the growing inflammation.