ON THE PHYSICAL HISTORY OF CHAPTER THE FIRST. INTEREST OF THE STUDY OF MAN. As a motto for our Articles on this subthe line of the poetject, we might most appropriately adopt "The proper study of mankind is man.' We do not, however, wish it to be inferred that our own subject is above all others that which ought peculiarly to interest mankind. In fact, the expression of the poet is rather an assertion of what is, than what ought to be. There are many reasons why man should be, to himself, the most interesting and absorbing of all studies. Surrounded by an external creation constantly suggestive of thoughts of the beautiful, the grand, and the sublime— man yet turns his mind upon himself, and finds that it is in his own wonderful nature that he must look for the true source of the beauty and the wonder of the external world. The heavens and the earth are alike unfolded to the eye of lower beings; but they kindle no emotions in their consciousness, and add no happiness through thought to their being. ple, and we shall undertake much more. Though we shall depend less upon external aid, we shall have fewer difficulties to conquer. We shall not only have a better estimate of our strength, but we shall exert it with greater calmness. The world is apt to think that simplicity indicates a want of perceptive power, and very readily INTRODUCTORY.-ON THE OBJECTS AND laughs at the ignorance indicated. It is is indeed true that simplicity is often in blessed ignorance of many idle, frivolous, and vicious things, but it is the better enabled to know things of an infinitely greater value. Above all things, it enables the possessor to know himself or herself. If ignorant of a crowd of details which it cannot comprehend, how often is this compensated by a clear and perfect knowledge of what is noble, sublime, and good. It frequently happens that persons of very estimable character in many respects, run into error by a want of simplicity, even in a moral point of view. They occupy themselves with subtle distinctions, and give themselves up to anxious investigations; burden themselves with numerous minute observances, and obstruct the action of their faculties by unnecessary rigour, and by a minute and tyrannical self-scrutiny. They forge for themselves useless chains, and impose upon themselves gratuitous and unnecessary duties. They unjustly suspect themselves, raise doubts, imagine possible interpretations of their most praiseworthy actions, and have an excessive distrust of themselves. Thus real self-advancement is frustrated by the very means taken to secure it. The conception of what constitutes real good becomes confused, and the power to seize it is weakened. The practice of the minor virtues is neglected, and extraordinary occasions are sought for. Such people forget that perfection depends neither upon a happy concourse of circumstances, nor on the size of the theatre. And here we are struck by a fact which tells powerfully for the truth of the principles we are advocating, viz., that whilst simplicity of character is one of the fundamental essential conditions of moral advancement, it is accessible to all men, whatever may be their rank or circumstances, and even more easily attainable by those least favoured of fortune. The very obscurity in which they are placed favours the preservation of simplicity. The thought and consciousness of man, his soul and understanding, have ever been to him a theme of intensest interest, and must, whilst the human race continues on the earth, occupy a first place in its regards. Setting aside the thought by which man is enabled to reason and study as a part of his nature, his relation to the rest of creation must give him a deep interest in his race. When we regard the external world with attention, we see clearly that the distinction between the material world and that of organic existences is very great. However wonderful may be the movements of the heavenly bodies, and the grand features which mark the physical geography of the surface of the earth, we instinctively feel that there are higher agencies and a more mysterious power at work in the growth and functions of the simplest plant of the field. We speak of rivers, seas, rocks, and mountains as existing, but of plants as living. We feel that there is more proof of Wisdom and Goodness in the creation of a plant, than of a stone. We then pass on from plants to animals, and we observe in passing up from class to class, till we arrive at the highest-the gradual complication of their structure, the increase of their functions, the numerous new relations they sustain to the mineral and vegetable world, and we feel that animals are more wonderful structures than plants, and they accordingly take a stronger hold of our sympathies, and demand a larger part in our affections. If then we carry on our view to man, and find him embracing all that is wonderful in the creations below him, and exhibiting new and unlooked-for adaptations to his position in the world-we shall see how it is that man, as an object external to himself, becomes one of the most interesting and absorbing subjects of his own study. Our object, however, in these Papers, is not to speak of man generally, but in a particular point of view. The study of the frame of man constitutes the science of human anatomy; the functions it performs, that of physiology. The laws which regulate the exercise of his mind and feelings constitute the field of inquiry for the metaphysician; and the actions of man, as they originate in moral causes in nations and communities, constitute the science of history. But, independent of all these, we find that the relation of man to the rest of creation-the effects of physical causes upon his habits and manners-the varieties of structure and appearance which he presents on the surface of the earth-and, in fact, all that relates to his external physical character, constitutes a subject of deep interest, and one that is at the present moment engrossing a greater amount of attention than it has ever before done in the history of the world. The causes of this attention are numerous and interesting, as showing what are the questions involved in this subject. To a few of these we shall allude: : In the first place, we may make the assertion that there is more than one species of man inhabiting the earth. We shall have, presently, to examine this question, and we refer to it here as showing the ground which our subject will take us over. The unity, or multiplicity of the human species, is one of great interest, and also of importance. The supposed multiplicity has been adopted as an argument against the truth of the doctrines of Christianity; and the alleged specific distinctions between the Negro and the Anglo-Saxon have been employed to justify the condition of slavery which the latter race, in America, has succeeded in forcing on the former. The great increase in our knowledge of the races of men, through travellers and missionaries, have very much contributed to increase the interest in this subject. Travellers have also gone out prepared to make the necessary inquiries, and have had their attention directed to those points which could throw light on some of the interesting problems to be solved in the physical history of man. The spread of the missionary spirit in Great Britain, during the last fifty years, has contributed a vast mass of useful materials from all the countries which have been visited by the preachers of the Gospel. The materials thus collected have been collated, and an increased interest given to the history of tribes and races in various parts of the world. Nor should we omit to mention the increase of our colonial empire. Every day is bringing our soldiers and sailors into more close contact with people whose names, only a few years ago, were unknown among us. In the narratives of voyages, settlements, expeditions, and governorships, we have had lately constantly introduced to us, new races, with new habits and languages; and thus has increased the stock of information, as well as our interest about the physical history of man. The question of race, however, is not confined to distant parts of the world. In various parts of America we see the red, the black, and the white man living in the same communities. Although in Europe we have no large part of the population either red, or black, yet we have distinctive races. The Celtic Irish, the Saxon English, and the Hebrew, are familiar enough to ourselves; and the distinction of Saxon, Celt, and Jew, is seen as prominently on the continent of Europe. Further distinctions exist there, as we shall see, and the study of these distinctions have gained a deep interest in these days; as it is the opinion of many, who have studied these subjects gravely, that the late revolutions on the Continent have been more the result of few substances from which it can be condifference of race than any other cause. On this ground, at least, we may explain some of the more striking events of the last two years, and the increased interest which has been taken in our subject. Again, the application of the study of languages, as a means of ascertaining the relation of races, and the success which has attended these researches, have contributed much to increase the interest taken in the physical history of man, as well as to direct towards this subject the minds of a number of intelligent men, between whose pursuits and those of the naturalist, and the natural philosopher, it acts as a bond of union and a mutual ground of research. Lastly, we may mention emigration. Large numbers of our countrymen are every year seeking homes in untried lands, and amongst different races of people, and the desire of knowing something more of the latter has prompted the perusal of works on the Natural History of Man. Besides this, another question of interest arises to the European emigrant, and that is -in what climates, and under what circumstances, can his race expect to succeed, and carry on the great objects of existence? These are some of the circumstances which have lately tended to give an impulse to the study of the physical history of mankind, and it will be seen, from these introductory remarks, that, independent of its special interest as a study, important practical consequences result from a knowledge of the physical conditions which influence the races of men. veniently separated, so as to exhibit its properties in the free state. One mode of obtaining it has already been mentioned, viz., from the red oxide of mercury (page 24). If a portion of this substance be introduced into the tube apparatus (Fig. 3, p. 26), and strongly heated by the flame of a spirit-lamp, the oxygen will be given off in the form of gas, and may be collected in tubes, or bottles, over the pneumatic trough; the mercury being set free at the same time, and collecting in liquid globules on the cooler part of the tube. This, however, is not to be recommended as a practical mode of obtaining oxygen; for oxide of mercury is rather an expensive substance. I mention it here chiefly for the sake of once more calling your attention to the nature of chemical decompositionto the splitting up of a substance into two others differing altogether in appearance and in general properties from each other, and from the original substance,—and also for the sake of pointing out an important law of chemical combination, which this decomposition, from its extreme simplicity, will perhaps illustrate better than any other. The law in question is, that chemical combination always takes place in definite proportions; or, in other words, the proportions in which elements unite to form a given compound are fixed and invariable. Thus, if twenty-seven grains of the red oxide of mercury were decomposed in the manner just described, and the mercury and oxygen separated from it, were accurately weighed, which may be done, though I cannot enter into the mode of doing it at present, it would be found that the twenty-seven grains of the red oxide were made up of twentyfive grains of mercury and two grains of oxygen; and if any other quantity of the oxide were similarly treated, the quantities of mercury and oxygen obtained from it would bear the same proportion to each other. The red substance always contains of its weight, or 7 per cent of oxygen. This fixed proportion of the elements of a compound constitutes one of the most important distinctions between chemical combination and ordinary mixture. Substances may be mixed in any proportion whatever. For instance, any quantity of milk may be mixed with any quantity of water; or again, you may mix flowers of sulphur and iron filings in any proportions whatever; but put the mixture into an iron ladle over the fire, and presently you will see it all become red hot, and if you then leave it to cool, you will find that the sulphur and iron have united to form a new substance different from either of them. Now, in this substance, called sulphide of iron, the sulphur and iron are united exactly in the proportion of four parts of sulphur to seven of iron; any quantity of sulphur more than sufficient to form this proportion will not unite with the iron, but will be driven off in vapour; and any excess of iron will remain behind quite unaltered. These examples will suffice for the present to give an idea of the LAW OF DEFINITE PROPORTIONS; it is quite universal; there is no exception to it; every chemical combination and decomposition that we come to will furnish an additional instance of it. To return to the preparation of oxygen -one of the most convenient and economical sources from which this element can be obtained is the black oxide of manganese. This substance, which is found ready formed in nature, in great abundance, and is used for a variety of purposes in the arts, is composed of twenty-seven parts of a metal called manganese, and sixteen parts of oxygen; so that every forty-three grains of it contain sixteen grains of the latter element. Now, when this oxide is heated to bright redness, it gives up, not the whole, but only one-fourth of its oxygen-that is to say, four parts out of the sixteen; and there is left a dark brown substance, containing twenty-seven parts of manganese, united with twelve parts of oxygen, which cannot be separated by any degree of heat whatever. Here, then, we see an essential difference between the oxygen compound and the two metals-mercury and mangathe union between oxygen and mercury being completely broken up, even by a moderate heat, whereas a much higher temperature effects but a partial separation between oxygen and manganese. It appears, then, that the force by which the elements of compounds are held together admits of different degrees. nese; water escapes rapidly, and causes the substance to froth up considerably, frequently stopping up the gas-delivery tube, and occasioning great inconvenience. The oxide must be put into an iron bottle, having a flexible tube adapted to it, and heated in the fire. The gas may be collected over water, in the pneumatic trough. A third mode of obtaining oxygen is from a substance called chlorate of potassium; or, more commonly, chlorate of potash. This substance is composed of oxygen, the metal potassium; and a third element called chlorine. It contains rather less than two-fifths of its weight of oxygen, the whole of which is given off on the application of a moderate heat. For small quantities, the tube-apparatus (Fig. 3) may be used, and heat applied by means of a spiritlamp; the tube should be of hard glass, free from lead-common flint-glass is almost sure to melt. For generating larger quantities of the gas, the most convenient apparatus is a Florence-flask, mounted, as shown in (Fig. 2). It should be coated at the bottom with fire-clay, to enable it to withstand the heat better, and heated over a charcoal fire. This is the best of all methods of obtaining oxygen if it be required perfectly pure: the quantity which it produces is likewise very considerable; an ounce of the chlorate of potassium will yield about two gallons of the gas. The residue is a white solid substance, containing nothing but chlorine and potassium. The decomposition of the chlorate of potassium is, however, effected far more easily, and at a much lower temperature, if it be mixed with a little black oxide of manganese in fine powder. In that case it is not necessary to coat the flask with fireclay, and it may be heated by a spirit-lamp without risk of melting the glass. The materials should be dried before they are introduced into the flask, otherwise there will be danger of fracture from drops of water running down the sides and coming in contact with the heated part. The action of the manganese in this process is somewhat mysterious; it undergoes no change whatever, the whole of the oxygen evolved The oxide of manganese, used for obtain-proceeding from the chlorate of potassium. ing oxygen, should be, not in the state of powder, but in small lumps (it is sold in both forms); for the powder is always damp, and when it is strongly heated the This, however, is only one out of many cases in which the decomposition of one substance is facilitated by the mere presence of another. The oxygen obtained by this process is not quite so pure as that which is evolved from chlorate of potassium alone; for the oxide of manganese is always, more or less, contaminated with particles of carbonaceous matter, and these, by contact with the free oxygen, are burnt and converted into carbonic acid, which, being a gas, passes over with the oxygen, and renders it impure. The same impurity is generally present in oxygen obtained by heating oxide of manganese in an iron vessel. In fact, as already observed, the only method of obtaining oxygen perfectly pure is to heat the chlorate of potassium alone; but if perfect purity is not an object, the method last described is by far the best that can be adopted, at least for preparing noderate quantities of the gas; for very large quantities, the oxide of manganese, being a much cheaper substance than chlorate of potassium, is the source most generally used. Oxygen, in the free state, is a transparent and colourless gas, perfectly destitute of taste and smell. It has never been reduced to the liquid or solid state by cold or pressure; in fact, when separated from other substances, we know it only as a gas. It is somewhat heavier than common air; a vessel capable of containing 10,000 grains of air will hold 11,057 grains of oxygen gas; so that the weight of a given volume of common air is to that of an equal volume of oxygen, as 10,000 is to 11,057; or as 1 is to 1.1057. Now, the weight of any substance compared with that of an equal volume of another substance, selected as a standard, is called its specific gravity; and for gases it is usual to take atmospheric air as the standard or unit of specific gravity. Referred to this unit, the specific gravity of oxygen is 1.1057. The practical method of determining the specific gravities of gases will be explained hereafter; for the present it is sufficient to give a distinct idea of the meaning of the term. The most striking property of oxygen has already been mentioned (p. 24), viz.that it supports combustion much more readily and vividly than common air; so that a slip of wood introduced into it with the merest point in a state of glow, immediately bursts out into flame, and burns with great brilliancy; the kindling of the flame is always attended with a slight detonation. All substances capable of burning in the air, burns with increased brilliancy and rapidity in oxygen gas; and many which will not burn continuously in common air, are rapidly consumed when introduced into oxygen, with a single point in a state of glow. If a thin iron wire, bent into a spiral form, and having a small piece of German tinder attached to its lower end, be thrust into a large cork, wide enough to cover over the mouth of a bottle containing oxygen, but not to fit into it; the tinder set on fire, and the wire introduced into the gas; heat will soon be imparted from the tinder to the wire, sufficient to make it burn with great brilliancy. As the iron burns it combines with the oxygen, and forms an oxide, which falls off from time to time in fused drops. It is best to have a little water at the bottom of the bottle, to prevent these hot globules from coming in immediate contact with the glass; without this precaution the bottle is almost sure to crack. A piece of steel watchspring, introduced into the gas in the same manner as the iron wire, burns with peculiar brightness, throwing off the most splendid scintillations. The oxide of iron formed in this combustion is quite different in appearance from the metal itself, being destitute of the high lustre peculiar to metals, and so brittle that it crumbles with ease between the fingers. Every twenty-nine parts of it contain twenty-one of iron, and eight of oxygen: it is identical in composition with the black scaly substance, which forms on the surface of a bar of iron heated to redness, and then left to cool in the air or plunged into water. A strip of zinc-foil, set on fire by holding it in the flame of a candle, and then plunged into oxygen gas, burns with a bright white light, and is coverted into snow-white flakes of oxide of zinc; every five parts of which contain four of zinc and one of oxygen. If a piece of charcoal be attached to the end of a stout iron wire, thrust through a large cork, then held in the flame of a candle or spirit-lamp, till one or two points of it become red hot, and plunged into a vessel of oxygen, it will burn with a quiet red light if it consists wholly of wood-charcoal, but with brilliant scintillations if it has bark attached to it. This combustion differs from those above described, in not yielding any visible product; the charcoal |