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laws of proportional combination which are universally received as true by chemists. They are four in number, and refer to combination by weight; the laws of combination by volume being excluded from our present inquiry. Three of them were discovered by Dalton ; all of them were brought into new prominence by his labours; and his atomic theory, or rather hypothesis, as it should be called, is an endeavour to explain them, by assuming a peculiar ultimate constitution of matter, which absolutely necessitates their existence. These laws are based upon one, deeper and more fundamental than themselves, which is assumed in their enunciation, and is to the following effect :The same compound consists invariably of the same components. Water, for example, always consists of oxygen and hydrogen; common salt, of chlorine and sodium ; vermillion, of sulphur and mercury. Exceptions to this law were at one time thought to exist, in the case of certain minerals and native gems,

such as garnet, which seemed to exhibit constant physical characters, and yet to vary in their constituent ingredients. But Mitscherlich's discovery of Isomorphism not only solved the difficulty attending the consideration of these, but in the end supplied new confirmation of the law which at first it seemed to contradict. This, then, premised, we may enter at once on the consideration of the following laws:

The first of these is generally named the law of Definite proportion, but should rather be called the law of Constant proportion. It teaches, that the elements which form a chemical compound are always united in it in the same proportion by weight. Water not only consists invariably of oxygen and hydrogen, but the weight of oxygen present is always eight times greater than that of hydrogen. Whether we obtain it from lake, or river, or sea, or glacier, or iceberg; from rain, or snow, or hail, or dew ; from the structures of plants or the bodies of animals; whether it has been formed ages ago by the hand of nature, or is produced on the instant by mingling together its elements in the most random way, the ratio of its components is immutably the same: eight-ninths of its weight are always oxygen, and the remaining ninth, hydrogen. It is the same with every compound. Common salt always contains 35 parts of chlorine to 22 of sodium; marble, 22 of carbonic acid to 28 of lime; vermillion, 16 of sulphur. to 101 of mercury. In virtue of this law, a number can be found for every body, simple or compound, expressing the ratio in which (or in a multiple or submultiple of which) it combines with every other. Any series of numbers may be taken to represent these combining ratios, provided the due proportion is maintained among them, so that the number NO. I.

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for oxygen shall be eight times greater than that for hydrogen, that for nitrogen fourteen times greater, that for sulphur sixteen times, that for iron twenty-seven times, and so on, according to the relations which analysis brings out. Different scales of combining numbers are in use among chemists; but the only one we need consider is that which makes hydrogen 1, and counts from it upwards. The numbers in this scale are all small, and do not, in the majority of cases, go beyond two integers.*

It must not be forgotten that such tables represent relative, not absolute weights. Of the smallest possible quantity of oxygen which can combine with the smallest possible quantity of hydrogen, we know nothing; all that we are certain of is, ihat it is eight times greater than that of hydrogen, whatever that be. None of the numbers taken singly has any absolute value: the 16, for example, which, in tables of the kind we are discussing, stands against sulphur, does not represent 16 grains, 16 millionths of a grain, or any other absolute quantity; its value appears only when it is taken in connexion with the number attached to hydrogen, to which the quite arbitrary value of 1 has been given. We may give any value we please to any one of the elementary bodies we choose to fix

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for a commencement, and call it 1, 10, 100, , ), or any other integer or fraction ; but here our liberty ceases. The relation between the numbers is absolute, though their individual value is not; and from the settled figure we must count upwards or downwards, or both ways, so as to maintain inviolate the relative values throughout the series.

The law we are discussing, as we have already stated, is generally called that of definite proportion, but, as we think, erroneously; for it asserts something more than that the proportion in which the elements of a compound unite is definite ; it affirms, also, that it is constant, or always the same. The elements of a compound must be united in definite proportion. A definite weight of water, for example, must consist of a definite weight of hydrogen and of oxygen; but the proportion of these elements might be quite variable, so that one specimen of water should be found to contain 1 hydrogen to 8 oxygen; another, 8 hydrogen to 1 oxygen; a third, a moiety of either ingredient; and so on, ad infinitum.

* In conformity with the universal practice of chemists, in illustrating the laws of combining proportion, we have here, and elsewhere throughout this paper, employed round numbers, cutting off the decimal fractions, by which the exact combining proportions exceed or fall short of these. The equivalent of oxygen, for example, is not 8, but 8.01; that of nitrogen, not 14, but 14.06 ; and so on with many others. The equivalents of a few of the elementary bodies are round numbers: carbon is 6; calcium, 20 : the greater number are not.

The native garnet, to which reference has already been made, is always a definite compound; but the proportion of its ingredients varies within wide limits, so that while one specimen contains 27 per cent. of a certain constituent alumina, another does not contain 1 per cent. The alum of the dyer may in the same way contain a proportion of peroxide of iron, varying in different specimens from 1 to 90 per cent. ; and differences in the ratio of ingredients as great as these occur in all the combinations of what are called isomorphous bodies. These garnets and alums, however, are in reality mixtures in variable proportions of quite constant compounds, and offer no exception to the law we are discussing ; but they illustrate what is manifestly quite possible, that constancy in physical character, and constancy in the nature of the constituent ingredients, might co-exist with inconstancy in the proportion of the latter. Now Dalton's first law affirms, in contradiction to this possibility, that the proportion of elements in a compound is in every case as constant as their nature; a truth which the title, 'Law of definite proportion,' does not bring out, whilst that of constant proportion not only does, but in addition includes all that the former expresses; for a constant proportion must of necessity be a definite one also.

For these reasons, we press upon the reader the propriety of avoiding the singular and almost unaccountable confusion which exists in many of our best works in the use of the word definite, as equivalent to constant, and name the law-that of constant proportion.

This law applies to all bodies, organic and inorganic, native and artificial, so that in the light of it our earth, with its atmosphere, may be considered as the sum or complement of an almost infinite number of compounds adjusted by weight, and told to the tale; and in a sense as mathematically true as it is poetically sublime, we may understand the declaration of an inspired writer, that God has weighed the mountains in scales and the hills in a balance.'

The law of constant proportion was known before Dalton's time, and had been distinctly announced by several chemists in different countries towards the close of last century. We can scarcely doubt that it had been fully apprehended, in many quarters, before it was specially proclaimed. Every chemist who undertook the analysis of a substance must have blindly or intelligently taken for granted that it would prove definite in composition; and most of them, we may readily believe, connected with this a more or less clearly discerned expectation that it would prove constant in composition also. This length, certainly, Bergman the Swede, our own Cavendish, Lavoisier, and many others, had reached, in their observations and speculations on the combinations of bodies; but it was made the subject of special demonstration by two German chemists, Wenzel and Richter, and by a French chemist, Proust, who published their respective works between the years 1777 and 1792. The views of the German chemists will come better under our notice when discussing the third law of combining proportion; those of Proust deserve more particular mention here, as they were published in consequence of a discussion carried on between him and the celebrated French chemist, Berthollet, as to the existence of such a law as the one we are considering. Berthollet asserted that the number of compounds which any two elements can form with each other is quite unlimited, and that constancy of physical characters, such as specific gravity, colour, taste, &c., is no sign of constancy in chemical composition. Proust affirmed, on the other hand, that the number of compounds formed by two elements, such as iron and oxygen, is always limited, and often very small; and that so long as the physical characters remain uncnanged, the chemical composition is equally invariable. The evidence adduced by him was so ample and incontrovertible, that the discussion ended in satisfying every chemist of the truth of his views.

The second law of combining proportion is related to the circumstance, that the same elements, in almost every case, combine in more than one proportion to constitute several compounds. Even the beginner will be prepared for this, if he is aware that the chemist has, in the meanwhile, reduced all kinds of matter to some fifty-six primary ones, and has the whole world to account for out of these. This law is named that of Multiple Proportion, and enforces the remarkable truth, that when one body combines with another in several proportions, the higher ones are multiples of the first or lowest. Oxygen and hydrogen, for example, which in water are united in the ratio of eight of the former to one of the latter, unite to form a second compound, named the peroxide of hydrogen, in which the oxygen is to the hydrogen as 16 to 1; or, the hydrogen remaining the same, there is exactly twice as much oxygen as in water. There are two compounds of hydrogen and carbon remarkable as being the bodies which suggested this law to Dalton. In the one of these, (olefiant gas,) there are six parts, by weight, of carbon, to one of hydrogen; in the other, (marsh gas, or fire-damp,) there are six

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parts of carbon to two of hydrogen; or, the weight of carbon being the same in both, there is exactly twice as much hydrogen in the first as in the second. One of the most remarkable examples of this law occurs in the compounds of nitrogen and oxygen, which are five in number. The proportion of nitrogen is the same in all, and may be represented by the number 14, while that of the oxygen, which in the lowest may be expressed by 8, in the second is 16, or twice 8; in the third, 24, or three times 8; in the fourth, 32, or four times 8; and in the fifth, 40, or five times 8; the higher proportions are multiples of the lowest, by 2, 3, 4, and 5, at which last number, in this case, they stop. In every series of compounds we find the same law operating. If a substance can combine with more than eight parts of oxygen, the least next quantity it combines with is 16. It never combines with 8 and frds, 8 and {ths, 8 and faths, or any other fraction whatever; but if it overstep the 8, goes right on to the 16 before it is again saturated. It may go past the 16, but in that case it cannot stop at any

intermediate ber, but must proceed to 24. It need not halt at 24, however, if it can go on to 32; or at 32, if it can combine with 40; and it may pass at once from 8 to 40, or to any other quantity, however large, provided it be a multiple of the original 8. The only unalterable decree is, that whatsoever smallest quantity of one body another can combine with, every higher compound must contain in increasing multiples.

In all the cases referred to, binary compounds have, for the sake of simplicity, been taken for illustration, and they have been such, that one of the elements has remained constant in quantity, while the other has increased in the higher or more complex compounds, by multiples of the quantity found in the lowest or simplest. But cases are quite common where both of the elements of binary compounds, and all those of more complex ones, occur in multiples of their smallest combining quantities. One illustration from a small series of binary compounds may suffice. There are three well-known compounds of iron and oxygen. In the first, we have 27 parts of iron to 8 of oxygen; in the second, 54 of iron to 24 of oxygen, or the proportion of iron is doubled, and that of oxygen tripled; in the third, we have 81 iron to 32 oxygen, or the iron tripled and the oxygen quadrupled.

This law reigns through all nature, and is so manifest, that it scarcely calls for fuller illustration. Those who are quite unfamiliar with chemical speculation, however, may perhaps be able to grasp it more firmly by means of the following comparison :A compound body is with great propriety likened to a chain, while the separate links of which the latter is made up represent

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