finding ourselves on the road towards a decrease in the number of our elements, we shall have another, or perhaps two more, to account for and to harmonise.

We now pass to the consideration of researches having no direct connection with the question we have just been handling, but which still indirectly testify in favour of the compound nature of our so-called elements. A law has been proposed which exhibits these "elements" not as a haphazard assemblage of independent bodies, whose number, properties, and atomic weights might have been other than we find them, but as a definite series, or rather group of series, whose members bear to each other relations somewhat similar to the successive grades-e. g., of oxidation-of some one supposed element. The law in question, though it does not remove the teleological difficulties inherent in the respective quantities and the distribution of the simple bodies, solves, at any rate, some of the hitherto unanswered questions which they have put before us. It does more; it enables us to declare not merely that a link is wanting in the series, but to foretell with tolerable accuracy not alone its atomic weight and its specific gravity, but even certain of its reactions. The prevision of phenomena not yet observed has been rightly declared by methodologists to be one of the principal distinctions between a science, in the strict sense of the term, and a mere accumulation of unorganised knowledge. We still hear mention, from time to time, of the splendid triumph achieved by Astronomy, when Leverrier -having from certain observed facts deduced the existence of a planet as yet unknown-was able to calculate its distance, its mass, its orbit and probable position, and when his announcements were found verified on the telescopic examination of the part of the heavens indicated. Such a fulfilment of his forecasts was a verification of astronomical science perfectly intelligible to the outside public. Prof. Mendeleeff, by the application of his "periodic law," was able to foretell distinctly, in 1869, the properties of a metal then unknown, to which he gave the name of "eka aluminium." On the evening of August 27th, 1875, M. Lecoq de Boisbaudran, being engaged with the examination-chemical and spectroscopic-of a blende from the mine of Pierrefitte, discovered a new metal, to which he has given the name of "gallium," in honour of his country. He does not appear to have been acquainted with the predictions. of M. Mendeleeff, which, till their startling verification, had not by any means attracted the attention which they undoubtedly merit. The more thoroughly, however, M. Lecoq

de Boisbaudran succeeded in purifying the body which he had obtained, and in accurately determining its properties and reactions, the more closely did it approximate to M. Mendeleeff's "eka aluminium." It must be distinctly understood that the prediction had not been couched in loose generalities like the prophecies of the late Francis Moore, Physician. In 1869 M. Mendeleeff wrote as follows:-" Its atomic weight will be El=68; its oxide, El,O,; its salts will present the formula EIX,. Thus its (only ?) chloride will be EICI,, yielding on analysis 39 per cent of metal and 61 of chlorine, and will be more volatile than ZnCl2. sulphide, El2S3, or oxysulphide, El,(S,O),, will be precipitable by sulphuretted hydrogen and insoluble in ammonium. sulphide.

39

Its

"The metal will be easily obtained by reduction; its specific gravity will be 5'9, consequently its atomic volume will be 115; it will be almost fixed and fusible at a low temperature. It will not become oxidised in contact with the atmosphere, and at a red-heat it will decompose water. The pure metal melted will be slowly attacked by the acids and alkalies. The oxide, El,O,, will have the specific gravity 55, or thereabouts; it should be soluble in strong acids, form an amorphous hydrate insoluble in water, but soluble in acids and alkalies. The oxide will form neutral and basic salts, El(OH,X), but not acid salts; its alum, EIK(SO4)212H2O, will be more soluble than the corresponding salt of aluminium and less crystallisable. The basic properties of El2O, being more decided than those of Al2O3, and less than those of ZnO, it will be precipitable by carbonate of baryta. The volatility, as well as the other properties of the saline compounds of El, being the mean between those of aluminium and those of indium, it is probable that the metal will be discovered by means of spectrum analysis, as was the case with indium and thallium."

This very definite account may be read in "Liebig's Annalen" (Supplement-Band viii., p. 133, 1871), and it has indeed been most strikingly confirmed by the properties of the metal as observed by M. Lecoq de Boisbaudran. We must particularly bear in mind that this accord between prediction and observation is evidently becoming more complete as the new metal is obtained in larger quantities and in a state more closely approximating on purity. The celebrated French chemist, indeed, remarks-" Supposing the forecasts of M. Mendeleeff verified altogether, I should have been led to seek for gallium in the precipitates formed by ammonia, and not, as I have done, in the ammoniacal VOL. VII. (N.S.)

X

solutions. In fact, the properties of the hypothetical metal ought to present the mean between those of aluminium and indium,' metals whose oxides are almost completely insoluble in ammonia." Yet in a footnote he very materially qualifies this deliverance. He there states-" Oxide of indium is generally considered almost insoluble in ammonia, a property which is utilised in its separation. As for alumina, its solubility in ammonia, though slight, is sensible. It remains to be seen whether the great delicacy of the spectral reaction of gallium, and the minuteness of the quantities upon which I have operated, may not have caused me to over-rate the relative insolubility of gallic oxide in ammonia." We cannot here help pointing out that the solubility of hydrated alumina in ammonia is sufficiently great to vitiate an analysis, even for technological purposes, unless certain well-known precautions are observed. Hence gallia may still be taken up by ammonia to an extent amply sufficient for spectroscopic purposes, even if considerably less soluble. in that medium than is alumina.

M. Lecoq de Boisbaudran further observes that without the particular method followed in the present investigation neither the theories of M. Mendeleeff nor his own would have, for a long time, led to the discovery of gallium. With all due deference we must submit that this point is utterly beside the question. If certain theories enable us to foretell correctly the properties of a metal as yet undiscovered, their value is established, and whether the ultimate discovery of such metal was due to the prediction is a mere secondary consideration.

M. Mendeleeff has also announced the probable existence of another metal, to which he gives the name of "ekasilicium," Es=72, forming an oxide ESO2. Its properties ought to be intermediate between those of silicium and tin, and it is to be especially sought for among arseniferous and titaniferous minerals or residues. To the discovery of this metal-if metal it may be called, since even tin is relegated among the non-metallic bodies by chemists whose opinions are entitled to respect-we must look forward with anxious interest, not so much for its own sake as for the light which it must throw upon the theory in question.

We must now proceed to an exposition of the law which, in one instance at least, we have seen so signally verified. M. Mendeleeff sets out with a brief reference to the labours of those chemists who have preceded him in this line of enquiry. Gladstone, Cooke, Dumas, Pettenkofer, have all pointed out that the atomic weights of certain groups of the

elements stand in a simple regular ratio to each other. Thus in the calcium group, taking the old atomic weights, we find a progressive increase of approximately 24, the other properties undergoing a correspondingly progressive change. In the sulphur group-sulphur, selenium, and tellurium-the increase of atomic weight on the present scale is approximately 47, with again a progressive modification of properties. Similar developments, which need not here be particularised, have been demonstrated in the potassium, the chlorine, and the phosphorus groups. It has also been noticed, if we are not mistaken, that—although no definite ratio be discoverable-the atomic weights of all the common elements are low, and those of the rarer high. It appears, further, that only the elements of low atomic weight enter into the composition of organised beings. With the very limited exception of copper (Cu=63) found in the chocolate nut, in the blood of certain crustaceans, and in the feathers of the touraco, and of zinc (Zn=65) in the ash of a pansy, iron (Fe=56) has the highest atomic weight of the organic elements. Again, the elements with high atomic weights, beginning with vanadium (V=51) and chromium (Cr=52), with perhaps the single exceptions of manganese and iron, may be regarded as poisonous whenever they exist in a soluble condition. But although such generalisations have not been wanting, there has been no attempt at bringing all the apparently independent groups into harmonious connection. M. Mendeleeff considers that the regular dependence of the properties on changes of the atomic weight appears most clearly on the consideration of dissimilar elements, by the study of which he was led, in 1869, to the discovery of his "periodic law." This law he expressed in the following words:"The properties of simple bodies, the constitution of their combinations, as well as the properties of the latter, are periodic functions of the atomic weights of the elements." There is one term here so generally misunderstood and misapplied, not merely by persons of good general education, but even by scientific writers, that the meaning of M. Mendeleeff's law will scarcely be understood at first sight. The word "period" is commonly used to signify any portion of

* It may be said that we are using the term "organic element," or organogen," in a manner different from its usual acceptation. We are perfectly aware of this difference; but we submit that every element really assimilated by plant or animal, and not merely lodged in its tissues as a foreign and hostile intruder,-e.g., mercury in the bones of a votary of blue pill and calomel -has the right to be considered an "organogen." Lead (Pb=207) has been found in the metallic state in the intestinal canal of insects but there is no evidence whatever in support o its assimilation.

time which we may wish to particularise; but if we remember its derivation we see that it means a "journey round," and that it is applicable only to such portions, of time or of anything else, as exhibit some series of changes tending first in a certain direction and then returning to, or at least towards, the point of departure. Thus a year is legitimately a "period," because in it certain phenomena-astronomical, meteorological, and organic-go through a circle of changes which necessarily ends where it began; but a stretch of 10, or 20, or 100 years cannot be called a period, unless we can discover in it some phenomenon which has its increase and decrease, or its recurrence in such a term of years. In like manner, if we take a group of animals, of minerals, or of elementary bodies, and find that in them some one attribute increases and then decreases again to its former condition, or suffers any other cyclical variation, we may call such a group a "period," and the change in question "periodic." We trust that none of our readers will feel aggrieved at being thus reminded of what doubtless most of them are aware, since a correct understanding on this point is absolutely necessary for the intelligibility of Mendeleeff's law. At the same time we should be very happy to find some word incapable of being misunderstood.

We now turn to Table I., which exhibits the known elements arranged in the arithmetical order of their atomic weights. In the second and third columns we find all whose weights range from 7 to 36. Here we perceive that the characters of the elements change gradually and regularly with alternating magnitudes of the atomic weights. These changes are periodic, taking place in both columns in the same manner, so that the corresponding members are analogous. If we compare respectively

Li

Be B C N

Fl

Na Mg Al Si P S CI

we must admit that there is, between the members of each of these pairs, a resemblance, which in the cases of carbon and silicon, fluorine and chlorine, is especially striking. We notice, further, that the atomic weight of each member of the first group differs from that of the corresponding member of the second almost exactly by one and the same number, viz., 16. A correspondence of this latter nature, indeed, can no longer be traced between the corresponding members of the remaining columns. If, however, we add the atomic weight of an element in column 3 to that of its representative in column 5, and divide by 2, we obtain an