probability that the number of oxygen atoms in a molecule of ozone stands to the number in a molecule of ordinary oxygen in the proportion of three to two. Thus, ozone and ordinary oxygen consist of molecules, the composition of which differs, not in respect of the quality but of the number of atoms which they contain. Imagine the oxygen atoms as so many soldiers belonging to one army, although this comparison be a rough one, it is not so inappropriate as might be imagined; for in fact, in chemical action, these atoms do fight against the alliance between other kinds of atoms, and must vanquish their opposition before they can take up and maintain new positions. A certain weight of oxygen consists of an unspecified number of these atoms or soldiers; but in equal weights of ordinary and of ozonized oxygen, the same number is arranged in a different manner. A given weight of common oxygen contains a certain number of these atoms or warriors, which are marshalled in a certain number of battalions or molecules; an equal weight of ozone contains the same number of exactly similar atoms or warriors, which are, however, placed in a smaller number of molecules or battalions. A molecule of ozone is numerically a stronger battalion than a molecule of oxygen. By the help of this crude simile, it is easy to understand how the same element can pass from one of these conditions into the other, and how, according to the different marshalling of the said atoms, the same element may have different chemical effects upon other bodies-may attack the molecules or battalions of which they consist in a different manner.

The explanation just offered of the possibility of similar atoms being arranged in different kinds of molecules, is manifestly applicable also to dissimilar atoms. Imagine different kinds of elementary atoms-carbon, hydrogen, oxygen atoms for instance, to be represented by different kinds of soldiers, as infantry, cavalry, riflemen. Equal weights of acetic acid and sugar contain the same quantities of carbon, hydrogen, and oxygen, i.e. the same number of atoms of the three elements, or of warriors of the three different arms. But the arrangement of these warriors in military order-in battalions or regiments-is dissimilar in the two bodies. A molecule of sugar contains a greater number (at least thrice, perhaps six times as great) of atoms of carbon, hydrogen, and oxygen as a molecule of acetic acid. Although the composition of acetic acid is identical with that of sugar, both as regards quantity and quality of the contained elements, yet the molecules of the two substances are of dissimilar formation, i.e. they contain the same elementary atoms in different numbers though in the same proportions, and we know with still greater certainty that in aldehyde, only half as many atoms of carbon, hydrogen, and oxygen are united to form a molecule, as in acetic ether or butyric acid, which in all other respects have exactly the same composition, containing the same elements in the same proportions.

But what is the cause of the difference between the two lastnamed substances-butyric acid and acetic ether, the molecules of which consist of equal numbers of the same kinds of atoms? Obviously the atoms contained in a molecule may differ, not only in number and quality, but also in the mode in which they are grouped together. To employ the simile once more, the battalions are divided into companies, and it is evident that this mode of division may differ, whilst the number and kind of elementary atoms remain the same. Two battalions containing equal numbers of warriors of three kinds and in like proportions may have very different internal formations. The soldiers of each class may, for instance, be massed in separate companies, or they may be mingled indiscriminately throughout the whole battalion; again, the number of companies and the mode in which the different classes of warriors are arranged may differ widely in the two battalions. These differences of internal arrangement may greatly affect their respective aggressive movements and powers of resistance, and when the battalion is vanquished and dispersed, its previous formation will affect the combinations which may be formed from its fragments.

Thus, even in modern chemistry, the fundamental idea that the varieties of matter depend upon differences of chemical composition, is still maintained, although not precisely in the same sense in which it was understood forty years ago; its scope is wider, and it has received new developments in special directions. Pure chemistry— as distinguished from its technical applications-is at present occupied with the working out of this idea in the most varied directions. The different bodies found in nature are being interrogated with the view of ascertaining if any description of atom can be found which has not hitherto been met with, and to render the list of elementary substances more complete, new and more searching methods of investigation are being devised, in order to discover any kind of matter which has hitherto remained hidden or unnoticed. Many laws relating to the method of arrangement of the elementary atoms in molecules have been already discovered, and the existence of others foreshadowed. Certain peculiarities of the different kinds of elementary atoms are becoming more and more apparent-for instance, that some will enter in couples only into the composition of a molecule, and that elementary atoms present different numbers of sides, so to speak, for the attachment of other atoms. How the elementary atoms are grouped into the proximate constituents of various complex substances, and how certain chemical properties, such as those of acids, depend upon a special arrangement of atoms; these are examples of the problems which are now being assiduously investigated. Also the dependence of many of the physical properties of substances on their chemical composition, in the widest sense of the term, has been proved, and has been the subject of continual investigation, resulting in ever-extending knowledge, The

limits of this article prohibit the further pursuit of the ideas which guide modern chemists in their investigations, but we have followed these latest developments sufficiently to show that, notwithstanding their widely-different aspects, one and the same leading idea underlies the chemistry both of the past and of the present.

V. THE IRON ORES OF GREAT BRITAIN. By ROBERT HUNT, F.R.S., Keeper of Mining Records. In the year 1866 more than ten million tons of Iron ores were submitted to the action of fire in 613 blast furnaces, and from them we obtained about four millions and a half tons of Pig Iron. The great importance of these minerals, regarding them merely as sources from which we draw the material for the manufacture of our almost infinite variety of machines and tools, our rails, our armour-plates, and nearly every description of implement and cutting instrument, necessarily renders any examination of the phenomena connected with their occurrence in Nature of considerable interest. We find Iron disseminated through every rock, and in various conditions of aggregation in almost every geological formation, playing often a very important part in giving character to the mass. Indeed, the fact that this metal is found in each of the three Kingdoms of Nature indicates some especial function, which is not yet clearly appreciated, of equal importance in the organic and the inorganic worlds. It is, however, only with the occurrence of Iron in rock-masses that this paper will deal. It is intended especially to examine the peculiar conditions under which some of the varieties of Iron ores occur, and to discuss the circumstances which probably attended the formation of many of our ferruginous deposits, whether occuring as nodular concretions, intercalated with our coal-beds, as crystalline mineral forming lodes in the older rocks, or as sedimentary beds, spread over wide areas of very different geological ages.

The following list gives the varieties of commercial Iron ores produced and used in this country, showing the average percentage yield of Iron of each variety, and the proportions in which they are employed in the Iron manufactures of these Islands: