In a series of experiments made some ten years ago, I showed that the attraction for each other of the particles of a substance in a liquid form was as great as that of the same substance in a solid form. Consequently, the distinction between liquidity and solidity does not consist in a difference in the attractive power occasioned directly by the repulsion of heat; but it depends upon the perfect mobility of the atoms, or a lateral cohesion. We may explain this by assuming an incipient crytsallization of atoms into molecules, and consider the first effect of heat as that of breaking down these crystals, and permitting each atom to move freely around every other. When this crystalline arrangement is perfect, and no lateral motion is allowed in the atoms, the body may be denominated perfectly rigid. We have approximately an example of this in cast-steel, in which no slipping takes place of the parts on each other, or no material elongation of the mass; and when a rupture is produced by a tensile force, a rod of this material is broken with a transverse fracture of the same size as that of the original section of the bar. In this case every atom is separated at once from the other, and the breaking weight may be considered as a measure of the attrac tion of cohesion of the atoms of the metal.

The effect, however, is quite different when we attempt to pull apart a rod of lead. The atoms or molecules slip upon each other. The rod is increased in length, and diminished in thickness, until a separation is produced. Instead of lead, we may use still softer materials, such as wax, puty, &c., until at length we arrive at a substance in a liquid form. This will stand at the extremity of the scale, and between extreme rigidity on the one hand, and extreme liquidity on the other, we may find a series of substances gradually shading from one extremity to the other.

According to the views I have presented, the difference in the tenacity in steel and lead does not consist in the attractive cohesion of the atoms, but in the capability of slipping upon each other. From this view, it follows that the form of the material ought to have some effect upon its tenacity, and also that the strength of the article should depend in some degree upon the process to which it had been subjected.

For example, I have found that softer substances, in which the outer atoms have freedom of motion, while the inner ones by the pressure of those exterior are more confined, break unequally; the inner fibres, if I may so call the rows of atoms, give way first, and entirely separate, while the exterior fibres show but little indications of a change of this kind.

If a cylindrical rod of lead three quarters of an inch in diame ter be turned down on a lathe in one part to about half an inch, and then be gradually broken by a force exerted in the direction of its length, it will exhibit a cylindrical hollow along its axis of

half an inch in length, and at least a tenth of an inch in diameter. With substances of greater rigidity this effect is less apparent, but it exists even in iron, and the interior fibres of a rod of this metal may be entirely separated, while the outer surface presents no appearance of change.

From this it would appear that metals should never be elongated by mere stretching, but in all cases by the process of wiredrawing, or rolling. A wire or bar must always be weakened by a force which permanently increases its length without at the same time compressing it.

Another effect of the lateral motion of the atoms of a soft heavy body, when acted upon by a percussive force with a hammer of small dimensions in comparison with the mass of metal,for example, if a large shaft of iron be hammered with an ordinary sledge,―is a tendency to expand the surface so as to make it separate from the middle portions. The interior of the mass by its own inertia becomes as it were an anvil, between which and the hammer the exterior portions are stretched longitudinally and transversely. I here exhibit to the Association a piece of iron originally from a square bar four feet long, which has been so hammered as to produce a perforation of the whole length entirely through the axis. The bar could be seen through, as if it were the tube of a telescope.

This fact appears to me to be of great importance in a practical point of view, and may be connected with many of the lamentable accidents which have occurred in the breaking of the axles of locomotive engines. These, in all cases, ought to be formed by rolling, and not with the hammer.

The whole subject of the molecular constitution of matter offers a rich field for investigation, and isolated facts, which are familiar to almost every one when attentively studied, will be made to yield results alike interesting to abstract science and practical art.

ART. VI. ON THE OCCURRENCE OF THE ORES OF IRON IN THE AZOIC SYSTEM.-BY J. D. WHITNEY.*

THE object of the present communication is to call attention to the geological position and mode of occurrence of one of the most interesting and important classes of the ores of iron, namely, those which are associated with rocks of the Azoic System.

The term Azoic, first employed by Murchison and De Ver

* Proceedings of the American Association for the Advancement of Science, Ninth Meeting, held at Providence, R, I., August, 1855, p. 209.

neuil in their description of the geology of the Scandinavian Pe ninsula, has been adopted by Mr. Foster and myself in our Re ports on the Geology of the Lake Superior Land District, and has been shown by us to be applied with propriety to a series of rocks which cover an immense space in the Northwest. We have called attention to the fact, that this system of rocks, wherever it has been demonstrated to exist, has been found characterized by the presence of deposits of ores of iron, developed on a scale of magnitude beyond anything which occurs in any of the succeed. ing geological groups or systems of rocks.

In illustration of these views, we have briefly described some of the great ferriferous districts of the world, and particularly those of Lake Superior, Scandinavia, Missouri, and Northern New York, all of which exhibit a most marked analogy with each other, both in regard to the mode of occurrence and the geological position of the ores. The two last-named regions, however, not having been thoroughly examined by us in person, we were obliged to content ourselves with information obtained from others, in making a comparison of their most striking features.

Strongly impressed with the interest attaching to this subject, I availed myself of the first opportunity, after the publication of our Report, to visit the iron regions of Missouri and Northern New York, from the last-named of which I have just returned, after a careful examination of the most important localities where ore is now mined in that district. While it is intended to take another opportunity for giving a minute and detailed account of this region, I may be permitted to recapitulate here the principal points maintained by Mr. Foster and myself, to the general correctness of which my more recent explorations have furnished me with additional evidence.

We maintain therefore,

1. That deposits of the ores of iron exist in various parts of the world, which in extent and magnitude are so extraordinary as to form a class by themselves. The iron regions mentioned above, offer the most striking examples of the deposits now referred to.

2. That the ores thus occurring have the same general character, both mineralogically and in their mode of occurrence, or their relations of position to the adjacent rocks.

3. That these deposits all belong to one geological position, and are characteristic of it.

The extent of the workable deposits of the ores of the useful metals is usually quite limited. Most of the veins which are wrought in mines throughout the world are but a few feet in width, often not more that a few inches. This is true of the ores occurring in veins. In sedimentary metalliferous deposits, such as those of the ores of iron in the carboniferous, the horizontal

extent is often very considerable; but the vertical range is so limited, that the most extensive basins may be in time exhausted, when worked on so extensive a scale as is the case in some of the celebrated iron districts of Great Britain. The deposits of iron in the azoic, however, are many of them developed on a scale of such magnitude, that the term "mountain masses may be applied to them without exaggeration, while, from the very nature of their occurrence, they must extend indefinitely downwards, and cannot be exhausted. Thus the great iron mountain of Gellivara, in Sweden, has a length of three or four miles, and a width of not less than a mile and a half. Of course such a mass of ore, without limit in depth, might be worked on the most enlarged scale for any length of time without fear of exhaustion. The same may be said of some of the iron knobs and ridges of Lake Superior and of Missouri. They form veritable mountains of ore, and ages must elapse before their dimensions will have been perceptibly diminished. This is not necessarily the case with all the localities of ore of these districts. Indeed, in Northern New York and in Scandinavia, although there are accumulations of iron which may be measured by hundreds of feet, or even by miles, yet those which are best known and most worked are of much more reasonable dimensions.

The character of the ores thus occurring is mineralogically peculiar. They consist uniformly of the oxides, either the magnetic or the specular. Hydrous ores, carbonates and the like, are altogether wanting, unless it be upon the borders of the ore deposits, where a secondary metamorphic action between the ferriferous mass and the adjacent rocks may have taken place. The oxides found in this geological position are in general remarkably free from all injurious substances, such as sulphur, arsenic, lead, or zinc, and usually the approach to chemical purity in the ores is in proportion to the extent of the mass, the largest deposits being the purest. The principal foreign ingredient mixed with these ores is silica, which is always present, although frequently in minute quantity. Indeed, the analyses of the Lake Superior and Missouri ores show, in some instances, a surprisingly near approach to a state of absolute purity. It would not be difficult in some localities to procure large quantities of an ore not containing more than two or three tenths of one per cent. of foreign matter, and that exclusively silica. The purity of the ores may be inferred from the high character and value of the iron manufactured from them when they have been skilfully worked, as, for instance, in Sweden. Some samples of iron manufactured from Lake Superior ore have, when tested, exhibited a degree of tenacity unequalled by that from any other part of the world. The ores of Lake Superior and Missouri are mostly peroxides; those of Northern New York almost exclusively magnetic; while in Scandinavia the magnetic and specular ores are both of

frequent occurrence. Those of New York are often coarsegrained and highly crystalline, while the peroxides of Lake Superior and Missouri are rarely distinctly crystallized, but are very compact.

The mode of occurrence of these ores in the regions above mentioned is so peculiar, that, from this point of view alone, it is apparent that these deposits should be classed together as distinct from those in the later geological formations. In all the characteristics of true veins, the great masses of ore now under consideration are wholly wanting. Some of the least important of them approach much nearer to segregated veins, and might with propriety be classed with them, were they not developed on so large a scale as to render it difficult to conceive of segregation as a sufficient cause for their production.

In the case of the most prominent masses of ore of these regions there is but one hypothesis which will explain their vast extent and peculiar character. They are simply parts of the rocky crust of the earth, and, like other igneous rocks, have been poured forth from the interior in the molten or plastic state. No other origin can be assigned to the dome-shaped and conical masses of Lake Superior and Missouri, or to the elongated ridges of the first-named region. The Iron Mountain of Missouri forms a flattened dome-shaped elevation, whose base covers a surface of a little less than a square mile, and which rises to a height of 200 feet above the general level of the adjacent country. The surface of the mountain, where bare of soil, is found to be covered with loose blocks of peroxide of iron, without any admixture of rocky pebbles or fragments, which increase in size in ascending to the summit, where large blocks of ore many tons in weight lie scattered about, and piled upon each other. It is a most singular fact, that the ore is nowhere seen in place about the mountain, although the whole mass evidently consists of nothing else. Near its base, an excavation of seventeen feet deep has been made, which exhibits nothing but small, somewhat rounded fragments of ore closely compacted together, without any other substance present except a little red, ferruginous clay, which seems to have been formed by the friction of the masses against each other. This feature in the Iron Mountain is one of peculiar interest, and one which it seems difficult to explain. Evidences of drift action in this region are exceedingly faint. The ore itself is one which seems little likely to undergo decomposition from any exposure to atmospheric changes. The blocks upon the summit, although somewhat moss-grown, have their angles and edges but little rounded. As a key to the origin of the ore, we find in close proximity on the north a long eleva tion of a reddish porphyry of unmistakably eruptive character, connected with the Iron Mountain by a narrow ridge of a rock composed of iron ore and feldspathic rock, showing that the