with the larger metamorphoses which we have been considering in the present article.

In conclusion, however, let the reader be advised, if not yet familiar with the practical work of geology, to accept no theory, and be bound by no respect for established authorities in this matter. Geology is a subject for study, and not for dogmatic statement. It is not desirable to raise another such storm as that which disturbed the serenity of the scientific horizon when the aqueous and igneous theories were first propounded. We need not shut our eyes to truth because it seems opposed to opinion, but we should observe closely and avoid coming to conclusions too rapidly. Metamorphism is a great and difficult subject.

ATMOLYSIS.

BY W. B. TEGETMEIER.

In the previous volumes of the INTELLECTUAL OBSERVER* will be found notices of the discovery of the phenomena of Dialysis, by Mr. Graham, the Master of the Mint; of its application in the explanation of natural occurrences, such as the formation of minerals, and also of its practical utility when applied to the arts of life and civilization. Mr. Graham has been continuing these investigations with that profound originality of research that has caused him to be regarded by scientific men as one of the most illustrious investigators in the highest branches of physical science.

Those

The phenomena attending the passage of liquids through thin membranes constitute what is termed dialysis. under consideration at present, depend on the passage of gases through porous plates.

Mr. Graham commenced this train of investigation many years since, and, from time to time, some of his results have been made known. The instrument originally employed in these researches was termed a diffusiometer, and consisted of a plain, cylindrical glass tube, one inch in diameter, by ten inches in length. This, by being closed at one end by a thin plug of plaster of Paris, was converted into a receiver capable of being used over mercury. At the present time, compressed graphite has been substituted for the plaster of Paris; this is obtained by cutting slices with a fine steel spring-saw off the compressed blocks used for the manufacture of pencils. These slices, by rubbing down on a dry sandstone, can be obtained

* INTELLECTUAL OBSERVER, vol. i. pp. 156 and 381; vol. ii. p. 224.

of a degree of thickness not exceeding one-half a millimetre, about the thickness of an ordinary wafer: one of these plates attached by cement to the end of a glass tube closes it and forms a diffusiometer. During the process of filling over a mercurial trough, the porous plate is covered with a thin sheet of gutta-percha; on removing which, diffusion immediately takes place.

If one of these tubes be filled with hydrogen, it is found that the gas will totally diffuse itself into the air in the space of about forty minutes; about one-fourth as much air passing through in the reverse direction, and the mercury rising against the influence of gravity, to supply the space left by the more rapid diffusion of the hydrogen.

It is found that the pores of the compressed graphite are so minute, that the gas in a mass does not pass through, but that the molecules only pass by a sort of intestine motion. According to the hypothesis advanced to account for this circumstance, gases consist of perfectly elastic atoms which move amongst each other with different degrees of velocity in the different gases; and when the gas is confined in a close vessel, the atoms impinge against the sides and against each other, but that this occurs without any loss of velocity owing to their perfect elasticity.

In

If, however, the sides of the vessel are porous, the atoms moving against the open channels escape, and, in the same manner, the air or any gas which may be external to the vessel, enters. When the same gas is on both sides of a porous vessel, the interchange takes place, but it is not at tended with any alteration of bulk of the contained gas. the case of some gases which diffuse at the same rate, as for example, nitrogen and carbonic acid, the interchange takes place without any change of volume. But with gases having an unequal molecular velocity, as air and hydrogen, the diffusion is unequal.

The further investigations of Mr. Graham relate to the laws affecting the passage of a gas, either by pressure or by its own elastic force, through a graphite plate in one direction only, a vacuum being maintained on the other side. A gas may pass into a vacuum in three different modes:

1. When a gas passes into a vacuum through a minute aperture, as a hole in a plate of platinum foil, its rate of passage is regulated by its specific gravity. "A gas," says Mr. Graham, "rushes into a vacuum with the velocity which a heavy body would acquire by falling from the height of an atmosphere composed of the gas in question, and supposed to be of a uniform density throughout. The height of the uniform atmosphere will be inversely as the specific gravity of

the gas, the atmosphere of hydrogen being, for instance, sixteen times higher than that of oxygen; but as the velocity acquired by a heavy body falling is not as directly the height, but as the square root of the height, the rate of flow of different gases into a vacuum will be inversely as the square root of their densities. The velocity of oxygen being 1, that of hydrogen will be 4, the square root of 16. This law has been experimentally verified," and the mode of passage is termed Effusion.

2. If the aperture be in a plate of increased thickness, the law of effusion no longer holds good. When the length of the tube exceeds the diameter by 4000 times, a new ratio is established, that of the Capillary Transpiration of Gases. The rates of transpiration are not governed by specific gravity, and are singularly unlike those of effusion, the transpiration velocity of oxygen being 1, that of chlorine is 1.5, and that of hydrogen 2:26. These ratios appear in constant relation to no other known property of the gases, and they form a class of phenomena remarkably isolated. Exceedingly minute capillary tubes, however numerous, offer practically a perfect impediment to the passage of gas by transpiration.

3. The diffusive, or molecular movement of gases, enables them readily to pass through plates of graphite which are, practically, impenetrable to gas by either of the two preceding modes; this penetration appears to be due entirely to their own proper molecular motion, entirely unaided by transpiration, and uninfluenced by pressure. The graphite appears, as it were, to become a molecular sieve, allowing molecules only to pass through, and, consequently, hydrogen is found experimentally to pass through a graphite plate with precisely the same velocity, whether it is passing into a vacuum or into air.

These abstract investigations of Professor Graham have already been pressed into the service of practical science. An instrument termed an Atmolyser has been designed, by means of which mixed gases of different diffusibility can be separated. The most remarkable effects of separation are produced by the tube atmolyser; this is simply a narrow tube of unglazed earthenware, similar to a tobacco pipe stem, two feet in length, which is placed in a shorter tube of glass, and secured in its position by corks, so as to resemble in appearance a Liebig's condenser; the glass tube is then exhausted of air by being connected with an air-pump, and a mixed gas is allowed to flow through the earthenware tube, when the more diffusive gas is rapidly abstracted. Thus, when an explosive mixture of two volumes of hydrogen and one of oxygen is passed, the gas issuing from the tube consisted of oxygen, with less than 10 per cent. of hydrogen-a mixture which could not be ignited.

These considerations of these phenomena have led Mr. Graham into certain speculations respecting the constitution of matter. It is conceivable that the various kinds of matter, now regarded as different elementary substances, may possess one and the same ultimate molecules in different conditions of movement. The essential unity of matter is a hypothesis in harmony with the uniform action of gravity on all bodies; we may imagine one substance only to exist, namely, ponderable matter, and that this is divisible into ultimate atoms, uniform in size and weight; if these atoms were at rest the uniformity of matter would be perfect. But they always possess more or less motion due to some original primordial impulse; this motion gives rise to volume; the more rapid the motion the greater the space occupied by the atom. Thus, matter of different density forms different substances, that are usually regarded as different inconvertible elements, and this hypothesis may be pursued through the various phases of combination; the different states of solid, liquid, and gas, and the colloid and crystalline forms of matter.

Recent as are these discoveries and speculations of the author, they have already a practical bearing on the communication of heat to or from gas or vapours, by contact with solid or liquid surfaces; for the impact of the gaseous molecule on a surface possessing a different temperature from itself appears to be a condition of the transference of heat from one to the other. The more rapid the molecular movement of the gas, the more frequent the contact, and, consequently, the more rapid the communication of heat. Hence, the great cooling power of hydrogen as compared with that of air or oxygen. These gases have the same capacity for heat, as regards equal volumes; but a hot body placed in hydrogen is really touched 3.8 times more frequently than it would be if placed in air, and 4 times more frequently than it would be in an atmosphere of oxygen. This property of hydrogen recommends the application of that gas to the air or caloric engine, where the object is alternately to heat and cool a confined volume of gas with great rapidity.

There can be no doubt but that engines worked by steam, the employment of which is always attended with so great a loss of heat, will eventually be superseded by air or caloric engines, where there is no loss by condensation. Theoretically, air engines are perfect, and the practical difficulties that prevented their adoption are being gradually overcome.

THE TINNEVELLY PEARL BANKS.

BY CLEMENTS R. MARKHAM, F.S.A., F.R.G.S.

FROM time immemorial the pearl fishery in the narrow sea which separates India from the island of Ceylon has been famous in all the marts of the old world, and has rivalled the still more renowned fishery of Bahrein, in the Persian Gulf. Opinions have always varied respecting the value of the pearls from these fisheries. Tavernier, the old travelling jeweller, said, in 1651, that the pearls from the sea that washes the walls of Manaar, in Ceylon, are, for their roundness and water, the fairest that are found, but rarely weigh three or four carats. Master Ralph Fitch, a London merchant, who made a voyage to the Indies in 1583, says, on the other hand, that, though the pearls of Cape Comorin are very plentiful, they have not the right orient lustre that those of Bahrein have. Whatever the truth may be respecting the water and orient lustre of the pearls of these rival fisheries, there can be no doubt that a vast concourse of merchants and others has been annually attracted to the fisheries in the Gulf of Manaar from the most ancient times, which is sufficient evidence of their value.

The Ceylon fisheries have retained their old reputation down to modern times. But it is to the smaller and hitherto less productive pearl banks, on the opposite side of the Manaar gulf, off the shores of the Indian Collectorate of Tinnevelly, that the reader's attention is requested. An experiment, with a view to the improvement of the fishery, has now been commenced there, which possesses considerable scientific and general interest.

In the golden age of the Tamil people of Southern India, the Tinnevelly pearl fishery, then established, as Ptolemy states, at Kôru, the more modern Coil, paid tribute to the Pandyon kings of Madura; and at this period, we are told by the author of the Periplus of the Erythræan Sea, none but condemned criminals were employed in the fishery. Marco Polo, in the end of the thirteenth century, mentions the land of Maabar,* where many beautiful and great pearls are found off the coast. The merchants and divers, he says, congregated at Betaler, in April and May, and he relates how the divers, called Abraiamain, performed incantations to preserve themselves from the attacks of great fish in the depths of the sea.

* Maabar of Ibn Batuta and Marco Polo is the southern region of the Coromandel coast, comprised in the modern districts of Madura and Tinnevelly. Colonel Yule has suggested that the word may be Arabic (Ma' abar, a ferry), in reference to the passage or ferry to Ceylon.