but it is to be hoped that theoretical fears on this subject will soon be dissipated by successful experience. The use of petroleum as a fuel for steam engines seems to be approaching practical application.

The substitution of steel for iron in various parts of locomotives, and for rails, has added greatly to the permanence of the machinery, and diminished the wear and tear in a remarkable degree. The extensive use of steel in ship-building, especially since the Bessemer process has come into vogue, has contributed much to the strength and safety of sea-going vessels, with diminished weight, and seems likely to restrict the composite system of wood and iron construction to those navigating smooth waters. The battle of the guns versus armor-plates is still waged with great vigor, and the victory just now appears to be on the side of the steel projectiles and chilled shot of Maj. Palliser and others; but this will only give rise to improved machinery, a better selection of material, and better processes of manufacture on the part of the armor-plate makers.

The new gunpowder of Capt. Schultze, made from wood, by a process similar to that of making gun-cotton, bids fair to rival the old explosive for certain purposes. Nitroglycerine and gun-paper have also been successfully introduced, the former for blasting, and the latter for small arms.

In respect to light, heat, chemical affinity, electricity, and magnetism, universal attributes of matter in all its forms, it may be considered as proved that all these forces are so invariably connected inter se and with motion, as to be regarded as modifications of each other, and as resolving themselves objectively into motion, and subjectively into that something which produces or resists motion, and which we call force.

Recent researches go to show that magnetism is cosmical, and not merely terrestrial. One of the startling suggestions made by Mayer, as a consequence resulting from the dynamical theory of heat, is that, by the loss of the vis viva occasioned by friction of the tidal waves, as well as by their forming a drag upon the earth's rotary movement, the velocity of the earth's rotation must be gradually diminishing, and that thus, unless some undiscovered compensatory action exist, this rotation must ultimately cease, and changes hardly calculable take place in the solar system. M. Delaunay and Mr. Airy consider that part of the acceleration of the moon's mean motion, not at present accounted for by planetary

disturbances, is due to the gradual retardation of the earth's rotation.

According to Mr. Grove, in his Inaugural Address to the British Association for 1866, from which we quote largely, there are some objections, though not insuperable, against the theory of Mayer, that the heat of the sun is caused by friction or percussion of meteorites falling upon it; but these cosmical bodies have not been ascertained to impinge upon the sun in a definite direction from their gradually lessening orbits. And M. Faye, who has recently investigated the proper motions of the sun-spots, has pointed out many objections to this theory, and attributes them to some general action arising from the internal mass of the sun.

Assuming the undulatory theory to be true, and that light must lose something as light, in its progress from distant luminous bodies, it becomes an interesting question what becomes of the enormous force of light lost, and heat radiated into space, which do not apparently return in the same forms. Force cannot be annihilated; its modes of action in this case are only changed. This is one of the most interesting problems of celestial dynamics, which we wait for some Newton to solve.

The doctrine of the correlation of forces is steadily gaining ground. Many points of great practical importance are connected with this subject, as whether we can produce heat by the expenditure of other forces than those locked up in our coal-beds and forests; whether we can absorb and store up for future use, by chemical or mechanical means, the rays of the sun now wasted for human purposes in the desert and the tropics.

The researches of Prof. Tyndall on radiant heat, and the discoveries of Graham on the increased potential energy of atmospheric air when passed through films of caoutchouc, it becoming richer in oxygen by losing half its nitrogen, are interesting as indications of means for storing up force. The magneto-electric machine of Mr. Wilde, and the electrical machine of Mr. Holz, show how mechanical may be advantageously converted into electrical force. The greatest practical conversion of force is exemplified in the fact that the chemical action of a little salt water upon a few pieces of zinc, as shown in the Atlantic cable, has bound the two hemispheres together by electrical action.

The remarkable results of spectrum analysis, from the labors of Kirchhoff, Bunsen, Huggins, and Miller, have thrown a flood of light upon the structure of the heavenly bodies. These conclusions will be found under the head of "Celestial Chemistry."

The old theories of geological convulsions and cataclysms by which the inequalities of the earth's surface and the many breaks in the geological record were explained, are now supplanted by the modern view of Lyell and others, which refers the changes in the past to causes similar to those now in operation. With this, since the researches of Darwin, has become connected the question, whether, in a geological formation unmistakably continuous, the different characters of the fossils represent absolutely permanent varieties, or may be explained by gradual modifying changes. It is quite possible that many modifications of size and form, regarded as permanent, and on which specific differences have been assumed, may be due to changes in the conditions of existence. The opponents of Darwin's theory have a strong point in the fact that, with the present knowledge of fossil forms, the physical breaks in the strata make it impossible to fairly trace the order of succession of organisms; but, notwithstanding the imperfection of the geological record, the belief widely prevails among geologists that the succession of species bears a definite relation to the succession of strata.

Since Sir John Herschel, more than thirty years ago, proposed to explain the climatal perturbations on the earth's surface, with the attendant geological phenomena, by changes in the eccentricity of the earth's orbit, cosmical studies have been more intimately associated with geology. Mr. Croll has recently shown reason to believe that the climate in the frigid and temperate zones of the earth would depend on whether the winter of a given region occurred when the earth at its period of greatest eccentricity was in aphelion or perihelion - if the former, the annual average of temperatare would be lower, —if the latter, it would be higher than when the eccentricity of the earth's orbit was less, or approached more nearly to a circle he calculates the difference in the amount of heat, in these two positions, as nineteen to twenty-six. He thus explains the glacial, carboniferous or hot, and the normal or temperate periods, which we observe in geological records; he estimates that it is certainly not less, and probably much more, than one hundred thousand years since the last glacial epoch.

The progress of physiology during the last two years has been great, principally owing to microscopical and chemical investigations. The discovery of development by cells, evincing a simple, uniform law, underlying and working out the very different forms

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and structures of vegetable and animal life, marked a new era in physiological science. Says Prof. Huxley, Surely the knowledge that the tough oak plank, the blade of grass, the lion's claw, the contracting muscle, and the thinking brain, all emanate from simple forms which, so far as we can tell, are perfectly alike, and further, that the entire plant or animal also emanates from a single form or cell which is undistinguishable from the rudiments of its several parts, is as full of interest, and as suggestive of high thought as any one of the fragments of knowledge which man has worked out for himself in the whole range of physical science; and what better exercise can there be than teaching the operation of the great law of uniformity?

Organic chemistry has accumulated a vast array of facts which its professors are bringing to bear upon some of the most important questions in physiology, and their habits of investigation and knowledge of the nature of the forces acting within the body have made them umpires in many of the sanitary and even medical questions of the day. Such is the rapid advance of the chemical knowledge of common things, that physicians must be chemists to that degree as to be able to answer questions arising regarding the air, water, food, drink, and medicine which, by means of forces that exist in them, act upon the forces within the human body, and give rise to the phenomena of health and disease. From the researches of Traube, Playfair, E. Smith, Fick and Wislicenus, Frankland, and others, we know that the amount of labor which a man has undergone in twenty-four hours may be approximately arrived at by an examination of the chemical changes which have taken place in his body; "changed forms in matter indicating the anterior exercise of dynamical force." All will admit that muscular action is produced at the expense of chemical changes, but until recently it was generally believed that muscular power is derived from the oxidation of albuminous or nitrogenous substances; but more recent researches, detailed in the text, show that the latter is only an accompaniment and not the cause of the former, and that muscular force is supplied by the oxidation of carbon and hydrogen compounds. Messrs. Fick and Wislicenus, from their experiments in ascending the Faulhorn, state that "so far from the oxidation of albuminous substances being the only source of muscular power, the substances by the burning of which force is generated in the muscles are not the albuminous constituents of those tissues, but non

nitrogenous substances, either fats or hydrates of carbon, and that the burning of albumen is not in any way concerned in the production of muscular power."

The theory of Darwin, that species are not rigidly limited, and have not been created at various times complete and unchangeable, but have been gradually and indefinitely varied, from external circumstances, from natural efforts to accommodate themselves to surrounding changes, and from the necessity of yielding to force in the struggle for existence, has continually gained ground, and now numbers among its advocates many of the first naturalists of Europe and this country. The opponents of this theory have their strong points in accommodating definitions of a species, the phenomena of hybridity, and the non-occurrence of these changes before our eyes. If species were created as we now see them, the more we subdivide them by extended observation the more we increase the number of the supposed creations; and yet we have no well authenticated instance of a new creation, and in no other operations of nature such a want of continuity, such a perpetually recurring creative miracle. The tendency seems to be to the belief that there are no such natural divisions as species, genera, families, etc., but that they are merely convenient terms for subdivisions, having a permanence which may outlive many generations of man, and yet which are not absolutely fixed. Such is the length of geological periods now admitted, that the phenomena of hybridity may be legitimately explained on the theory of the continuity of succession; the infecundity may just as well be due to physical differences arising from long-continued variation, as to an original organic constitution; indeed, the acknowledged degrees of hybridity are best explained on Darwin's theory. Darwin insists upon time for the changes by natural selection; and no one will pretend, at the present day, to date back the earth's history only a few thousand years. Geology teaches that hundreds of thousands of years do not limit the period of the earth's existence as an abode for living organisms. In the early days of geological science, the numerous gaps in the record of fossil forms would have been a strong argument against the theory of Darwin; certain species seemed to become extinct and new ones to appear without connecting links; but, as page after page of this geological record has been discovered, the gaps become less numerous and less abrupt, and the intermediate forms are gradually being added to form the continuous series.