Popular Science Monthly/Volume 57/July 1900/Some Phases of the Earth's Development in the Light of Recent Chemical Research
|SOME PHASES OF THE EARTH'S DEVELOPMENT IN THE LIGHT OF RECENT CHEMICAL RESEARCH.|
JOHNS HOPKINS UNIVERSITY.
IN the following pages an effort is made to apply some of the results of recent chemical research to the earlier-history of the earth. It is hoped that the main facts brought out may be readily grasped by those who have never studied chemistry, and that each link in the chain of events will be made evident to those who have mastered the rudiments of this science.
Chemical action involves change of composition. Substances more or less complex may be broken down into simpler substances, or from several simpler substances a complex substance may be built up. From the complex ore of copper found in nature the simple element copper is obtained. From the elements sulphur and oxygen and the simple substance water the complex sulphuric acid is built up. Within the last few years the high temperature of the electric arc—the heat generated by a powerful electric current playing between two carbon poles—has been employed in bringing about chemical changes which do not occur at ordinary temperatures nor at those obtainable by burning fuel. The electric furnace is used industrially to make calcium carbide from lime and coke, carbon silicide (carborundum) from coke and sand, and the metal aluminum from its compounds.
Chemical changes at high temperatures have long been an object of research, but it was not until the introduction of the electric furnace that it was possible to command temperatures high enough to make exhaustive studies. In the last few years several chemists, especially Moissan, of Paris, and his pupils, have done systematic work with the aid of the arc furnace. The furnace used in the laboratory for high temperature work is a small and simple apparatus; Moissan's furnace is a block of quick lime a little longer and wider than a page of this magazine and about three inches thick. A rectangular cavity is cut on the upper surface of this block. A similar block forms the cover. In opposite grooves between the top and bottom piece are placed the carbons, such as are used in ordinary arc lights. The arc plays across the cavity in such a manner that the substance to be heated is not brought into the arc itself, which is vaporized carbon, but below it. The cavity thus represents a tiny reverberatory furnace; the arc heats the roof and sides to an intense heat, which is radiated on the open dish or closed crucible or tube containing the substance heated. This is the simplest form of laboratory furnace. Various modifications are' used, but in all the size is small and the arrangement simple. A powerful arc plays in the smallest possible cavity with the object of attaining the maximum of temperature, expense and duration of material being secondary considerations. Lime and magnesia are the best materials, because they are at the same time the most refractory substances available and are poor conductors of heat. A furnace top one and one half inches thick may be heated by so powerful an arc that the melted quick lime drips from the inner surface, while the outer surface is scarcely warm to the touch of the hand. Moissan has utilized in these little furnaces currents of electricity of varied strength, the lowest being that given by a four horse power dynamo, the highest that generated by three hundred horse power. The highest temperatures obtained were about 3,500° centigrade (6,300° Fahrenheit), with the heat constantly increasing; the limit to the obtainable temperature—as far as the experimental evidence showed—was merely the lack of any known substance refractory enough to bear the heat; for at the temperature mentioned quick lime and magnesia not only melt but are changed into gases, so that the furnace was filled with the vapors of its own material.
The effect of the heat on single substances is very interesting. Refractory metals, such as iron, manganese, uranium, platinum, melt rapidly and then become gaseous; the most refractory non-metallic elements, silicon, boron, carbon, are also changed into the gaseous form. Very refractory compounds are broken down into simpler ones. Magnesium pyrophosphate yields phosphorus, magnesium oxide and oxygen. Asbestos—a magnesium silicate—gives as chief product magnesium silicide; the other substances formed being silicon, silicon dioxide and a little magnesium oxide.
Such are the astounding changes wrought by simple heat upon those substances which we are accustomed to regard as infusible. It must be remembered that the range of temperature which chemists employ in ordinary laboratory work is not very great and that the conditions of work in the laboratory and of nature's work on the earth's surface at the present day favor the formation of two classes of compounds—the oxides and their hydrates. Although air is a mixture consisting mainly of four parts of nitrogen and one of oxygen, atmospheric nitrogen is generally inert at ordinary temperatures, and it is the oxygen of the air which is the more important factor in the growth of living things and in changes in lifeless matter. Water, a compound of oxygen and hydrogen, is present everywhere, either in the liquid form or as vapor in the air; even in the flame of the hottest fires there is water vapor in abundance, since water is one of the chief products of combustion of most forms of fuel. Is it a wonder that under such conditions we find the earth's crust to contain the elements chiefly compounded with oxygen? Was this always so? Are we justified in supposing that conditions may have prevailed—nay, must have prevailed—in former times on the earth's surface, which gave to other elements as important or more important functions than to oxygen? The answer to these questions must he sought in the results of the chemistry of high temperatures.
First let us consider the conditions of existence of the omnipresent water. Water begins to break down into its components, hydrogen and oxygen, at 934° centigrade; at 2,500° centigrade (4,500° Fahrenheit) the decomposition is complete. In other words, water vapor cannot exist at temperatures above 2,500°, but the hydrogen and oxygen exist in the free state.
Astronomers tell us that refractory elements like iron, silicon and carbon, perhaps disassociated into still simpler substances, are present as vapor in the atmosphere of the sun and that many others of our well-known elements, including hydrogen, are also present in this glowing atmosphere, while the heat of the sun's surface and that of the hotter stars is vastly higher than that of the electric furnace. Geologists believe that the evidence at their disposal points to a similar period of great heat in the early history of the earth. It may be considered, then, that temperatures higher than those of the electric furnace prevailed in former times on the earth's surface.
Let us now return to the study of the results obtained with the electric furnace. The following reactions are especially important. If metals, or refractory non-metals, or metallic or non-metallic oxides, or complex silicates, are heated to the higher temperatures in contact with carbon, boron, silicon or compounds of these three elements with oxygen, the result generally is that* very refractory carbides, borides or silicides of the metals or non-metals are formed. In other words, those complex substances which form the chief constituents of the outer crust of the earth at the present day are decomposed at high temperatures, and simple compounds of two elements—so-called binary compounds—are formed. Four classes of these binary substances seem to be especially stable at high heat—the carbides, borides, silicides and oxides; but the oxygen of the metallic oxides tends to pass off as an oxide of carbon, if carbon be present.
At somewhat lower temperatures nitrogen is very active and the nitrides of many metals are readily formed. An excellent example is shown by heating a mixture of carbon and of an oxide of titanium (titanic acid). When heated by a feeble current the acid is simply reduced, forming a lower oxide of titanium; with a more powerful current the mass is completely changed into the nitride of titanium, the nitrogen coming from the air; with a very powerful current this is changed into pure carbide, as the nitride cannot exist at the higher temperature, and the nitrogen escapes, carbon taking its place. At still higher temperatures hydrogen acts on many metals, forming hydrides. The carbides and other compounds of some metals are not stable at high temperatures, being reduced by gaseous carbon to the free metals, which remain then in the gaseous form.
At that period of the earth's history when the temperature was as high as that easily obtained in the electric furnace, we have the sanction of geologists for picturing the earth's surface as an ocean of molten matter surrounded by a glowing atmosphere. This molten surface must have consisted of binary compounds such as those mentioned above, and probably contained some refractory elements, metals and non-metals, in the free state. The atmosphere contained free hydrogen, oxygen and nitrogen, gaseous binary compounds like the oxides of-carbon, metals in the gaseous form and many non-metallic elements like sulphur and chlorine. In the atmospheric region furthest removed from the molten surface violent chemical reactions occurred between the heated elements, forming compounds which were again dissipated into their elements by the heat given off in the act of formation or radiated from the glowing surface below.
Under the enormous pressure of this atmosphere the liquid surface of the earth solidified at very high temperature. Whether the earth's mass solidified from the centre outward or by forming a solid crust over a liquid interior, is a question to be decided by physicists and geologists. We will consider only the outer crust and the atmosphere. As the surface and the atmosphere above it gradually cooled, the formation of nitrides, and later of hydrides, sulphides and chlorides, occurred.
The conditions now attained may have been fairly stable as long as the temperature of the surface and lower regions of the atmosphere were high enough to prevent the union of the atmospheric oxygen and hydrogen, or to decompose the water forming in the outer regions of the atmosphere. As soon, however, as by further cooling, water came into contact with the earth's surface, very violent reactions occurred, which were supplemented by other equally violent reactions when the cooling process permitted the formation of the ordinary mineral acids.
The reactions of water and of acids on many of the binary compounds are so important in determining the present composition of the earth's crust that they must be considered in detail. The carbides, nitrides, chlorides, sulphides and hydrides of most elements, and some silicides, are decomposed by water, or else by dilute acids, forming the hydrogen compounds of carbon, nitrogen, chlorine, sulphur and silicon respectively, and the oxide or hydroxide of the other element. Thus calcium carbide and water give calcium hydroxide and acetylene, a hydro-carbon. Aluminum carbide yields alumina and methane (marsh gas), another hydro-carbon, the chief constituent of 'natural gas.' Other carbides yield crude petroleum. The nitrides yield ammonia, which is the hydrogen composed of nitrogen. The chlorides give hydrochloric acid, the sulphides sulphuretted hydrogen and the silicides the hydrogen silicide. The metallic hydrides yield free hydrogen.
The violence and the magnitude of some of these reactions almost baffle the imagination. Let the reader drop a piece of calcium carbide as large as a small marble into a little water in a cup; there is a rapid action, a gas (acetylene) is given off, which burns with a smoky flame if a lighted match is held over the cup. (The experiment should be tried in the open air.) So much heat is generated in the reaction that the cup becomes hot. Nearly four per cent, of the earth's outer crust is calcium; all this was at this period of the earth's history in the form of carbide. Imagine all the vast limestone mountain ranges of the present day as carbide, and try to realize the effect when water fell on any considerable area. The heat generated would be so enormous that in a moment the acetylene would ignite and burn, forming oxides of carbon and water vapor, which would in turn decompose, throwing the jets of glowing hydrogen and oxygen vast distances into the atmosphere, there to cool and reunite to water. The decomposition of other carbides, of the hydrides and silicides, as well as-the formation of hydroxides by the action of the lighter metals on water, would produce similar phenomena, as the substances formed are combustible gases, or liquids or solids easily volatilized. This is no wild fantasy, but a conservative statement. Similar reactions are taking place at the present day in those stars whose cooling process has advanced far enough; a case in point is that of the so-called 'temporary stars.'
Extremely violent reactions are taking place constantly in the atmosphere of the sun. The sun's chromosphere, or outer layer of its atmosphere, consists mainly of hydrogen, and jets of glowing hydrogen are thrown to great heights above the chromosphere; these jets or 'prominences' have been frequently observed to have a height of 100,000 miles, and prominences of more than double this height are reported by observers. The most conservative estimates assume temperatures of the sun's surface so enormous that that of the electric furnace is insignificant in comparison, and we can have no conception of the chemical changes occurring under such conditions. Whether one believes, with Lockyer, that the chemical 'elements' are disassociated by the sun's heat into simpler substances or not, it is clear that very violent chemical reactions are in progress, and if we realize that the known chemical reactions increase in intensity with increase in temperature, it does not seem strange that at the sun's temperature the reactions occurring should cause disturbances like those observed.
Returning to the earth, let us consider the products of these violent reactions. The hydrogen and hydrides of boron, silicon, sulphur and carbon, combined with the oxygen of the atmosphere, forming water and boric, silicic, sulphurous and carbonic acids, which in turn acted on the metallic oxides and hydroxides, forming sulphites, carbonates, borates and simple and complex silicates; some quickly, some slowly, some at low temperatures and atmospheric pressure, others at high temperatures in liquid or semi-liquid condition and under the pressure of rock masses above. To determine the relative age of existing rock layers, or the mode of their formation, whether by eruptive action, by surface heat, by deposition of finely divided material under water, or by metamorphic changes of the cooled silicate under subsequent action of water, pressure and heat, is the province of the geologist. The present writer refrains from an opinion whether any of the first formed solid crust could or could not survive to the present day in its primary form, considering the exposure to water, acids, heat and pressure which it suffered.
Yet an idea may be formed of the condition of the earth's surface when it had cooled so far that the more violent chemical action had ceased. It consisted chiefly of silicates, simple and complex; of some of the original binary compounds, which are scarce affected by water or acids, such as the silicide of carbon (carborundum), of stable oxides, chlorides and sulphides, with other compounds in smaller proportion, and free elements in greater proportion than at the present day. Everywhere, from crevices in the surface, hydrocarbons, phosphoretted hydrogen (phosphine) and ammonia were issuing as gases; the atmosphere was heavy with these gases and with carbon dioxide.
No scientific observations thus far show how or from what definite compounds plant life or animal life was first evolved from lifeless matter; but it is certain that the materials were much more abundant and the conditions more favorable at the period when it was evolved than at the present day. An ocean much warmer and less saline than now, a damp atmosphere like that of a hothouse, an abundance of plant food and a choice of raw material, were at hand. The chief foods required for plant life are nitrogen in the form of ammonia or nitrates, carbon dioxide, phosphorus as phosphates, sulphates of lime, of magnesia and of the alkalies, and water. As to the raw material for the first formation of the living cell, it is impossible to say what compounds of carbon were employed; suffice it to note that the known simple and complex binary compounds of carbon were there ready for use; the hydrocarbons, carbon monoxide and carbon dioxide were oozing from the earth's surface, from the ocean floor as well as from the land, or hanging heavy in the air above it. If warmth or increased pressure were desiderata, an ocean warm to its greatest depths could afford any pressure required. From the decomposition of the nitrides and phosphides below the surface, ammonia and phosphine were escaping into the ocean and into the air. The conditions then during long periods of time were especially favorable for marine life, and as sand and mud accumulated on the rocky surface of the earth, for land plants; the absence of a thick soil being more than compensated for by the abundance of plant food, notably of carbon dioxide and ammonia.
The statement may be found in excellent modern text-books of chemistry that ammonia is always formed by the decomposition of plants and animals, accompanied by the further statement that ammonia is a requisite for plant food. No plants—no ammonia; no ammonia—no plants. If this were true, the beginning of plant life would indeed have been a struggle for existence; that it is not true is shown above. This decomposition of nitrides has ceased practically on the actual surface of the earth at the present day because the nitrides have all been decomposed; yet it may be mentioned that specimens of rock freshly quarried in Sweden were recently found to give off ammonia when wet with water, showing the presence of nitrides. Below the actual earth's surface it is probable that nitrides still exist in large quantity, for ammonia is one of the constituents of volcanic gases; to believe that volcanic ammonia is a product of plant or animal decomposition is difficult; to suppose it formed by the action of steam on nitrides in the earth's interior is simple.
Much the same may be said of the presence of carbides. While they no longer exist on the surface, there is no doubt of their existence in the interior of the earth, and the volcanic gases contain their decomposition products. In this connection the theory—first put forward by Mendelèeff and since supported by Moissan—of the origin of petroleum, may be mentioned. These writers favor the hypothesis that it was formed by the decomposition of carbides by water under pressure; and while the evidence at hand perhaps favors the belief that the petroleum of the more important oil fields owes its origin to decomposition of the lower forms of marine animal life, yet there can be no doubt that petroleum may be formed by carbide decomposition, and it seems probable that natural gas is in part at least a result of the same action.