that continued prosperity has been due to the quality of the water supply. The water of the Nile is probably lower in salt content than any other used extensively for irrigation. This water carries during the flood season less than 150 parts per million of dissolved salts, and about 75 per cent of these salts are compounds of calcium and magnesium. Furthermore, the ancient system of irrigation, which has been gradually replaced during the last half century, was one in which it was absolutely essential to have an adequate drainage system through which to draw off the water from the flooded basins. As a result there was only a short period each year when the flood waters percolated into the subsoil. With perennial irrigation the subsoil has gradually become filled with water and its relief calls for a much more comprehensive and expensive type of drainage.

In some of our own irrigation enterprises there exists a very different set of conditions. Some of our important irrigation streams carry more than 1,000 parts per million of dissolved salts, of which more than half are compounds of sodium. Too often irrigation has been undertaken without making any provision for drainage until the need for drainage became painfully apparent. In other situations where the water supply has been inadequate, or where it has been necessary to lift it for long distances to reach suitable lands, it has been used so sparingly that all the water applied has been used by crop plants or evaporated from the soil surface. Under such conditions it is inevitable that the salts carried to the land by the water must remain in the upper layers of the soil and finally reach concentrations that become toxic to the plants or by reacting with the soil produce a condition of impermeability.

There is no sound reason for doubting that irrigation farming can be made as safe and as permanent as any other kind of farming if the essential conditions are complied with. In attempting to understand these conditions we may learn some useful lessons from the history of ancient irrigation enterprises as well as from the careful observation of the tendencies in more recent projects. There can be little doubt that the same fundamental laws of physics and chemistry that are operating now have been operating during ages long past. When the balance of salts in the irrigation water has been such that the permeability of the soil has been impaired by its use, the accumulation of salts has in time made crop production impossible. Such results have occurred in the past and are taking place at present.

While the alkali problem presents many difficulties and complications it is by no means insurmountable. Experience and scientific investigation are both contributing the knowledge with which the problem may be solved.

AN OUTLINE OF GEOPHYSICAL-CHEMICAL PROBLEMS.1

By ROBERT B. SOSMAN,

Geophysical Laboratory, Carnegie Institution of Washington.

The subject-matter of geophysical-chemistry may be defined as "the physical properties and chemical reactions of the substances and aggregates that make up the earth." It may therefore be roughly divided into two parts: A. Properties and reactions of materials accessible at the earth's surface. B. Properties and reactions of materials in the earth's interior.

Each of these may again be subdivided as follows:

1. Properties and reactions of individual chemical substances; for example, the silicate minerals.

2. Properties and reactions of aggregates; for example, oceanic water, silicate rocks.

3. Properties and reactions of larger units of matter; for example, glaciers, batholiths.

A. MATERIALS AT THE EARTH'S SURFACE.

CHEMICAL SUBSTANCES.

A relatively small number of "common" oxides serves to make up practically 98 per cent by weight of the outer 10 miles of the lithosphere. All the other elements and compounds known to chemistry are included in the remaining 2 per cent. From the geochemical standpoint, therefore, we may divide chemical substances into two classes," abundant" and "rare."

The" abundant" oxides are, in the order given by averages of a great many analyses of terrestrial rocks, as follows:

SiO2 About 60 per cent by weight.

Al2O, About 15 per cent by weight.

FeO
Fe,O,

3

About 6 per cent by weight.

1 Reprinted by permission from the Proceedings of the National Academy of Sciences, vol. 6, No. 10, pp. 592–601. October, 1920.

An understanding of the chemistry of these oxides and their combinations is essential to the progress of petrology. Their study should proceed from the simple to the complex, i. e., should begin with the individual oxides, then proceed to their two-component systems, then the ternary systems, and so on. Upon this fundamental basis is then erected the structure of physical properties for each system-densities at all accessible temperatures, mechanical properties, fluidity, surface tension, specific and latent heats, etc.

The study of these systems may be divided on practical grounds into (1) investigations of the anhydrous oxides and silicates (taking in the first eight oxides in the list above); (2) investigations involving hydrous silicates, as well as combinations containing both carbon dioxide and water.

(1) Anhydrous silicates.-Work on the first group involves hightemperature researches under ordinary atmospheric pressure conditions, except in the case of systems containing the oxides of iron, where the oxygen pressure must be controlled, and systems containing the alkali silicates, where attention to moisture, carbon dioxide, and volatility of the oxides is necessary in certain cases. Considerable progress has been made in the study of the anhydrous silicates. The phase rule diagrams of the four ternary systems of SiO, Al2O3, MgO, and CaO are now complete, and a large amount of data is at hand on the alkali feldspars, the forms of silica, portions of several quaternary silicate systems, etc.

(2) Silicates with volatile components.-Work on systems involving the volatile components CO2 and H2O must be done, for the most part, under pressure, and with apparatus designed especially for this purpose. The methods are well in hand and progress is being made in assembling experimental data. The theoretical side, involving the complications due to pressure as a variable in addition to temperature, is also being carried forward by several investigators.

So much for the 98 per cent. But the remaining 2 per cent contains many natural substances of such great economic as well as geologic interest that they must also receive attention. These may be roughly classified as in the following examples:

The sulfide ores (e. g., sulfides of iron, nickel, zinc, copper, lead, cobalt, cadmium, mercury, silver).-These must be studied both in

their dry melts (to obtain their fundamental characteristics) and in relation to water solutions under atmospheric pressure (problems of oxidation and secondary enrichment). A distinct problem of the sulfides is their relation to the silicates in the igneous rocks (differentiation of sulfide-bearing bodies, as at Sudbury, Ontario).

Volcanic gases and salts.-These are of particular interest in their relation to volcanic activity, as at Kilauea and Vesuvius. Research on gases, including the various gas mixtures evolved from volcanic vents, is of a peculiarly trying character on account of the invisibility and intangibility of the substances handled, as well as the difficulty of collecting and transporting samples of the natural products. A special phase of this work is the study of the complex gases given off by fumaroles and hot springs. In addition to chemical composition and equilibria of the gases, data are needed on the physics of the flow of such gases from vents, as related to volume, temperature, and pressure at the point of emission.

The volcanic "sublimates," such as sulfur, ammonium chloride, arsenic sulfide, copper chloride, magnetite, may be mentioned in this connection, as well as the minerals accompanying fumarole and hotspring activity.

The oxide ores (e. g., ores of iron, chromium, manganese, tin).— The study of these ores involves high-temperature investigations similar to those on the silicates, and also studies of the hydrated and colloidal oxides.

The natural hydrocarbons.-Organic chemistry of a very complex kind is involved in the formation and alteration of natural gas and petroleum, and many problems of physics and physical chemistry, such as adsorption, surface tension, and colloid phenomena, are also involved in their underground storage and movements.

Other substances-for example, the silicate ores, the carbonate ores, the titanium minerals-may be similarly grouped for purposes of experimental study, but it is hardly necessary here to make a complete inventory of such groups.

Running along with all these investigations is the general research necessary to develop experimental methods and apparatus, and to keep the general theory of physics and chemistry abreast of the newly accumulated facts.

AGGREGATES.

In the preceding paragraphs we have mentioned some of the researches that are necessary on the chemical substances of the earth's surface. We come next to aggregates, including the igneous rocks, the pyroclastic and sedimentary rocks, the oceans and other bodies of water, and the atmosphere.