of the soil solution, the latter's composition and concentration. Cameron, as a physical chemist, should have appreciated that fact, but didn't, probably partly because of the results of his analyses of soil extracts obtained by a rapid method of digestion, which precluded the possibility of attaining equilibrium in the system. He does not seem to have appreciated sufficiently, moreover, the importance of the small differences in concentration between the different soil extracts which he obtained nor of the effect of season, and hence of temperature and moisture, in accomplishing marked changes in the dynamic system of the soil. He did not realize, as we do now, that the power of different soils to supply anew nutrients which are removed from the soil by rapidly growing plants varies widely.

All of these points have been developed in the last six or eight years and still constitute vital subjects of investigation in their different bearings and relationships today. While, thus, physical chemistry, a better training of our investigators, and better facilities, have permitted us to make rapid progress in our views concerning soils as media for plant growth and to modify considerably many of Cameron's important teachings, the latter loom large in the annals of our science. They represent a fundamental contribution which necessitated a keen mind, clear thinking, unusual gifts as a chemist, and more than the usual modicum of courage to enable their proponent to escape from the moorings of conservatism, and facing alone the scientific world to break the shackles which had held in bondage the ideas of soils investigators for sixty years. In other words, it is Cameron's attitude rather than his experimental data which constitute his contribution. It does not detract from the credit which is due him to point out, as I have above, some of the serious errors into which he was led by his theories and his disputes. His position is, in my opinion, destined to be secure and eminent in the annals of scientific investigations.

The theories of Cameron and those of Whitney which have been directly or indirectly responsible for the investigations of the Bureau of Soils did not, as may well have been surmised, go unchallenged. It would be impossible in so short a time as is now at my disposal to review the record of the controversies which have raged between the Bureau of Soils on the one hand, and the soil chemists and agronomists of this and other countries on the other, during the last fifteen year period, and particularly during the first ten thereof. Such well-known soils investigators

as King, Hilgard, and Hopkins, and such organizations as the Association of Official Agricultural Chemists, have been aligned against Cameron and Whitney, and opposed to their theories. Due to the progress which I have just briefly sketched for you, which has been accomplished by soil chemists during the last six or eight years, the protests of conservative men and organizations have nearly all been silenced, however, and the general attitude adopted by Cameron with the modifications described, are accepted in this country today. Soil analysis by the strong acid digestion method and by the fusion method have been discontinued very largely and most chemists realize that it does little service to farming interests, and certainly to science, in the great majority of cases to determine how much of the elements essential to plant growth are to be found in soils. We have learned that we must devise methods for determining the composition and concentration of our soil solutions, must study the phase phenomena in such solutions, must learn what factors regulate all these in the field, and to supply the means by which they may be kept in optimum relations for plant growth. We owe much of our ultra-modern ideas on the soil solution, on methods for obtaining and studying it, and on soils in general, to a rapidly growing body of workers who are enabling us to shape Cameron's theories to the pattern of a vast body of new and striking facts. At the risk of making invidious distinctions by mentioning some and not others, I feel constrained to mention the outstanding work in the last five years of Bouyoucos, Morgan, Jordan, Burd, Hoagland and Stewart, Gillespie, Sharp and a few others who are applying with great success a few simple fundamental principles of physical chemistry to the study of soils. We owe a debt, further, to the plant physiologists who have been profiting, like the others, from the application of physical chemistry to their work. The results which they have obtained and which we are applying to good advantage in soils work are largely due to the outstanding investigations of Osterhout, Livingston, Dixon and Atkins, Stiles and Jorgensen, Brenchley, Shantz and of many others who are following in their footsteps.

From the combined results of modern plant physiologists and soil chemists, we are beginning to accord more importance to the role of the concentration of the soil solution as a medium of plant growth. In my opinion, a proper understanding of the effects of differences in concentration of the soil solution as affecting plants will probably go a great way toward solving some of the

most difficult problems in plant physiology with which we are confronted today. Particularly the problems involved in the so-called physiological diseases of plants would, it seems to me, be made easier by an understanding of the effect of the concentration of the nutrient medium on the diseased plants. For example, in such diseases as die-back and mottled leaf in citrus trees, it is not unlikely that we are dealing with concentrations of salts in the soil solution which are many times higher than those to which the roots of citrus trees are accustomed in their natural habitats, and hence derangement of the normal function of the leaves may result. This is particularly possible when, in addition to a high concentration of salts, a state of balance between the constituents of the solution obtains which is inimical to plants. This will serve as an illustration of the great possibilities which are in store for us in studies of the concentration and balance of the soil solution. The experimental data and observations which support such a theory cannot be reviewed here.

For the sake of completeness, another theory on the infertility of soils, which has been advanced in the last seven or eight years, should be mentioned. It is the one proposed by Bolley, to account for "flax-sick" and "grain-sick" soils. Bolley believes that such soils are rendered infertile through the accumulations, in large numbers, of parasitic fungi, introduced with poor seed, and self-propagating in the soil, which develop with the young plants and depress their growth. This theory has not received much serious attention, however, in spite of the fact that all agronomists and soil chemists seem to agree that the fungi in question may contribute to the depression in yields of those soils. With the fungi removed by the use of good seed, crop rotation, and tillage, the essential problem of maintaining the soil's power to produce still remains.

With the more intense study of the soil itself, which has characterized the last few years, there has gradually grown a more and more insistent demand for accurate methods in such study. One important basis for such demands should be referred to here, in view of its overwhelming significance for the future of all soil studies. I refer to the very marked variability which characterizes soil samples gathered in any field, no matter how apparently uniform, and no matter how close together the samples are taken. Only within the past several months, Frear has called attention to this great variability, and, independently and contemporaneously, one of my colleagues, Waynick, has

actually made a statistical study of one such case, the results of which not only are in agreement with Frear's view, but, in addition, lend mathematical precision to it. Moreover, it provides a basis by which such variability may be determined and a way is made open for the correct appraisal of the significance or insignificance of differences in the results obtained in experimental work.

I have given you in brief, and I fear only in fragmentary form, a survey of the important theories in the history of soils studies. Had I appreciated the true difficulty of the task of discussing them in so brief a space, I should probably not have undertaken it, since I realize how inadequately I have been able, for lack of time, to discuss some very interesting and important points which are involved. I trust that I have succeeded, however, in picturing to you the epochal monuments left by the great figures in the procession of thinkers and workers on soils, from the time of Virgil until today; to interpret for you the successive steps by which they have progressed; to appraise for you the relative importance of the contributions as I view them; and to give you a general idea of the status of our theories today. Above all, I desire to emphasize the fact that now, more than ever before, are we sadly wanting in the best kind of human material for our investigators in soils. The great triumphs of the past have indeed explained much, but, like all investigations, they have served to indicate much more that must yet be known and much that holds in store as great reputations and as many splendid victories as any of the phases of our problems which have been solved. But the problems which remain are of an intricacy and a difficulty probably surpassing anything which we have been obliged to face heretofore. We therefore need men who are second to none in imaginative capacity, in sheer mental vigor, and in training in physics, mathematics, and physical chemistry. To such, I can promise problems of an innate beauty, complexity, and fascination which are not surpassed in any other branch of science. I am optimist enough to believe that, though they have been slow in coming to us, we shall gradually impress such men with the importance of the work and enlist them in a service of the greatest interest to science, and hence of the greatest importance to mankind.

ATOMIC STRUCTURE.1

BY R. R. RAMSEY,

Indiana University, Bloomington.

Within the last twenty-five years there has been a radical change in our ideas of electricity and the structure of matter. The year 1896 is a memorable one in the history of physics. In the early months of 1896 Roentgen's discovery of the X-rays were heralded over the earth as a wonderful discovery enabling one to look through his pocketbook. In 1896 Henri Becquerel discovered the Becquerel rays. That is, that there is an invisible radiation given off from uranium that will darken a photographic plate, this discovery being the first in the science of radioactivity. In 1896 Zeeman discovered that when the source of light is placed in a magnetic field, the lines in the spectrum of the light are broadened and separated into two or more bands. This was an experimental verification of the electron theory of matter put forth by Lorentz a few years before, this theory being that atoms are made up of a number of electrons carrying negative charges of electricity rotating about a central body.

The word electron and a rough idea of an electron had been put forth in 1874 by Dr. Johnstone Stoney of Belfast. In 1879 Sir William Crooks had demonstrated his tubes and the fourth state of matter. Lorentz had given his theory a few years later. But it was not until after 1896 that the theory was looked upon as having much experimental foundation. Thus the electron theory of matter may be said to be based upon three fundamental experiments, X-rays, radioactivity and the Zeeman effect. From these and subsequent experiments we are forced to believe that electricity is made up of electronic charges. That a wire carrying a current becomes hot by the bumping, sliding or friction, if you wish to call it friction, of these electrons or carriers of negative charges which move in the opposite direction to that which the current is said to move.

Our idea of atomic structure is crude and has changed many times in the last twenty years. No doubt it will change many times in the future. Our model atom must explain all known phenomena, and must be changed or reconstructed to explain any new fact that is discovered experimentally.

We cannot hope to dissect an atom and analyze its various parts as we would a machine, an engine, say. We are groping in the dark. Suppose you never saw an engine, never had read

Read before the Physics and Chemistry Section of the Indiana State Teachers Association, Indianapolis, November 1, 1917.