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of "standard mixtures" having a known hydrogen ion concentration or pH value. These investigators established their formulas very precisely by the use of a potentiometer method employing a hydrogen electrode. According to Clark, the pH values in the freshly prepared mixtures may be considered reliable to a few hundredths of a pH unit. Probably the widest application of these standard solutions is, in connection with color indicators, for comparisons with solutions having an unknown hydrogen ion concentration.
The fact that standard mixtures can readily be prepared, combined with the further fact that the hydrogen electrode is an appliance which is simple and convenient to use, leads to a very obvious suggestion. This is to utilize the hydrogen electrode as a means for comparison of an unknown with a standard solution. It should extend materially the usefulness of the standard solutions to which reference has been made.
We may suppose that we desire to titrate a solution of unknown pH value to a definite hydrogen ion concentration. From the curves of Sörensen's article, or from the formulas of Clark and Lubs, we select the particular solution having a pH value which corresponds to the point to which we desire to titrate. This solution is placed in one vessel with a hydrogen electrode, and connection is established between the standard solution and the unknown, in a second vessel, by means of a salt bridge of saturated potassium chloride, so that concentration potentials may be eliminated.* Another hydrogen electrode is placed in the solution of unknown concentration, and the two electrodes are connected through a tapping key and a galvanometer of high resistance. Appropriate protective resistance may also be put in this circuit. The process of titrating to the desired end point then consists merely of adding the titrating solution until, upon tapping the key, no deflection of the galvanometer is
• Private communication.
The suggestion of using an agar-agar salt bridge to minimize diffusion effects (Falles and Vosburgh, J. A. C. S., 40, 1306, 1918) seems a good one.
observed. The inference is that zero potential difference between the hydrogen electrodes is an indication of equal hydrogen ion concentrations of the two solutions. The supposition may be verified by putting both electrodes into one or the other of the solutions and noting whether the galvanometer deflection remains zero.
It may be pointed out that such a titration can be carried out in any solution in which a hydrogen electrode will maintain its equilibrium, regardless of color, turbidity, or other experimental conditions. The electrolytic portion of the galvanometer circuit will, in most cases, have a low resistance, which insures the desirable condition for sensitiveness of response of the instrument. The method has most of the advantages of the potentiometer method over the colorimetric methods, with the obvious exception that it can be used only for titrating and that the titration can be carried only to the end point which is determined by the standard solution. With the potentiometer it is possible, of course, not only to titrate to any end point but also to make a direct measurement, without titration, of the pH value, whatever this may be.
It should be noted, finally, that in the titration described no calomel electrode is used, and that the accuracy with which the titration may be made is limited only by the accuracy with which the pH value of the standard solution is known.
PAUL E. KLOPSTEG LEEDS & NORTHRUP COMPANY, PHILADELPHIA, PA.
THE AMERICAN SOCIETY OF
THE second annual meeting of the American Society of Mammalogists was held May 3-5, 1920, in the American Museum of Natural History, New York City. Officers for the coming year are Dr. C. Hart Merriam, president; Mr. E. W. Nelson and Dr. Wilfred H. Osgood, vice-presidents; Dr. H. H. Lane, recording secretary; Dr. Hartley H. T. Jackson, corresponding secretary; Mr. J. W. Gidley, treasurer; Mr. N. Hollister, editor; Dr. Glover M. Allen, Dr. R. M. Anderson, Dr. Joseph Grinnell,
The Calvert Miocene formation and some of its mammals: WILLIAM PALMER. Thirty minutes. Illustrated with lantern slides.
On some early states in the evolution of mammalian dentition: WILLIAM K. GREGORY. Forty minutes. Illustrated with lantern slides.
Some scattered observations about narwhals: MOR-
Beginnings of the placental mammals: W. D.
TUESDAY, MAY 4
Afternoon Session, 2 P.M.
A dissection of a pigmy sperm whale (Kogia): C. L. CAMP AND J. P. CHAPIN. Fifteen minutes. Illustrated with lantern slides.
(a) Notes on New England. (b) Bison remains in New England. (c) Exhibition of specimens of Myotragus, the remarkable Pleistocene goat of the Balearic Islands: GLOVER M. ALLEN. Twenty-five minutes. Illustrated with photographs and specimens.
Blue-fox farming and the maintenance of the fur
Saving the Yellowstone elk herd: E. W. NELSON.
P. BLAKISTON'S SON & CO., Publishers, PHILADELPHIA
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THE SCIENTIFIC BASIS OF THE ILLINOIS SYSTEM OF PERMANENT SOIL FERTILITY1
It is practically impossible to cover, in an adequate way, the scientific principles underlying the Illinois system of permanent soil fertility in the brief space of time allotted me on the program. Nevertheless, I shall point out the fundamental principles underlying the system without attempting to illustrate the points made by definite data as I should like to do. Eighty years ago Liebig, the father of agricultural chemistry, made the following statement:
Agriculture is, of all industrial pursuits, the richest in facts, and the poorest in their comprehension. Facts are like grains of sand which are moved by the wind, but principles are these same grains cemented into rocks.
The great contribution made to American agriculture by the late Dr. Hopkins was the gathering together, classifying, interpreting and unifying, by his own investigations the known facts of agriculture, into a definite whole as practised and taught by him in the Illinois system of permanent soil fertility.
Many of the facts upon which the Illinois system rests have been known for many years and even centuries and have been developed by other men in other institutions and in other times. It remained, however, for Dr. Hopkins to bring together and unify these isolated facts into a definite workable system and by his own investigation to demonstrate clearly that the system could be understood and used by the average farmer on his own farm with very profitable results. In his interpretation of the facts upon which the system is based, all men have not agreed and some even still do not agree with him but the system rests on the
1 An address given at the Hopkins Memorial, January 22, 1920