Silica is found in natural waters in two forms, the one insoluble and the other soluble. In the latter it is usually known as silicic acid and is reported as Si02 in the ordinary water analyses. In solution it occurs in a colloidal form or in combination with the basic elements. Its presence in water used for steam purposes has always been considered as detrimental and conducive to the formation of a hard flinty scale. Silicic acid is said to be responsible for many boiler disturbances. When the acid is distilled with water solutions of nitrates and chlorides, nitric and hydrochloric acid are liberated.Turbidity in water is generally caused by the suspensions of very finely divided mineral matter, mainly clay. Clay may be defined as a mixture of minerals of which the most representative members are the silicates of aluminum, iron, and the alkalies, and the alkaline earths and organic substances. The hydrated aluminum silicate or kaolin (AhOa, 2Si02, 2H 20) is the most abundant compound.There are many instances in the literature emphasing the difficulties of clarifying water containing finely dispersed clay particles. Fuller and Ellrns' found that at times there occurred a turbidity in the Ohio River that was difficult to coagulate and required an abnormal consumption of alum. Black and Veatch! and Catlett" show the difficulties in the treatment of such water.Properties of colloids. As silicic acid and a clay suspension are considered to be present in water in a colloidal form, a brief discussion of the general properties of colloids will be desirable. Burton defines a colloid solution as
This paper constitutes a report on a part of the work which we are doing in studying the value of the Aptitude or Placement Examination and of semester grades in predicting the future performance of our students.OurFreshman classes areconducted similarly to those in other institutions.In 1926-27, 879 students who had not had chemistry registered for General Chemistry. In the first week these students were given the Iowa Chemistry Aptitude Test, CT-1,A. From the results of this test the students were sectioned for quiz and lahoratory into groups of twenty-five students. There was no sectioning in the two hours devoted to lecture, demonstrations, and tests. For laboratory and quiz the whole class was divided into six groups of 150 to 200 students, depending solely upon the afternoon the pupil took laboratory. This group was then subdivided according to the Aptitude Test or term grades into groups of twentyfive students each. Thus on one afternoon the best twenty-five students would be in one group, the next best in another, etc. The lahoratory and quiz for any one group was under the same instructor. One important difficulty of this method lies in the fact that there are five or six best groups of twenty-five students each, where there should be only two or three (10% of class) of these groups.This sectioning according to ability was carried out throughout the three terms. The score made in the first term was used as a basis for sectioning in the second term, and similarly the score in the second term was taken as the basis for the sectioning in the third.The examinations given were largely of the objective type and graded by means of scores. The term grades in per cent were obtained from score grades. In each examination scores were plotted and a curve drawn to see how well the markings were distributed. During the latter part of the last term a group of 164 students took the Iowa Chemistry Training Examination. These examinations were scored in the Departments of Chemistry and Education of the University of Iowa.'Of the 164 students discussed by Dr. Jacob Comog, only 131 started in the fall and completed the work in a regular manner. It is the performance of these 131 students which we will discuss. We believe these *Read before the Division of Chemical Education of the American Chemical Society at St. Louis, April 17, 1928.' Cornog and Stoddard, "Chemistry Training of High-School and College Students," THIS JOURNAL, 6,85-92 (Jan., 1929).
An explanation is given in this paper of why many turbid river waters require at certain periods excessive amounts of coagulants for clarification and how purification may be most efficiently accomplished. The action of electrolytes commonly occurring in natural waters upon the stability of colloidal clay suspensions and silicic acid and their effect when alum is used as a coagulant, is discussed. Colloidal clay and silicic acid are more prevalent in the surface waters of the Mississippi valley2 and the western states, and the need of research in this line has been emphasized in several papers.8The experimental work was carried out along 3 well defined lines, and is discussed in the following order: 1. The removal of silicic acid 1 Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Illinois.2 Water Supply Paper No. 236.
Report oj the Committee on Examinations and Tests T HE development of the Cooperative Chemistry Tests1 is a part of a cooperative endeavor of the committee on Examinations and Tests of the Division of Chemical Education and the American Council on Education through its Cooperative Test Service. As educational problems of wide significance arise, the organization obtains grants from foundations for the study of these critical problems. On July 1, 1930, the General Education Board, one of the Rockefeller Foundations, granted a sum of money to the 0 10 20 30 40 50 Percentile 60 70 80 90 100 Figure 1.-Distribution of Scores for 1935 and 1936American Council on Education to form a non-profit agency to stimulate the construction, distribution, and use of better tests at the high-school and college level.
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