Lever. IV. Based on buoyancy of liquids. V. Chain balance. VI. Hydrostatic balance. In order to understand the principle on which the first group of weighing machines is based it will be necessary first to consider the lever. A lever is any rigid bar or rod, the weight of which may be neglected, free to turn about a fixed point called the fulcrum. The forces acting on the lever are the weight, or resistance, the power, and the reaction of the fulcrum. Since these are in equilibrium, the resultant of the power, P, and the weight Q (compare figures below) must act through the fulcrum C, for otherwise they could not be balanced by the reaction. The product of the power and length Ca of the arm of lever from the fulcrum to the point at which the power is applied, and the product of the weight and the length of the arm Cb, are called the statical moments. In order, therefore, that the lever may be in equilibrium, the product of the power and the power arm must be equal to the weight times the weight arm. This is called the Law of the Lever and can be expressed by the following equation: Levers. Levers are divided into three kinds, according to the position Kinds of of the fulcrum with respect to the points of application of the power and of the weight: I. The fulcrum is between the power and the weight. II. The weight is between the power and the fulcrum. III. The power is between the fulcrum and the weight. Apparatus necessary: A model for demonstrating the principles of the Exercise I. lever, and weights. Exp. 1. Hang two weights on the 5th pin to the right, also two weights on the fifth pin to the left. Then the statical moment on the left is 2X5= 10; on the right it is also 2×5 and we have equilibrium. Exp. 2. Hang 3 weights on the 8th pin on each side. What are the statical moments in this case? The Lever. -- Exp. 3. Hang 3 weights on the sixth pin to the left, also 2 weights on the 9th pin to the right. What are the statical moments in this case? Why do we have equilibrium? Exercise II. Fig. 5. Lever. Exp. 4. Hang 4 weights on the 10th pin to the left, also 5 weights on the 4th pin to the right. How many weights must you hang on the 10th pin to the right in order to obtain equilibrium? Exp. 5. Hang 3 weights on the 9th pin to the left. Calculate how many weights you must hang on the fifth and on the third pins to the right in order to produce equilibrium. Prove by experiment. Exp. 6. Hang 1 weight on the 10th pin to the left and 2 weights on the 7th pin to the left. How many weights must be hung on the 4th pin to the right to restore equilibrium? On the sixth pin to the right? On the 8th pin to the right? Prove your experiment. Apparatus necessary: The same as for Exercise I, also a pulley. See Max Kohl, Präcisionsmechaniker in Chemitz, Germany. Catalogue, p. 35, Published Literary. Books Reviewed. AN INTRODUCTION TO CHEMICAL ANALYSIS FOR STUDENTS OF MEDICINE, PHARMACY AND DENTISTRY. By Elbert W. Rockwood, Professor of Chemistry and Toxicology and head of the department of Chemistry in the University of Iowa. Second revised edition. P. Blakiston's Son & Co., Philadelphia. Price $1.50 net. That a second edition of this book is issued three years after the appearance of the first is proof that it has found friends among the students who have been brought in touch with the volume. The author well says "The value of analytical chemistry in stimulating observation, power of discrimination, independence and self-reliance has long been recognized, as well as its services in affording an easy introduction to chemical work. It may however, be carried on as a handicraft without being of assistance in demonstrating the fundamentals of chemistry." He might safely have added that there is great danger of a students learning the general routine of analysis. and following its course empirically without gaining that stimulation. to his powers of observation which a wise and thoughtful study only can produce. It seems to the reviewer that the author has not altogether succeeded in showing how habits of empiricism may be avoided. While the methods of the book in general are fairly good, one can but feel that the author has endeavored to "make the course more interesting by showing some of its applications" at the expense of want of thoroughness on the part of the student, the result of which must be a failure to train him in that independence of thought and power of discrimination which a more rigorous course alone would produce. However, with the most perfect of text-books the desired ends will only be gained by the wise and careful direction of the instructor. Doubtless, this fact the author as an experienced teacher has learned. There are a few mistakes in printing chemical equations which doubtless will be corrected in future editions. The book is well printed on good paper, and makes a very neat volume. W. W. Daniells. CHEMISCHE TECHNOLOGIE. Von Dr. Fr. Heusler. Ein Bd., pp. XVI, 351, mit zahlreichen Abbildungen. Verlag von B. G. Teubner in Leipzig. 1905. M. 8.00., geb. M. 8.60. This volume in the third of "Teubner's Handbücher für Handel und Gewerbe," edited by Van der Borgt, President of the statistical bureau of Berlin, Dr. Schumacher, director of the commercial high school at Bonn, and Dr. Stegemann, privy council in Braunschweig. The first volume is entitled "Die Zuckerindustrie," the second "Versicherungswesen." Soon to appear are "Betrieb der Fabriken,” “Anlage von Fabriken," "Die Zuckerproduktion der Erde." As in course of preparation, eight other numbers, ranging from such general subjects as an introduction to political economy and a like treatise on chemistry to more technical subjects such as transportation and chemical industry, are enumerated. With the development of courses in commerce at our large universities, these volumes ought to meet with a hearty welcome on this side of the Atlantic. The rather sudden growth of these new courses, found many a teacher without adequate text or even reference books, particularly in the applied natural sciences. In the domain of chemistry and chemical technology the special development in this direction has been left largely to the colleges of engineering. The natural result has been that some of the best books on the subject are too technical on the side of engineering. While, no doubt, there is a considerable demand for the chemical engineer in this country, there ought to be a place as well for the technically trained chemist who is not primarily an engineer, also for the student of political economy and commerce, who is in possession of a good general knowledge of chemistry and technology without being a technical expert. This book is especially designed for chemical and commercial students, be they at college or in the laboratory or office. It presupposes an elementary knowledge of chemistry, organic as well as inorganic, but it is written so that even any person with a good general education may get much out of it without a knowledge of chemical |