TN the Philippine Journal of Science of February, 1914, the authors •*-published the results of an extensive series of observations on the amount of radium emanation in the atmosphere of Manila. The wellknown charcoal-absorption method was used and the main part of the investigation was preceded by numerous tests on certain points concerned with the accuracy of the method. The work at Manila was supplemented by an investigation of the variation of the emanation content of the atmosphere with altitude, observations being taken on Mt. Pauai, elevation 2,460 meters, and the results compared with those for sea level. 1 Although the especial points of the investigation were those above mentioned we felt justified at that time, since the period involved extended over almost a year and a half, in pointing out certain evident relations between the emanation content of the atmosphere and the different meteorological factors. The data at our command was not considered sufficient, however, to justify the drawing of more than very general conclusions. During the past year observations have been taken with the object of determining more definitely to what extent the amount of radium emanation in the air is dependent on weather conditions. At the same time tests have been made on several other points which have a more or less direct bearing on the question.The experimental method used was described in detail in our previous paper, but for the sake of completeness a brief summary of the description will be given, together with such modifications as experience had shown to be advisable.The method had its origin in the discovery by Rutherford that charcoal made from the shells of coconuts possesses the property of absorbing radioactive emanations. Eve 2 and Satterly 3 independently applied the discovery to the determination of the radium-emanation content of the atmosphere. 1
among its several phases, the determination of the number of free ions per unit volume in the earth's atmosphere together with the variation of this number due to altitude, location, and meteorological conditions.The first attempt to measure the ionization of the free atmosphere was made by Elster and Geitel 1 while investigating the causes of the discharge of electrified bodies surrounded by air in closed vessels. They found that when a piece of wire gauze, connected to an electroscope, was exposed to the open air both positive and negative electricity was discharged at a much more rapid rate than when placed in a closed vessel, also that the rate of discharge of positive and negative was generally different. The rate of discharge was taken as a measure of the ionization, but, since by this method they had no means of determining the volume of the air tested, quantitative measurements were impossible and only comparative values were obtained.A short time later an instrument was devised by Ebert 2 which made the determination of the number of positive or negative ions present per cubic centimeter in the atmosphere comparatively easy. With this apparatus determinations have been made at various points on the earths' surface by different observers, and although the individual results vary greatly the general conclusions that have been drawn are fairly uniform.During the past two years we have been investigating atmosphericelectric phenomena at Manila and vicinity and as a part of that work have made a large number of determinations of the free ions present. These observations were taken by an improved form of Ebert's apparatus made by Gunther and Tegetmeyer of Braunschweig, Germany. It has a vertical rather than a horizontal collecting cylinder, and a Wulf 3 electrometer in the place of the Elster and Geitel electroscope. The theory of the instrument is well known, but a brief discussion seems of value here for the sake of completeness.
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