Transcriber’s note: Table of Contents added by Transcriber. CONTENTS Preventive Inoculation 115 Professor Ewart's Penycuik Experiments 126 Colonies and the Mother Country 139 The Future of the Negro in the Southern States 147 The Physical Geography of the Lands 157 The New York Botanical Garden 171 Gas and Gas Meters 179 The Sun's Destination 191 A Biographical Sketch of an Infant 197 Correspondence 206 Scientific Literature 213 The Progress of Science 219 THE POPULAR SCIENCE MONTHLY EDITED BY J. McKEEN CATTELL VOL. LVII MAY TO OCTOBER, 1900 NEW YORK AND LONDON McCLURE, PHILLIPS AND COMPANY 1900 COPYRIGHT, 1900, BY McCLURE, PHILLIPS AND COMPANY. [Illustration: PROFESSOR WOLCOTT GIBBS, PRESIDENT OF THE NATIONAL ACADEMY OF SCIENCES, EMERITUS RUMFORD PROFESSOR AND LECTURER ON THE APPLICATIONS OF SCIENCE TO THE USEFUL ARTS AT HARVARD UNIVERSITY.] THE POPULAR SCIENCE MONTHLY. JUNE, 1900. PREVENTIVE INOCULATION. (I.) BY DR. W. M. HAFFKINE. DIRECTOR-IN-CHIEF, GOVERNMENT PLAGUE RESEARCH LABORATORY, BOMBAY. It was due to certain particularly favorable circumstances that the first ideas on preventive inoculation were gathered from observations on smallpox patients. Such circumstances were presumably the following: _a._ It is a disease which attacks epidemically, in a short time and within a small area, large numbers of people, thus permitting of easy comparisons and suggesting conclusions from the facts observed. _b._ Its fatality is comparatively small, so that after each outbreak a large number of convalescent persons remain alive to serve as objects for future observation and comparison. _c._ These convalescents are marked and are thus easily distinguishable from the rest of the population who have not been attacked, and even the severity of the disease they have gone through is, so to say, written down on their faces and bodies. _d._ The disease is easily communicable, owing to the infectious matter appearing on the surface of the patient’s body in the pustules.