and less as the emanation unden\'ent radioactive decay, and it became motion less after about 30 seconds. Since this process was occurring very rapidly, Hahn and Sackur marked the position of the pointer on a scale with pencil marks. As a timing device they used a metronome that beat out intervals of approximately 1. 3 seconds. This simple method enabled them to determine that the half-life of the emanations of actinium and emanium were the same. Although Giesel's measurements had been more precise than Debierne's, the name of actinium was retained since Debierne had made the discovery first. Hahn now returned to his sample of barium chloride. He soon conjectured that the radium-enriched preparations must harbor another radioactive sub stance. The liquids resulting from fractional crystallization, which were sup posed to contain radium only, produced two kinds of emanation. One was the long-lived emanation of radium, the other had a short life similar to the emanation produced by thorium. Hahn tried to separate this substance by adding some iron to the solutions that should have been free of radium, but to no avail. Later the reason for his failure became apparent. The element that emitted the thorium emanation was constantly replenished by the ele ment believed to be radium. Hahn succeeded in enriching a preparation until it was more than 100,000 times as intensive in its radiation as the same quantity of thorium."

This collection of essays is a tribute to Stillman Drake by some of his friends and colleagues, and by others on whom his work has had a formative influence. It is difficult to know him without succumbing to his combination of discipline and enthusiasm, even in fields remote from Renaissance physics and natural philosophy; and so he should not be surprised in this volume to see emphases and methods congenial to him, even on topics as remote as Darwin or the chemical revolution. Therein lies whatever unity the discerning reader may find in this book, beyond the natural focus and coherence of the largest section, on Galileo, and the final section on Drake's collection of books, a major and now accessible resource for research in the field that he has made his own. We have chosen, as the occasion for presenting the volume to Stillman Drake, Galileo's birthday; Galileo has had more than one birthday party in Toronto since Drake came to the University of Toronto. As for the title, it reflects a shared conviction that experiment is the key to science; it is what scientists do. Drake has already asserted that emphasis in the title of his magisterial Galileo at Work, and we echo it here. Those who have had the privilege and pleasure of working and arguing with Stillman over the years know his tenacity, penetration, and vigour. They also know his generosity and humility. We owe him much.

and less as the emanation unden\'ent radioactive decay, and it became motion less after about 30 seconds. Since this process was occurring very rapidly, Hahn and Sackur marked the position of the pointer on a scale with pencil marks. As a timing device they used a metronome that beat out intervals of approximately 1. 3 seconds. This simple method enabled them to determine that the half-life of the emanations of actinium and emanium were the same. Although Giesel's measurements had been more precise than Debierne's, the name of actinium was retained since Debierne had made the discovery first. Hahn now returned to his sample of barium chloride. He soon conjectured that the radium-enriched preparations must harbor another radioactive sub stance. The liquids resulting from fractional crystallization, which were sup posed to contain radium only, produced two kinds of emanation. One was the long-lived emanation of radium, the other had a short life similar to the emanation produced by thorium. Hahn tried to separate this substance by adding some iron to the solutions that should have been free of radium, but to no avail. Later the reason for his failure became apparent. The element that emitted the thorium emanation was constantly replenished by the ele ment believed to be radium. Hahn succeeded in enriching a preparation until it was more than 100,000 times as intensive in its radiation as the same quantity of thorium."

This collection of essays is a tribute to Stillman Drake by some of his friends and colleagues, and by others on whom his work has had a formative influence. It is difficult to know him without succumbing to his combination of discipline and enthusiasm, even in fields remote from Renaissance physics and natural philosophy; and so he should not be surprised in this volume to see emphases and methods congenial to him, even on topics as remote as Darwin or the chemical revolution. Therein lies whatever unity the discerning reader may find in this book, beyond the natural focus and coherence of the largest section, on Galileo, and the final section on Drake's collection of books, a major and now accessible resource for research in the field that he has made his own. We have chosen, as the occasion for presenting the volume to Stillman Drake, Galileo's birthday; Galileo has had more than one birthday party in Toronto since Drake came to the University of Toronto. As for the title, it reflects a shared conviction that experiment is the key to science; it is what scientists do. Drake has already asserted that emphasis in the title of his magisterial Galileo at Work, and we echo it here. Those who have had the privilege and pleasure of working and arguing with Stillman over the years know his tenacity, penetration, and vigour. They also know his generosity and humility. We owe him much.

These remarks preface two volumes consisting of the proceedings of the Third International Conference on the History and Philosophy of Science of the International Union of History and Philosophy of Science. The conference was held under the auspices of the Union, The Social Sciences and Humanities Research Council of Canada, and the Canadian Society for History and Philosophy of Science. The meetings took place in Montreal, Canada, 25--29 August 1980, with Concordia University as host institution. The program of the conference was arranged by a Joint Commission of the International Union of History and Philosophy of Science consisting of Robert E. Butts (Canada), John Murdoch (U. S. A. ), Vladimir Kirsanov (U. S. S. R. ), and Paul Weingartner (Austria). The Local Arrangements Committee consisted of Stanley G. French, Chair (Concordia), Michel Paradis, treasurer (McGill), Franyois Duchesneau (Universite de Montreal), Robert Nadeau (Universite du Quebec a Montreal), and William Shea (McGill University). Both committees are indebted to Dr. G. R. Paterson, then President of the Canadian Society for History and Philosophy of Science, who shared his expertise in many ways. Dr. French and his staff worked diligently and efficiently on behalf of all participants. The city of Montreal was, as always, the subtle mixture of extravagance, charm, warmth and excitement that retains her status as the jewel of Canadian cities. The funding of major international conferences is always a problem.