This book is primarily about the methodological questions involved in attempts to understand two of the most peculiar phenomena in physics, both occurring at the lowest of temperatures. Superconductivity (the disappearance of electrical resistance) and superfluidity (the total absence of viscosity in liquid helium) are not merely peculiar in their own right. Being the only macroscopic quantum phenomena they also manifest a sudden and dramatic change even in those properties which have been amply used within the classical framework and which were thought to be fully understood after the advent of quantum theory. A few years ago we set ourselves the task of carrying out a methodological study of the "most peculiar" phenomena in physics and trying to understand the process by which an observed (rather than predicted) new phenomenon gets "translated" into a physical problem. We thought the best way of deciding which phenomena to choose was to rely on our intuitive notion about the "degrees of peculiarity" developed, no doubt, during the past ten years of active research in theoretical atomic and elementary particle physics. While the merits of the different candidates were compared, we were amazed to realize that neither the phenomena of the very small nor those of the very large could compete with the phenomena of the very cold. These were truly remarkable phenomena if for no other reason than for the difficulties encountered in merely describing them.

How happy it is to recall Imre Lakatos. Now, fifteen years after his death, his intelligence, wit, generosity are vivid. In the Preface to the book of Essays in Memory of Imre Lakatos (Boston Studies, 39, 1976), the editors wrote: ... Lakatos was a man in search of rationality in all of its forms. He thought he had found it in the historical development of scientific knowledge, yet he also saw rationality endangered everywhere. To honor Lakatos is to honor his sharp and aggressive criticism as well as his humane warmth and his quick wit. He was a person to love and to struggle with. The book before us carries old and new friends of that Lakatosian spirit further into the issues which he wanted to investigate. That the new friends include a dozen scientific, historical and philosophical scholars from Greece would have pleased Lakatos very much, and with an essay from China, he would have smiled all the more. But the key lies in the quality of these papers, and in the imaginative organization of the conference at Thessaloniki in summer 1986 which worked so well.

"Tile; D'apC:Tile; l. DpWTa ()coi 7rpo7rapod)w GBP D'T}K,mi'. "between us and Goodness the gods have placed the sweat of our brows". This quote from Isiodos, the first lyrical poet, is jotted on a sheet of paper found among the papers of Heike Kamerlingh Onnes at the Boerhaave Museum, Leiden. On this same sheet, one can also read quotes from Schiller, Goethe, Shakespeare, Homer, Pindar and Dante. Each quote is for somebody or something. It appears to have been a game played at least by Ehrenfest and Crommelin -an unmistakable sign of these two physicists's deep culture. This particular quote was for the "Werkplaats", the Physical Laboratory of the University of Leiden. Our purpose in putting together the Selected Papers of its first Director, Kamerlingh Onnes (1853-1926), is to try and articulate the dominant trends of a different type of culture at Leiden: its physics culture during the years that established low temperature physics as a distinct branch of physics. Our aims in choosing the particular papers are threefold. First, we wish to present the interconnectedness among the different research programs of Kamerlingh Onnes and to bring out the decisive role of the work initiated by van der Waals in determining the direction of nearly all of these research programs.

How happy it is to recall Imre Lakatos. Now, fifteen years after his death, his intelligence, wit, generosity are vivid. In the Preface to the book of Essays in Memory of Imre Lakatos (Boston Studies, 39, 1976), the editors wrote: ... Lakatos was a man in search of rationality in all of its forms. He thought he had found it in the historical development of scientific knowledge, yet he also saw rationality endangered everywhere. To honor Lakatos is to honor his sharp and aggressive criticism as well as his humane warmth and his quick wit. He was a person to love and to struggle with. The book before us carries old and new friends of that Lakatosian spirit further into the issues which he wanted to investigate. That the new friends include a dozen scientific, historical and philosophical scholars from Greece would have pleased Lakatos very much, and with an essay from China, he would have smiled all the more. But the key lies in the quality of these papers, and in the imaginative organization of the conference at Thessaloniki in summer 1986 which worked so well.

This book is primarily about the methodological questions involved in attempts to understand two of the most peculiar phenomena in physics, both occurring at the lowest of temperatures. Superconductivity (the disappearance of electrical resistance) and superfluidity (the total absence of viscosity in liquid helium) are not merely peculiar in their own right. Being the only macroscopic quantum phenomena they also manifest a sudden and dramatic change even in those properties which have been amply used within the classical framework and which were thought to be fully understood after the advent of quantum theory. A few years ago we set ourselves the task of carrying out a methodological study of the "most peculiar" phenomena in physics and trying to understand the process by which an observed (rather than predicted) new phenomenon gets "translated" into a physical problem. We thought the best way of deciding which phenomena to choose was to rely on our intuitive notion about the "degrees of peculiarity" developed, no doubt, during the past ten years of active research in theoretical atomic and elementary particle physics. While the merits of the different candidates were compared, we were amazed to realize that neither the phenomena of the very small nor those of the very large could compete with the phenomena of the very cold. These were truly remarkable phenomena if for no other reason than for the difficulties encountered in merely describing them.

"Tile; D'apC:Tile; l. DpWTa ()coi 7rpo7rapod)w GBP D'T}K,mi'. "between us and Goodness the gods have placed the sweat of our brows". This quote from Isiodos, the first lyrical poet, is jotted on a sheet of paper found among the papers of Heike Kamerlingh Onnes at the Boerhaave Museum, Leiden. On this same sheet, one can also read quotes from Schiller, Goethe, Shakespeare, Homer, Pindar and Dante. Each quote is for somebody or something. It appears to have been a game played at least by Ehrenfest and Crommelin -an unmistakable sign of these two physicists's deep culture. This particular quote was for the "Werkplaats", the Physical Laboratory of the University of Leiden. Our purpose in putting together the Selected Papers of its first Director, Kamerlingh Onnes (1853-1926), is to try and articulate the dominant trends of a different type of culture at Leiden: its physics culture during the years that established low temperature physics as a distinct branch of physics. Our aims in choosing the particular papers are threefold. First, we wish to present the interconnectedness among the different research programs of Kamerlingh Onnes and to bring out the decisive role of the work initiated by van der Waals in determining the direction of nearly all of these research programs.