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Precipitate Replications: The Cognitive Analysis of Michael Faraday’s Exploration of Gold Precipitates and Colloids

Precipitate Replications: The Cognitive Analysis of Michael Faraday's Exploration of Gold Precipitates and Colloids Ryan D. Tweney (tweney@bgnet.bgsu.edu) Ryan P. Mears (rmears@bgnet.bgsu.edu) Robert E. Gibby (gibbyro@bgnet.bgsu.edu) Christiane Spitzmulle r (spit@bgnet.bgsu.edu) Yanlong Sun (suny@bgnet.bgsu.edu) Department of Psychology, Bowling Green State University Bowling Green, OH 43403 USA Abstract In 1856, Faraday conducted a long series of experiments on the color of gold. We report replications of some of his experiments, permitting an understanding of his response to an important anomaly and the resulting conceptual reorganization of his ideas. Introduction In 1856, Michael Faraday (1791-1867) carried out nearly a year's worth of research on the optical properties of thin gold films (Faraday, 1857). In the course of this work, he discovered colloidal gold (the first metallic colloid) and what is now known as the “Faraday-Tyndall Effect,” the fact that colloids scatter light. Colloids were an unexpected consequence of his attempts to understand the unusual color properties of thin films of gold. These, in turn, were an important extension of his earlier attempts to understand the interaction of light and matter, and his speculations about the force-centered character of matter (James, 1985; Tweney, 2002). The present project was initiated by the discovery of over 600 surviving microscope slides and other specimens made by Faraday as part of his research, and now held at the Royal Institution in London (Tweney, 2002). The slides are numbered and indexed in Faraday’s Diary and represent nearly the complete set of metallic film specimens used by Faraday in 1856. However, only a few of his colloidal specimens (and none of his precipitates) survived; thus, one goal of the present effort is to restore lost specimens for analysis. Replication is also important even for specimens that still survive -- Faraday often subjected the specimens to destructive and/or damaging manipulations, and these manipulations also need replication. Four general procedures used by Faraday are currently being replicated by our group: (1) Precipitation of gold from solution, (2) “Deflagration” of gold wire, that is, exploding gold wire using sudden surges of current, (3) Producing colloids using reduction by phosphorous, and (4) Producing thin metallic films of gold using reduction by phosphorous. Here we present our replications of Faraday's precipitates. Besides restoring for analysis certain lost specimens, these allow insight into the tacit knowledge implicated in their preparation. More importantly, preparation of our own precipitates allowed analysis of aspects of Faraday’s research previously hidden from view, helping to account for an important conceptual change. In earlier accounts of Faraday's research, our group examined the way in which Faraday experimentally traversed a problem space of hypothesized and real results during his discovery of electromagnetic induction in 1831 (Tweney & Hoffner, 1987). Our analysis suggested that Faraday used a relatively narrow search strategy in the 1831 experiments, one in which potentially disconfirming evidence was initially ignored and only evidence which supported expectations was pursued. In later stages of the research, he made explicit attempts to disconfirm. This confirm early-disconfirm late strategy resembled heuristics observed by Klahr in the Big Trak studies (Klahr, 2000) and by Dunbar (1995) in an in vivo study of laboratory molecular biologists. Not all aspects of scientific thinking can be characterized as search through multiple problem spaces (Kurz & Tweney, 1998), and this is especially true of Faraday’s work. For example, Gooding (1990) replicated Faraday's 1821 discovery of electromagnetic rotations and argued that identification of circular rotatory motions could only have come about by means of an eye-hand-brain interaction of a very dynamic character. Rather than testing hypotheses, Faraday instead had to make the meaning of the otherwise chaotic appearances presented by the experimental apparatus. Similarly, Cavicchi (1997), partly by conducting replications, showed that Faraday's experimentation during his 1845 discovery of diamagnetism proceeded not by progressively refining