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Rh In 1858 a few voices were heard in Germany in praise of Mayer, but the one who did most to bring him historical justice was John Tyndall, who in 1862 lectured before the Royal Institution on Robert Mayer. I shall remain silent on the extremely bitter controversy, between Tyndall and Tait, regarding Mayer's researches. Tait and William Thomson placed a low estimate on Mayer's work and brought the charge that Tyndall, by praising Mayer, was belittling the work of Joule. One would suppose that gross historical errors would have been eliminated by this time from a subject like the conservation of energy, about which so much has been written. Such is not the case. Professor E. T. Glazebrook, writing in the great "Dictionary of National Biography" (article, "Joule"), said in 1892, just before the publication of Mayer's correspondence, that Mayer in 1842 endeavored to measure the heat produced in the compression of air, but committed the very serious mistake of assuming without experimental evidence that "all the mechanical energy spent in compressing the air was used in producing change of temperature." This same criticism was passed upon Mayer by Joule, Tait and Helmholtz. A fundamental question is here involved, but the charge is not true. As early as September 12, 1841, in a. letter to his friend Professor Baur, Mayer explained Gay-Lussac's experiment of 1807 on the flow of gas into a vacuum, and drew upon it to complete his argument on the equivalence of the work of compression and the heat generated by the compression. Gay-Lussac had found that when a gas expands into a vacuum it undergoes no change in temperature large enough to be detected by his thermometers. Hence, during compression practically all the work done upon the gas goes to produce change of temperature, and Mayer's argument is sound. That the criticism of Mayer's reasoning is invalid is not generally known to recent writers on the subject, but can now be verified by any one who will examine Mayer's collected works and letters, edited by Weyrauch in 1893. It should be added, however, that Mayer himself is partly to blame for the strictures passed upon his paper of 1812. Gay-Lussac is not mentioned and the whole matter is disposed of in a single sentence, though that sentence, we admit, is somewhat Germanic in its structure and linear dimensions.