The unsatisfactory state of our knowledge of the Mechanical Equivalent of Heat and, inseparably connected therewith, of the capacity for heat of water, is the more surprising when we consider the large number of physicists who have devoted their attention to this subject during the century just closed. Since the remarkable pioneer experiments of Count Rumford, undertaken just 100 years ago, to determine the nature of heat, the subject has been advanced step by step by different investigators. Conspicuous among these we may mention Begnault, who gave us the first idea of the mode of the variation of the specific heat of water with temperature, without, however, giving us any knowledge of the mechanical equivalent of heat; Joule, who gave us the first measurements of the mechanical equivalent without attempting to study the thermal unit at different temperatures; Rowtland, who by the remarkable accuracy of his experiments gave us not only a direct determination of the mechanical equivalent, but also the variation of the thermal unit over a limited range. More recently we have the exceedingly careful experiments of Miculescu, of Griffiths, of Schuster and Gannon, and of Reynolds and Moorby. It is evident from only a cursory glance at the work of these and the host of other investigators, that the science of calorimetry must he regarded as incomplete and approximate so long as its fundamental unit remains in doubt. To obtain, as is urgently needed, a complete series of determinations of the capacity for heat of water over the entire range of temperature is manifestly impossible by the older methods of calorimetry. A new method has long been required, more completely free from the influence of extraneous surrounding conditions.
On the direct determination of the electrostatic moments of molecules
