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BOAKE, ROBERTS, & CO. (LIMITED), Stratford, London, E. งล It is comparatively recently that the thermostat came into general use, and many improvements have been effected in the last few years. A collection has therefore been made of recent devices in the hope that a collection of the somewhat scattered notes on the subject may prove useful. In the preparation of these notes I am indebted to several of those named below for information in regard to various improvements and modifications. The Thermostat. - Copper appears to be the most satisfactory material for a thermostat. Enamelled iron usually cracks and rusts after a short time, particularly if used at higher temperatures. It is desirable that the thermostat should contain so much water that no appreciable alteration in temperature is effected when the experi mental apparatus is introduced. The level of the water in the thermostat should be kept constant, and an apparatus for this purpose is figured in a paper by Lumsden (Fourn. Chem. Soc., 1902, lxxxi., 352). If the thermostat is near a water-tap, the level may be kept constant with the arrangement monly used for hot waterbaths. com Glass-fronted Thermostats. Most of the German-made thermostats have the glass cemented into a groove in the metal. This gives a water-tight joint, but the cement is somewhat readily fractured. A more satisfactory FIG. 1. method is the use of a strip of rubber between the glass and metal, the glass being clamped to a flange on the side of the bath. Several of this type are in use in Edinburgh and Aberdeen, and are much superior to those with cement (made by Fraser, 16, Haddon Street, Aberdeen). Flow of Liquid at Constant Temperature.-A thermostat arranged to supply a stream of water at constant temperature is described by Lowry (Trans. Faraday Soc., 1907, iii., 119). Regulators. Since the value of the thermostat depends on the constancy of the temperature, it is natural that many varieties of regulators should be described. In all modern forms, the expansion of a liquid is used to actuate a cut-off for the heat supply. Choice of a Liquid. -The liquid to be used in the regulator should have a low specific heat, a high coefficient of expansion, and preferably a low density. As these are the main factors in the choice of a liquid, a few figures may be given for the specific heats and coefficients of expansion of some common liquids. The numbers are approximate, and refer to temperatures about 25°. A Paper read before the Faraday Society, December 6, 1911. Benzene Toluene Ethyl alcohol Ethyl ether .. Chloroform Carbon tetrachloride 0.2 The very low specific heat of mercury is counterbalanced by its great density, which necessitates the use of thickwalled apparatus. Most organic liquids show an expansion with temperature of the same order as those quoted, but A FIG. 2. It is paraffin oils are much lower and are unsuitable. mainly a matter of convenience which of the above organic liquids is used, but that with the lowest density will be slightly more efficient than the others. Bulb of Regulator.-The greater the surface exposed, the more sensitive will be the regulator; consequently, several have recommended the use of a spiral to contain the liquid (Gouy, Journ. de Phys., 1897, [3], vi., 479, and others). Lowry has described a convenient form of spiral (Trans. Chem. Soc., 1905, lxxxvii., 1030). The sensitiveness of the regulator is increased by use of a metal spiral instead of glass, and a satisfactory connection can be made with rubber, if a water trap is introduced between mercury and the organic liquid, so that the water layer covers the junction between metal and glass. Method of Cutting off the Gas Supply.-Two forms of "cut-off" with novel features may be mentioned. That |