=Gas Pyrometers.=—Wiborgh, Bristol, and others have constructed pyrometers in which the pressure of an enclosed gas is recorded by a Bourdon pressure-gauge, the scale of which is calibrated so as to read temperatures. A porcelain bulb, terminating in a capillary tube which is connected to the gauge, is used to contain the air or other gas; but at temperatures above a red heat the readings become uncertain, owing either to leakages or the distortion of the bulb. The most suitable material for the bulb (alloy of platinum, 80 per cent., and rhodium, 20 per cent.) is too costly to use industrially, and would deteriorate under the influence of furnace gases. In the Bristol recording instrument the moving index of the pressure-gauge terminates in a pen, which touches a chart-paper revolving by clockwork. Good results are obtained up to 400° C., but beyond this the indications are uncertain, and the instrument is more correctly described as a recording thermometer. =Wiborgh’s Thermophones.=—These consist of infusible clay cylinders, 2·5 cms. long and 2 cms. in diameter, which contain an explosive. When placed in a hot space, the explosion occurs after a definite time, the interval being less at high temperatures than at lower, as the rate at which heat is conducted through a solid varies directly as the difference between the external and internal temperatures. The interval elapsing between placing in the furnace and the subsequent explosion is noted on a stop-watch to the nearest 1/5 second, and from the observed time the temperature is obtained from a table, drawn up from the results of experiments under known conditions. If the cylinders be kept dry, an observer experienced in the use of thermophones may secure a reading to within 40° C. =Joly’s Meldometer.=—This device, due to Dr Joly, is intended for laboratory determinations of melting points. It consists of a strip of platinum, heated by electricity, upon which a tiny fragment of the material is placed, which is viewed through a microscope. The temperature of the platinum is regulated by means of a rheostat in the circuit, and in making a test the temperature is gradually raised until the material is observed to become globular, or to flow over the platinum strip. The temperature at which this occurs is deduced from the linear expansion of the platinum strip, which is measured by a micrometer attached to the instrument. When carefully used, very accurate determinations may be made by the meldometer, the results, moreover, being obtained rapidly, and with the use of the minimum of material.