568 HEAT of water infinitely near their separating interface. The rest of the water is merely a means of measuring hydrosta- tically the fluid pressure at the interface. When the temperature is so high as to make the pressure too great to be conveniently measured by a water column, the hydrostatic measurement may be done, as shown in the annexed drawing (fig. 7), by a mercury column in a glass tube, surrounded by a glass water jacket not shown in the drawing, to keep it very accurately at some definite temperature so that the density of the mercury may be accurately known. The simple form of steam thermometer repre sented with figured dimensions in fig. 6 will be very convenient for practical use for temperatures from freezing to 60. Through this range the pressure of water-steam, reckoned in terms of the balancing column of waterof maximum density, in creases (Table V.) from 6^ to 202 -.3 centimetres; and for this therefore a tube of a little more than 2 a-g CM. BORE. Fig. 7. metres will suffice. From 60 to 140 the pressure of steam now reckoned in terms of the length of a balancing column of mercury at increases from 14 - 88 to 271 8 centimetres ; and for this a tube of 280 centimetres may be provided. For higher temperatures a longer column, or several columns, as in the multiple manometer, or an accurate air pressure-gauge, or some other means, such as a very accur ate instrument constructed on the principle of Bourdon s metallic pressure-gauge, may be employed, so as to allow us still to use water and vapour of water as thermometric substance. High- 42. At 230 C., the superior limit of Regnault s high- pressure pressure steam experiments, the pressure is 27 53 atmos, but there is no need for limiting our steam thermometer to this temperature and pressure. Suitable means can easily be found for measuring with all needful accuracy much higher pressures than 27 atmos. But at so high a temperature as 140, vapour of mercury measured by a water column, as shown in the diagram (fig. 8), becomes available for purposes for which one millimetre to the degree is a sufficient sensibility. The mercury-steam- pressure thermometer, with pressure measured by water- column, of dimensions shown in the drawing, serves from 140 to 280 C., and will have very ample sensibility through the upper half of its scale. At 280 its sensibility steam thermo meter. will be about 4| centimetres to the degree ! For tempera tures above 280 sufficient sensibility for most purposes is obtained by substituting mercury for water in that simplest form of steam thermometer shown in fig. 6, in which the pressure of the steam is measured by a column of the liquid itself kept at a definite tempera ture. When the liquid is mercury there is no virtue in the particular temperature 0C., and a stream of water as nearly as may be of atmospheric temperature will be the easiest as well as the most accu rate way of keeping the mercury at a definite temperature. As the pressure of mercury steam is at all ordinary atmospheric temperatures quite imper ceptible to the hydrostatic test when mercury itself is the balancing liquid, that which was the chief reason for fix- ing the temperature at the interface between liquid and vapour at the top of the pressure-measuring column when the balancing liquid was water ( 40) has no weight in the present case; but, on the other hand, a much more precise Fig. 8. definiteness than the ten degrees latitude allowed in the former case for the temperature of the main length of the manometric column is now necessary. In fact, a change of temperature of 2 2 in mercury at any atmospheric temperature produces about the same propor tionate change of density as is produced in water by a change of temperature from to 10, that is to say, about Jg- per cent.; but there is no difficulty in keeping, by means of a water jacket, the mercury column constant to some definite temperature within a vastly smaller margin ,of error than 2 2, especially if we choose for the definite temperature something near the atmospheric temperature at the time, or the temperature of whatever abundant water supply may be available. If the vertical tube for the pres sure-measuring mercury column be 830 centimetres long, the simple mercury-steam thermometer may be used up to 520 C., the highest temperature reached by Begnault in his experiments (Table V. below) on mercury-steam. By using an iron bulb and tube for the part of the thermometer exposed to the high temperature, and for the lower part of the measuring column to within a few metres of its top, with glass for the upper part to allow the mercury to be seen, a mercury-steam-preesure thermometer can with great ease be made which shall be applicable for temperatures giving pressures up to as many atmospheres as can be measured by the vertical height available. The apparatus may of course be simplified by dispensing with the Tor ricellian vacuum at the upper end of the tube, and opening the tube to the atmosphere, when the steam-pressure to be measured is so great that a rough and easy barometer observation gives with sufficient accuracy the air-pressure at the top of the measuring column. The easiest, and not necessarily in practice the least accurate, way of measuring very high pressures of mercury-steam will be by enclosing some air above the cool pressure-measuring column of mercury, and so making it into a compressed-air pressure- gauge, it being understood that the law of compression of
the air under the pressures for which it is to be used