with sufficient prism dispersion just to separate the various bright lines of the source from one another.

My former assistant, Professor D. H. Marshall, made for me, in 1870, a series of careful measurements of the change of plane of polarisation of the lines C and F of hydrogen by this method, using a vacuum tube with a narrow bore, no slit, and a prism of small angle. It was found to give fair but not excellent results. Although no greater thickness of quartz was employed than the plates supplied along with Duboscq's saccharimeter, the planes of polarisation of C and F were separated by the thickest of them upwards of 130°; but the determination of the exact point of extinction is not easy. In measuring with practically homogeneous light, like that of a spiritlamp with chloride of sodium or of lithium, the prismatic dispersion was, of course, not required. The great merit of the rotatory polarisation process consists in the fact that there is scarcely any additional loss of light incurred by using a foot or two of quartz instead of a few millimetres, and thus in proportion increasing the amount of rotatory displacement; while the thicker the quartz the less is the inevitable percentage error of observation. Also the position of each bright line is determined in terms of a standard quartz-rotation, and needs no comparison spectrum. It remains to be seen whether, on trial, it may be found possible to have a great length of quartz cut with sufficient accuracy, and whether the bright lines are narrow enough for this mode of observation. I have ordered a 6-inch cylinder of quartz, and hope soon to have observations made with it. Meanwhile it seems likely that this combination of polarising and analysing prisms, with a quartz plate, and a small direct vision spectroscope (with very wide slit), may be well adapted for measurements of position of the bright lines in the spectra of auroras, comets, and nebulæ, where it is not easy to employ either a comparison spectrum or a wire micrometer.

Monday, 1st March 1880.

PROFESSOR GEIKIE in the Chair.

The following Communications were read:

1. On Steam-Pressure Thermometers of Sulphurous Acid, Water, and Mercury. By Sir W. Thomson.

The first annexed diagram represents a thermometer constructed to show absolute temperature realised for the case of water and vapour of water as thermometric substance. The containing vessel consists of a tube with cylindric bulb like an ordinary thermometer; but, unlike an ordinary thermometer, the tube is bent in the manner shown in the drawing. The tube may be of from 1 to 2 or 3 millims. bore, and the cylindrical part of the bulb of about ten times as much. The length of the cylindrical

part of the bulb may be rather more than 10 of the length of the straight part of the tube. The contents, water and vapour of water, are to be put in and the glass hermetically sealed to enclose them, with the utmost precautions to obtain pure water as thoroughly freed from air as possible, after better than the best manner of instrument makers in making cryophoruses and water hammers. The quantity of water left in at the sealing must be enough to fill the cylindrical part of the bulb and the horizontal branch of the tube. When in use the straight part of the

tube must be vertical with its closed end up, and the part of it occupied by the manometric water-column must be kept at a nearly enough definite temperature by a surrounding glass jackettube of ice-water. This glass jacket-tube is wide enough to allow little lumps of ice to be dropped into it from its upper end, which is open. By aid of an india-rubber tube connected with its lower end, and a little movable cistern, as shown in the drawing, the level of the water in the jacket is kept from a few inches above to a quarter of an inch below that of the interior manometric column. Thus, by dropping in lumps of ice so as always to keep some unmelted ice floating in the water of the jacket, it is easy to keep the temperature of the top of the manometric water-column exactly at the freezing temperature. As we shall see presently, the manometric water below its free surface may be at any temperature from freezing to 10° C. above freezing without more than per cent. of hydrostatic error. The temperature in the vapour-space above the liquid column may be either freezing or anything higher. It ought not to be lower than freezing, because, if it were so, vapour would condense as hoar frost on the glass, and evaporation from the top of the liquid column would either cryophorus wise freeze the liquid there, or cool it below the freezing point.

The chief object of keeping the top of the manometric column exactly at the freezing-point is to render perfectly definite and constant the steam-pressure in the space above it.

A second object of considerable importance when the bore of the tube is so small as one millimetre, is to give constancy to the capillary tension of the surface of the water. The elevation by capillary attraction of ice-cold water in a tube of one millimetre bore is about 7 millims. The constancy of temperature provided by the surrounding iced water will be more than sufficient to prevent any perceptible error due to inequality of this effect. To avoid error from capillary attraction the bore of the tube ought to be very uniform, if it is so small as one millimetre. If it be three millimetres or more, a very rough approach to uniformity would suffice.

A third object of the iced-water jacket, and one of much more importance than the second, is to give accuracy to the hydrostatic measurement by keeping the density of the water throughout the long vertical branch definite and constant. But the density of water

at the freezing point is only per cent. less than the maximum density, and is the same as the density at 8° C.; and therefore when per cent. is an admissible error on our thermometric pressure, the density will be nearly enough constant with any temperature from 0° to 10° C. throughout the column. But on account of the first object mentioned above, the very top of the water-column must be kept with exceeding exactness at the freezing temperature.

TORRICELLIAN VACUUM

In this instrument the "thermometric substance" is the water and vapour of water in the bulb, or more properly speaking the portions of water and vapour of water infinitely near their separating interface. The rest of the water is merely a means of measuring hydrostatically 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 second annexed drawing (fig. 2), 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 represented with figured dimensions in the first diagram will be very convenient for practical use for temperatures from freezing to 60°. Through this range the pressure of vapour of water, reckoned in terms of the balancing column of water of maximum density, increases from 6 to 202.4 centimetres; and for this, therefore, a tube of a little more than 2 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 0° 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 accurate 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-pressure Steam Thermometer.

At 230° C., the superior limit of Regnault's high-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. 3), 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 will be about 4 centimetres to the degree! For temperatures above 280° sufficient sensibility for most purposes is obtained by substituting mercury for water in that simplest form of steam thermometer shown in fig. 1, in which the pressure of the steam is measured by a column of the liquid itself kept at a definite temperature. When the liquid is mercury there is no virtue in the parti

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