HEAT 561 temperature, provided the substance is kept in the double condition of part liquid and part steam ; in other words, in this case the pressure depends upon the temperature alone and is independent of the volume. In the steam-pressure thermometer, therefore, there is no delicate measuring of volume of the thermometric substance, and the vessel containing it is not in the shape of bulb and stem ; but .0- the instrument consists essentially of a means of measuring
- ic the pressure of the thermometric substance, with a test
that it is really in the twofold condition of part liquid and part steam, whether by seeing it through a glass containing- vessel, or by a proper hydraulic appliance for ascertaining that the pressure is not altered by rarefaction or conden sation when the temperature is kept constant. Realized thermometers of this species, quite convenient for many practical purposes, with steam of sul phurous acid, of water, arid of mer cury, to serve for different ranges of temperature, from below - 30 C. to above + 520 C., are described in 39-44 below. 18. In respect to general con venience for large varieties of uses, whether for scientific investigation, or for the arts, or for ordinary life, liquid thermometers are generally and with good reason preferred ; but the general preference of either mercury or spirits of wine for the liquid, which is so much the rule, is not (20 below) so clearly reasonable. For ordinary uses in which the thermometer has to be moved about and placed in various positions, gas thermometers are much less convenient, because the} require essentially an accurate measurement of pressiye, and gene rally for this purpose a column of liquid. But when the thermometer is to be kept always in one position, as for instance when it is devoted to testing the temperature of the air in doors or out of doors, Amonton s air or gas thermometer is really as convenient and as easily read as any liquid thermometer can be : but even it, simple as it is, involves a triple division of the hermetically sealed space, with three different conditions of occupation, one part occupied by the thermometric substance, another by the pressure-measuring vapourless liquid, 1 and the third vacuous : and it is by so much the less simple than the liquid thermometer that in the liquid thermometer the enclosed space is divided into only two parts, one occupied by the thermometric liquid, and the other by its steam, with 1 An instrument closely resembling that shown in the drawing (fig. 2), but with common air instead of hydrogen, was made for the writer of this article, by Casella, about fifteen years ago, and has been used for illustrations in the natural philosophy class in Glasgow university ever since. It is probably an exceedingly accurate air thermometer. When it was set up in the new lecture-room after the migration to its present locality in 1870, the tube above the manometric liquid column was cleared of air. To do this the instrument must be held in such a sloping position, with the closed end of the tube down, as to allow the bubble of air always found in it to rise and burst in the bulb. If now the instrument is placed in its upright position, the liquid refuses to leave the top of the tube, and it would remain filling the tube (probably, for ever ?) if left in that position. No violence of knocking which has been ventured to try to bring it down has succeeded. To bring it down i bubble of air must be introduced. The bubble must be very small, so that the pressure of the air which fills it may become insensible when this air expands into the space of the tube left above the mano metric column after it descends to its proper thermoscopic position, bpecial experiments made for this article in September 1879 showed A 5 o o z - 1*1 o CE -- E 39 i
10 CM. (- _ 1 I 1 "" 20 I U : ! LJ u u r Ct
o
i C Q 1 i - i
1 Fig. 2. or without some admixture of common air, For accuracy the air or gas thermometer is superior, we might almost say incomparably superior, to the mercury thermometer, and, though in a much less degree, still decidedly superior to even the most accurate liquid thermometer, on account of the imperfect constancy of the glass containing-vessel. 1 9. If we were quite sure of the bulk measurement given by the glass bulb and tube, liquid thermometers would be quite as accurate as gas thermometers. For there is no diffi culty in giving any required degree of sensibility to the instrument by making the bulb large enough ; and the quality of the liquid itself, hermetically sealed in glass, may be regarded as being as constant as anything we know of in the material world. But, alas for thermometry, the glass measure is not constant ! In fact, glass is a substance of very imperfect elasticity (ELASTICITY, 4, vol. vii. p. 797) ; and it is found that the bulb of a thermometer is not always of the same volume at the same temperature, but that, on the contrary, it experiences uncertain changes exceedingly embarrassing in thermometry. In the course of a few months after a thermometer is filled and sealed, the bulb generally shrinks by some uncertain amount of from i-<y<y(5-7y t ^rnrw f its bulk, sometimes even in the course of years to almost -5-5-773-0-. This has been discovered by a gradual rising of the freezing point, in new mercury thermometers, generally as much as from - to |- C., sometimes to as much as 1, which corresponds to a shrinkage of -yVlrw as the bulk-expansion of mercury is when its temperature is raised from 0to 100C. (Table II. below) T i. T , or 01815, of its bulk at 0. After a few months or a few years this progressive shrinkage ceases to be sensible ; but if the thermometer at any time is exposed to the temperature of boiling water or any higher tem perature, an abrupt sub-permanent enlargement of the bulb is produced, and the freezing point, if tested for by placing the thermometer in ice and water, is found to be lowered ; then again for weeks or months or years there is a gradual shrinkage, as shown by a gradual rising of the freezing point when the thermometer is tested again and again by placing it in ice and water. A very delicate mercury thermometer, which has been kept for years at ordinary atmospheric pressures when out of use, and never when in experimental use exposed to any temperature higher than about 30 C., or much lower than the freezing point, becomes very constant, and probably may never show any change of as much as ~$ of a degree C. in its freez ing point or in its indication at any other absolutely definite temperature, within some such range as from - 20 or- 10 C. to + 30 or + 40 C. But the abrupt and irregular changes, produced by exposing the thermometer to tempera tures much above or much below some such limited range as that, constitute a very serious difficulty in the way of accurate thermometry by the mercury-in-glass thermometer. that in the nine years during which the instrument had remained undis turbed in the lecture-room a very sensible quantity of air (enough to render the temperature indication about 35 C. too low) had leaked from the bulb through the sulphuric acid into the tube above the liquid column. This air was eliminated, and the instrument reset up for use, an operation completed in a minute at any time if need be. Some care ful experiments were then made by Mr Macfarlane to ascertain if the pressure of vapour or gas from the sulphuric acid, in the tube, was sensible, with a happily decisive result in the negative. The bulb was kept at a very constant temperature by cold water; the uppermost few centimetres of the liquid column, and the whole of the tube above it, were heated to about 100 C. by steam blown through a glass jacket-tube, fitted round it for the purpose. The height of the manometric column remained sensibly unchanged ! Further experiments must be made to ascertain whether or not there is enough of variation of absorption of the air by the sulphuric acid with variation of temperature, and enough of the consequent variation of pressure in the bulb, to vitiate sensibly the thermometric use of the instrument. If, as seems improbable, the answer to this question be unhappily affirmative, a satisfactory negative might be found by substituting hydrogen for common ait.
XI. 71
