Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/596

562 HEAT Although, the greatest care has been bestowed by Regnault, Joule, and all other accurate therrnometric. experimenters to avoid error from this cause, we have still but little definite information as to its natural history in thermo meters of different qualities of glass, different shapes of bulb, and constructed differently in respect to processes of glass-blowing, boiling the mercury, and sealing the stem. Ve do not even know whether the excess of the atmospheric pressure outside the bulb over the pressure due to mercury and Torricellian vacuum inside is influential sensibly, or to any considerable degree, in producing the gradual initial shrinkage. If it were so we might expect that the effect of heating the thermometer up to 100 or more at any time would be rather to produce an accelerated shrinkage for the time than what it is found to be, which is a return towards the original larger volume, followed by gradual shrinkage from day to day and week to week afterwards, A careful comparison between two thermometers constructed similarly in all respects, except sealing one of them with Torricellian vacuum and the other with air above the mercury, would be an important contribution to knowledge of this subject, interesting, not only in respect to thermometry, but also to that very fundamental question of physical science, the im perfect elasticity, of solids (see ELASTICITY, 4). 20. The error of a thermometer due to irregular shrink ages and enlargements of the bulb is clearly the less the greater is the expansion of the thermometric fluid with the given change of temperature. By the investigation of 30 we can calculate exactly how much the error is for any stated amount of abnormal change of bulk in the bulb. Bat it is enough at present to remark that for different liquids in the same or in similar bulbs the errors are very nearly in the inverse proportions of the expansions of the liquids. Now (Table III. below) in being warmed from 0" to 1 C. alcohol expands 6 times as much as mercury, methyl butyrate 7 times, and sulphuric ether 8 times. Hence if irregular changes of bulk of the bulb leave, as they probably do in practice, an uncertainty of -j^ths of a degree in respect to absolute temperature by the bast possible mercury-in-glass thermometers used freely at all temperatures from the lowest up to 100 C., the un certainty from this cause will be reduced to ^th of a degree by using alcohol, or -g^-th by using methyl butyrate instead of mercury ; it may therefore, in a glass ther mometer of alcohol or of the butyrate, be considered as practically annulled ( 19) after a few weeks or months have passed, and the first main shrinkage is over. 21. An alcohol-in-glass thermometer is easily made strong enough to bear a temperature of 100 C., as this gives by the pressure of the vapour an internal bursting pressure of not quite an atmosphere and a quarter in excess of the atmospheric pressure outside. The boiling point of methyl butyrate (Table III. below) is 102 C.; a ther mometer of it may therefore be used for temperatures con siderably above 100 C., but how muoh above we cannot tell, as we have not experiments as yet on the pressure of its vapour at temperatures above its boiling point. The pressure of vapour of sulphuric ether (Table VI. below) is too great to allow a thermometer of this liquid sealed in glass to be used much above 60 or 70 C., but for low temperatures it makes a very valuable thermometer. It was used in 1850 by W. Thomson in finding by experi ment the lowering of the freezing point of water, predicted theoretically (see THERMODYNAMICS) by J. Thomson in 1849 (Trans. R S. E.}, and gave a sensibility of 128 divisions to 1 C. Glass thermometers with ether, or chloroform (whose expansion is about 4 per cent, greater than that of ether), were used by Joule and W. Thomson in experiments 1 on changes of temperature experienced by l Phil. Trans, for 1860, p. 325. bodies moving through air, in one of which the sensibility was as great as 330 scale divisions to the 1 C. All these liquids and many others in the modern chemist s repertory of oils and ethers and alcohols, besides the superior sensibility which they give by their greater expansions, have a great advantage over mercury for some thermometric purposes in their smaller specific gravity. This allows the bulb to be larger, with less liability to break or to give disturbed read ings through distortion by the weight of the contained liquid. 22. Liquids which wet the glass have another great advantage over mercury in their smaller capillary attrac tion (see CAPILLARY ACTION, vol. v. p. 56 ; and MATTER, PROPERTIES OF) and in the constancy of their 180 angle of contact with the glass, instead of the much greater absolute intensity of capillary attraction in the mercury, and its very variable angle of contact, averaging about 45 when the mercury is rising, and about as much as 90 when it is falling. On account of these variations the bulb of the mercury thermometer is subjected to abrupt variations of pressure when the mercury is rising or falling. The greatest and least pressures due to this cause are experi enced when the angle of contact is respectively least and greatest, and differ by the pressure due to a vertical column of mercury equal in height to the difference of depressions of mercury in a capillary tube of the same bore as the thermometer stem when the angle of contact is changing from one to the other of the supposed extreme values. Hence the mercury in a thermometer rises and falls by jerks very noticeable in a delicate thermometer when looked at with a lens of moderate magnifying power, or even with the naked eye. Dr Joule informs us that this defect is much greater in some thermometers than in others, and that he believes it is greatly owing to the tube being left unsealed for too long a time after the introduction of the mercury (by which it is to be presumed something of a film of oxide of mercury is left on the glass to reappear on the surface of the mercury when it sinks as it cools after the sealing of the end). In Joule s own thermometers not the smallest indication has ever been detected of what he calls " this untoward phenomenon, which is calculated to drive an observer mad, if he discovers it towards the close of a series of careful experiments." Their admirable quality in this respect is no doubt due to the great care taken by the maker, Mr Dancer, under Joule s own instructions, to have the mercury and the interior of the bulb and tube thoroughly clean, and to guard it from exposure to any " matter in its wrong place " until completion of the sealing. But no amount of care could possibly produce a mercury thermometer of moderate dimensions moving otherwise than by jerks of ever so many divisions, if its stem were of fine enough bore to give anything approaching to two or three hundred divisions to the centigrade degree. 23. One chief objection to the use of alcohol or othervola- tile liquid for the thermometric substance in ordinary glass thermometers is the liability to distillation of some of the liquid into the stem and head reservoir, unless the glass above the level of the liquid be kept at least as warm as the liquid. On this account a spirit thermometer is not suitable for being plunged into a space warmer than the surrounding atmosphere with the stem simply left to take the temperature to which it comes in the circumstances. But whether for elaborate experimental use, or for the most ordinary thermometric purposes, there is little difficulty in arranging to keep the part of the stem which is above the liquid surface somewhat warmer than at the liquid surface, and this suffices absolutely to prevent the evil of distillation. The only other objection of any grave validity 2 2 There is one other objection which, though often stated as very grave against the thermometric use of any other liquid than mercury, we do

not admit to be so. It is that when the temperature is rapidly sink-