Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/597

HEAT 563 against the use of highly expansive liquids instead of mercury is the difficulty of allowing for the expansion of the liquid in the stem, if it is not at the same temperature as the bulb. With the same difference of temperatures in different parts of the instrument, the error on this account is clearly in simple proportion to the expansibility of the liquid ; and therefore, the residual error due to want of perfect accuracy in the data for the allowance will, gene rally speaking, be greater with the more expansive than with the less expansive liquid. But in every case in which the bulb and stem can all conveniently be kept at one temperature, a thermometer having for its thermometric substance some highly expansive oil or alcohol or ether, or other so-called organic liquid of perfectly permanent chemical constitution, cannot but be much more accurate and sensitive than the mercury thermometer, which has hitherto been used almost exclusively in thermometric work of the highest rank. We shall see ( 62 and 64-68) that the ultimate standard for thermometry, according to the absolute thermodynamic scale ( 34) r is practically attained by the use of hydrogen or nitrogen gas as the thermometric substance, but that for ordinary use a gas thermometer can scarcely be made as convenient as one in which the thermometric substance is a liquid. For practical thermometry of the most accurate kind it seems that the best plan will be to use as ordinary working standard thermometers highly sensitive thermometers con structed of some chosen " organic " liquid, and graduated according to the absolute thermodynamic scale, by aid of the thermodynamically corrected air thermometer ( 62) used as ultimate standard of reference. The great con venience of the mercury thermometer in respect to freedom from liability to distillation and smallness of error on account of difference of temperature between the bulb and stem renders it the most convenient for a large variety of scientific and practical purposes in which the most minute accuracy or the most extreme sensibility is not required. 24. Without any thermodynamic reason for preferring air to mercury as thermometric fluid, Regnault preferred it for two very good reasons. (1) Its expansion is 20 times that of mercury and 160 times the cubical expansion of glass, and therefore with air the error due to irregularity in the expansion of the glass is 20 times smaller than with mercury, and small enough to produce no practical defalca tion from absolute accuracy in thermometry, as he found by elaborate and varied trials. So far as this is concerned, some highly expansive organic liquids would answer nearly as well as air for thermometric fluid, and would have the advantage of giving a thermometer much more easily used. (2) For an ultimate standard of reference air has the advantage over organic liquids generally, that different samples of it taken at different times, or in different parts of the world, and purified of water and carbonic acid 1 by well-known and easily practised processes, are sufficiently uniform to give thermometric results between which the ing, before becoming stationary, a little of the liquid Lags behind the descending free surface, detained on the glass, and, trickling slowly down to rejoin the main column, must be waited for before the sta tionary temperature can be correctly read. We believe that if a fairly inviscid (or mobile) liquid such as alcohol or ether orbutyrate of oxide of methyl be used, there will be practically no time lost from this cause, and certainly no accuracy lost when proper care is taken by the ob server. The observer must be on his guard against a possibly false steadi ness, through the falling of temperature being momentarily balanced in its effect on the free surface by the trickling down of liquid from the glass above, when the free surface is still above, or, it may be, has gone down to a little below, the true position for the final temperature. Henceforth, to avoid circumlocutions, the unqualified word " air" will be used to denote atmospheric air taken in any part of the world, and deprived of carbonic acid and whatever vapour of water it may have contained, by aid of hydrated lime, or caustic potash, or some other suit- able reagent for removing the carbonic acid, and quicklime, or chloride of calcium, or sulphuric acid, or phosphoric acid, for removing the water. accordance is practically perfect, provided the thermo metric plan according to which the different samples are used is the same, or as approximately the same as is easily secured in practice. Two plans for the thermometric use of air naturally present themselves : (I.) augmentation of volume of air kept in constant pressure; and (II.) augmenta tion of pressure of air kept in constant volume. Regnault Reg- tried both plans, but found that he could only arrange his nault s apparatus to give good results by the second, and on it there- n fore he founded what he called his "normal air therm o- mome t e meter." For the sake of perfect definiteness he chose, as the density of the air in his normal thermometer, the density which air has when at the temperature of melting ice and under the pressure of one atmo. He adopted the centigrade scale in respect to the marking of the freezing and boiling points by and 100; and the principle which he assumed for the reckoning of other temperatures was to call equal those differences of temperature for which differences of pressure of the air in his normal thermometer are equal. Thus he was led to a definition of temperature expressed by the following formula : where II denotes the pressure one atmo, and p and jo IOO the pressures of the air of the normal thermometer at the temperatures denoted by t and by 100 respectively, the latter being the temperature of steam issuing from water boiling under the pressure of one atmo. By the most accurate observations which he could make Regnault found for his " normal air" ^> 100 = 1 3665 x II. Hence his ther mometric formula becomes 1 = 100 P ~ U = 272-85/^-1 3665 xn U 25. Regnault compared with his normal air thermo- meter thermometers on the same plan of constant volume, but with air at other than the normal density of 1 atmo, arid with other gases than air ; also air and gas ther- mometers on the plan of constant apparent volume as meters measured in a glass bulb and stem ; also a thermometer founded simply on the dilatation of mercury; also thermo meters of mercury in different kinds of glass, each graduated on the glass stem with divisions corresponding to exactly equal volumes of the bore; also overflowing thermometers (thermometres a deVersement), in which a bulb with a short piece of fine stem was perfectly filled with mercury at and the quantity of mercury expelled by the high tempera ture to be measured was weighed, instead of being volu- metrically measured by divisions of a long stem as in the ordinary thermometer. The whole of this thermometric investigation is full of scientific interest, and abounds with results of great prac tical value in respect even of the minutest details of Regnault s work. It will be found fully described in the first of his three volumes, entitled Relation des Experiences entreprises par ordre de Monsieur le Ministre des Travaiix Publics et sur la proposition de la Commission centrale des Machines a, Vapeur pour Determiner les Principals Lois et Donnees Numeriques qui entrent dans le Calcul des Machines a Vapeur, which were published at Paris in 1847, 1862, and 1870. Here we can but state some of t-he most im portant of the general conclusions: (1) The air thermometers with pressure at 0of from 44 to 149 centimetres of mercury agreed perfectly with the nor mal air thermometer calculated according to the same for mula (2), and nearly the same numerical coefficient 272 85. A slightly larger value 272 98 (or -0036632- 1 ) gave the best agreement for the 44 c.m. pressure, and the somewhat smaller value 272-7 (or -003667" 1 ) for the pressure 149 c.m. (2) The hydrogen gas thermometer, with pressure one

atmo at 0, and with its indications calculated according tc