II* 



THKHMOMETER. 



THERMOW 



muscles of tho human body. It ha* also been employe.! by Mr. Juulo 

 in confirming tin- .deduction* of Professor Thomson'* tlieory of the 

 tK^n.l effects of* stretching solid bodies already alluded to. Mr. 

 Joule ha* found that if * *trip of vulcanised india-rubber U stretched 

 by might which double! it* length, an elevation of iti temperature 

 by 50* C, shortens it by ai much a* Ath of iU whole length ; and 

 IVli.er ha* ilwcovered that a cooling effect U produced by an electric 

 current in flowing from bumuth to antimony acroat a curface of 

 contact. Thi* remit ha* been applied by Professor Thonuon to the 

 esUbliahmsat of a " Hechanical Theory of Thermoelectric Current*" 

 (' Tranaaction* of Koyal Society of Edinburgh,' 1851). 



The term pyro+ltctrieity if sometime* applied to the phenomena 

 produced by heating certain mineral*, auch ai boraoite, topas, axinite, 

 mesotype, tourmaline, prehnite, calamine, and sphene. It wu shown 

 by (Epinui, to far back a* 1757, that the two electricities are to be 

 found on opposite side*, or ends, of the same piece of tourmaline when 

 heated, and it wu proved by Hatiy that electric crystal* exhibit 

 different set* of crystalline faces on the two aide* on which the contrary 

 electric effect* are observed. It is remarkable, also, a* lately pointed 

 out by Sranberg, that in bismuth and antimony, where there is one 

 particular plane of cleavage more brilliant than the rest, when ban of 

 the metal* are placed with this plane of cleavage perpendicular to the 

 direction of the current, such bars are more highly negative than in 

 any other position, whilst if a less brilliant plane of cleavage be placed 

 across the line of current, the bar is more highly positive than in any 

 other position. 



THEKMIXMETER (from the Greek words topptt, ?tol, and utrpov, a 

 meature) is an instrument by which the temperatures of bodies are 

 ascertained. It consists of a glass tube with a capillary bore containing, 

 in general, alcohol or mercury, which expanding or contracting by 

 variations in the temperature of the atmosphere, or on the instrument 

 being immersed in the liquid or gas which is to be examined, the state 

 of the atmosphere, liquid, or gas, with respect to heat, is indicated by 

 a scale which is either applied to the tube or engraven on it* exterior 

 surface. 



The end proposed by a thermometer is the measurement of the 

 temperature of any body with relation to the temperature of some 

 other substance, as of water at the point of freezing ; but the measure 

 so obtained must not be understood to express the absolute quantity 

 or density of heat in any body, it being well known that different sub- 

 stances, though exhibiting the same apparent temperature, contain 

 very different quantities of heat according to their capacities for that 

 element. [SpECiric HEAT ; LATENT HEAT.] 



The thermometer must have been in use in the beginning of the 

 17th century, but it is not known, precisely, to whom the honour of 

 the invention is due. A physician of Padua, named Santorio, and 

 Cornelius Drebbel, of Alkmaar in Holland, are the persons to one of 

 whom that honour is, with most probability, ascribed ; and the former, 

 in hi* ' Commentaries on Avicenna' (1626), actually claims it for him- 

 self. It may, however, have happened with thin, as with other scien- 

 tific discoveries, that the idea of the instrument occurred to two 

 persons or more at or about the same time. 



The first thermometers were intended to indicate variation* in the 

 temperature of the atmosphere merely ; and the most simple of them 

 consisted of a hollow glass-ball at one extremity of a long 

 tube which was open at the opposite extremity. The air 

 within the ball and tube being rarefied by the heat of a lamp, 

 and the tube being in a vertical position, the open end was 

 plunged into a vessel containing a coloured spirit; as the 

 enclosed air cooled, the pressure of the atmosphere on the 

 liquid caused it to ascend in the tube till the expansive force 

 of the air in the ball and the upper part of the tube became 

 equal to the pressure. In this state, an increase of the 

 temperature of the atmosphere caused the air in the ball to 

 expand and press down the spirit in the tube ; on the other 

 hand, a diminution of temperature, by causing that air to 

 contract, allowed the external pressure to raise the spirit. 

 A scale was adapted to the tube in order to express the 

 degree of temperature by the number of the graduation at 

 the upper extremity of the spirit. 



An effort was made to render the instrument portable by 

 bending the lower part of the tube upwards, and terminating 

 thi* branch also with a ball ; and a small aperture was made 

 in the latter in order that the external air might have access 



Oriftnal 



to the lower surface of the spirit Mr. Boyle subsequently 

 modified the air-thermometer by making the tube quite 



" straight and open at both ends : the lower end was immersed 



nij*i HCI - it _* \ A _* i_ _ ii_ __ 1.1 . 





ig both air and coloured spirit, 



being formed with a neck which closely 

 encircled the tube, it wa* hermetically sealed to the latter. The 

 variations in the temperature of the atmosphere caused the air in tlic 

 vessel to expand or contract, and thus to press with more or lees force 

 on the surface of the spirit ; the Utter was consequently made to 

 ascend or descend ill tJM tube. 



The air-thermometer invented by Amontons (1 702) consisted of a 

 tube nearly four feet long, open at both ends, and curved upward* at 

 bottom, where it terminated in a ball Thi* tube carried a column of 

 mercury about 2( English inches high, so that the air in the ball was 



compressed by the weight of two atmospheres. A light body, iu wlu< li 

 was inserted the lower end of a wire, floated on the upper extremity <>f 

 the column of mercury in tin- tube ; and near the upper end 

 wire was an index by which the number of the graduation on a soak 

 was shown. The variations of the temperature of the air in the ball 

 caused the mercurial column to ascend or descend in the tube ; and 

 thus were produced corresponding movements in the index. By this 

 instrument it was proposed to measure high temperatures on a scale 

 whose length was only half of that which was required with the simple 

 air-thermometer. 



The defect* inseparable from all the above thermometers ar. . 

 the dilatations of the air are not proportional to the iucrcm. 

 heat ; that the length of the column of ipirit or mercury varies with 

 the temperature of the atmosphere ; also, that the air which is in '"- 

 tact with the surface of the spirit in the open vessel, in the first kind 

 of instrument, or with the top of the column of the spirit or mercury 

 in the others, exerts more or less pressure according to it* density ; 

 and thus the indications afforded by the thermometer are rendered 

 erroneous, or require corrections which it i* difficult to apply. The 

 air-thermometer proposed by Dubuat, and of which the following i* 

 a brief description, possesses some advantages above those which 

 have been mentioned ; but not being portable, it has never been 

 - on] > 'I. 



It consists of a column of mercury in a tube, like that of a baro- 

 meter, hermetically sealed at the upper end, and bent below so as to 

 form a short branch inclined at about 40* to the straight part of the 

 tube; this branch terminating with a hollow ball. The in. 

 occupies the straight part of the tube to the height of abcir 

 inches above the bend ; and at thi* bend it terminates without entering 

 into the ball, which, by the construction, i* a little above the bend. 

 The part of the tube which is above the column of mercury is free 

 from air, and when the bend is plunged in boiling water the tube is 

 to be in a slightly inclined position, so that a vertical line may pass 

 through the two extremities of the mercurial column ; then, upon the 

 ball becoming cool, and the elasticity of the air in it being diminished, 

 the weight of the mercury will cause it to descend in the long branch 

 and rise in the other. The mercury U to be prevented from entering 

 the ball by making the tube decline farther from the vertical p 

 so that the lower extremity of the mercury may remain in the vertical 

 line before mentioned ; and the temperature of the air is to be deter- 

 mined by the height of the top of the column of mercury above a 

 horizontal line passing through the lower extremity, that is, by the 

 cosine of the declination of the tube from the vertical. Since the air 

 in the ball preserves constantly the same volume, the elasticity com- 

 municated to it by the heat of the atmosphere, or by the fluid iu which 

 the instrument is plunged, is always in equilibrio with the pressure of 

 the column of mercury, which is the force acting against it, and is 

 proportional to the vertical height of that column. 



About the middle of the 1 7th century the members of the Artademla 

 del OimaUo caused thermometers to be constructed in which, instead 

 of air, alcohol or spirit of wine was employed. The fluid was intro- 

 duced, as at present, into a glass tube terminating at bottom in a 

 hollow ball, from which the air had been expelled by heat. The oppo- 

 site extremity of the tube was then hermetically sealed, and a scale of 

 equal parts was applied to the stem, by mean* of small beads of coloured 

 enamel, for the purpose of expressing the temperature of the atmo- 

 sphere, or of the liquid which was to be examined. Alcohol dilates 

 and contracts considerably with the variations of temperature to w liieh 

 it may be subject, though not in so great a degree as air. It is also 

 capable of measuring very low temperatures ; but as it is brought to a 

 boiling state much sooner than water, it cannot be [employed to 

 ascertain a high degree of heat. Spirit-thermometers were introduced 

 into thi* country by Mr. Boyle, and they are still used for low tem- 

 peratures. 



sir Isaac Newton, being dissatisfied with the smaUness of the range 

 of spirit-thermometers, employed linseed-oil in tubes for the purpose 

 of measuring degrees of temperature. This liquid has nearly the same 

 amount of expansibility by increment* of heat as alcohol ; and it i* 

 capable of bearing a considerable amount of heat and of cold without 

 either boiling or freezing; but from its viscidity it adheres so nuu-li to 

 the interior side of the tube as to render accurate observations quite 

 impossible, and on this account it has not since Newton's time been 

 employed for thermometers. 



The thermometer which is now in general use is a slender tube of 

 glass terminating in a bnll containing mercury, the air having been 

 expelled and the tube afterwards hermetically sealed. The idea of 

 employing this fluid for the purpose of measuring degrees of heat by 

 its expansion is supposed to have first occurred to Dr. H.illcy ; and tha 

 reason why it was not employed by that philosopher appears to have 

 been that the range of it* expansion is much less than that of alcohol. 

 According to Boerhaave (' Elements Chemise,' 1732), the honour of 

 having been the first to recommend a mercurial thermometer is to be 

 ascribed to Itotner, the discoverer of the motion of light, who is said 

 to have invented it in 1709; but it was not till the year 1724 that 

 such a thermometer was known in this country. In that year an 

 account of a mercurial thermometer which had boon invented by 

 Fahrenheit, of Amsterdam, in 1720, was read before the Royal Society, 

 and was published in the ' Philosophical Transaction*' (vol. xxxiii.K 



