THE 



847 



THE 



bel of Alkmaar in Holland, are the persons to one of whoa 

 that honour is, with most probability, ascribed, and th 

 former, in his ' Commentaries on Avicenna ' (1626), actu 

 ally claims it for himself : it may however have happene 

 with this, as with other scientific discoveries, that the ide 

 of the instrument occurred to two persons or more at th 

 same time. 



The first thermometers were intended to indi- 

 cate variations in the temperature of the atmo- 

 sphere merely ; and the most simple of them con- 

 sisted of a hollow glass-ball at one extremity of 

 a long tube which was open at the opposite ex- 

 tremity : the air within tlie 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 ; the pressure of the atmosphere on this 

 spirit 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 diminu- 

 tion of temperature, by causing that air to 

 contract, allowed the external pressure to raise 

 the spirit. A scale was adapted to the tube in Original Air 

 order to express the degree of temperature by Thsnnometer 

 the number of the graduation at the upper extremity oi 

 the spirit. 



An effort was made to render the instrument portable 

 by bending the lower part of the tube upwards and ter- 

 minating this branch also with a ball ; and a small aper- 

 ture was made in the latter in order that the external air 

 might have access 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 in a small glass vessel con- 

 taining both air and coloured spirit, and the vessel being 

 formed with a neck which closely encircled the tube, it 

 was hermetically sealed to the latter. The variations in 

 the temperature of the atmosphere caused the air in the 

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

 or less force on the surface of the spirit ; the latter was 

 consequently made to ascend or descend in the tube. 



The air-thermometer invented by Amontons (1702) con- 

 sisted of a tube nearly 4 feet long, open at both ends and 

 curved upwards at bottom, where it terminated in a ball : 

 this tube carried a column of mercury about 29 English 

 inches high, so that the air in the ball was compressed by 

 the weight of two atmospheres. A light body, in which 

 was inserted the lower end of a wire, floated on the upper 

 extremity of the column of mercury in the tube ; and near 

 the upper end of the wire was an index by which the num- 

 ber of the graduation on a scale was shown. The varia- 

 tions 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 tem- 

 peratures on a scale whose length was only half of that 

 which was required with the simple air-thermometer. 



The defects inseparable from all the above thermometers 

 are, that the dilatations of the air are not proportional to 

 the increments of heat, that the length of the column of 

 spirit or mercury varies with the temperature of the atmo- 

 sphere, also that the air which is in contact with the surface 

 of the spirit in the open vessel, in the first kind of instru- 

 ment, or with the top of the column of the spirit or mer- 

 cury, in the others, exerts more or less pressure according 

 to its density ; and thus the indications afforded by the 

 thermometer are rendered erroneous, or require corrections 

 which it is difficult to apply. The air-thermometer pro- 

 posed by Dubuat, and of which the following is a brief 

 description, possesses some advantages above those which 

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

 been employed. 



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

 of a barometer, 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 terminat- 

 ing with a hollow ball. The mercury occupies the straight 

 part of the tube to the height of about 29$ inches above 

 the bend ; and at this bend it terminates without entering 



into the ball, which, by the construction, is 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 is to be prevented from entering the ball by 

 making the tube decline farther from the vertical position, 

 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 determined 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 elas- 

 ticity communicated to it by the caloric in the atmosphere, 

 or by the fluid in 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 pro- 

 portional to the vertical height of that column. 



About the middle of the seventeenth century the mem- 

 bers of the Accademia del Cimento caused thermometers 

 to be constructed in which, instead of air, alcohol or spirit 

 of wine was employed. The fluid was introduced, as at 

 present, into a glass tube terminating at bottom in a hollow 

 ball, from which the air had been expelled by heat : the 

 opposite extremity of the tube was then hermetically 

 sealed, and a scale was applied for the purpose of express- 

 ng the temperature of the atmosphere, or of the liquid 

 which was to be examined. Alcohol dilates and contracts 

 considerably with the variations of temperature to which it 

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

 s also capable of measuring very low temperatures, but as 

 t is brought to a boiling state sooner than any other liquid, 

 t cannot be employed to ascertain a high degree of heat. 

 Spirit-thermometers were introduced into this country by 

 Mr. Boyle, and they are still used both here and on the 

 Continent. 



Sir Isaac Newton, being dissatisfied with the smallness 

 of the range of spirit-thermometers, employed linseed-oil 

 n tubes for the purpose of measuring degrees of heat : this 

 iquid has nearly the same amount of expansibility by in- 

 :rements of caloric as alcohol ; and it is capable of bear- 

 ng very high degrees of heat and cold without either 

 soiling or freezing ; but from its viscidity it adheres so 

 nuch to the interior side of the tube as to render accurate 

 )bservations quite impossible, and on this account it has 

 lot since Newton's time been employed for thermometers. 

 The thermometer which is now in general use is a slen- 

 der tube of glass terminating in a ball containing mercury, 

 he air having been expelled and the tube afterwards her- 

 metically 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. Halley ; and the reason why it was not. 

 employed by that philosopher appears to have 

 been that the range of its expansion is much 

 less than that of alcohol. According to Boer- 

 haave (Elementa Chemia;, 1732), the honour of 

 having been the first to recommend a mer- 

 curial thermometer is to be ascribed to Romer, 

 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 

 been invented by Fahrenheit, of Amsterdam, in 

 1720, was read before the Royal Society, and 

 was published in the 'Philosophical Trans- 

 actions' (vol. xxxiii.). The advantages of 

 mercury over alcohol and air, as a measure of 

 temperature, are that its expansions are more 

 nearly proportional to the increments of caloric 

 than those which take place in cither of the 

 other fluids ; it is easily deprived of air, and its 

 power to conduct heat being considerable, the 

 changes of itsvolume by changes of tempera- 

 UT. ture in the surrounding medium take place 

 more rapidly than those of any other fluid except the gaseg. 

 At first the scales for measuring the degrees of heat were 



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