THKHMOMKTKK. 



THERMOMETER. 



fnmt, in order that the uicn-ury or spirit may be easily distinguished 

 at certain dUtanoe, aa by approaching very near the instrument, the 

 heat of the observer's penon may affect the length of the column. 

 Mr. Sheepshanks found that tube* with round bores were far more 

 nearly true than those with flat one*. He also approves of bulbs 

 three or four-tenths of an inch in diameter. They should not be too 

 small, or the graduations of the scale will be close together, nor 

 too large, or the instrument will not be sensitive on account of the 

 large body of mercury required to be heated. The bulb should not be 

 blown by the breath, lest moisture be introduced, but by an india- 

 rubber bag. 



It is of course essential that the extent of the thermometer-scale 

 should be great enough to comprehend all the temperatures at which 

 the substances generally required to be examined exist in a state of 

 fluidity ; and this extent may be obtained when mercury is employed. 

 According to the experiments of Mr. Dalton, mercury does not boil 

 till it has acquired a temperature equal to 660 of Fahrenheit's scale; 

 and it does not freeze till it is subject to a degree of cold expressed by 

 30 divisions below the zero of that scale, (39) or 71 below the freez- 

 ing-point of water. Pure alcohol, on the other hand, has never been 

 frozen, though it has been exposed to a degree of cold exceeding that 

 which U expressed by 166 below the zero of Fahrenheit; (-166) 

 though at that remarkably low temperature it became viscid. Hence 

 a spirit-thermometer is to be preferred to one of mercury when it ia 

 intended to ascertain the temperature of the air in high northern or 

 southern latitudes : but since the spirit boils in air with a degree of 

 heat expressed by 173 of Fahrenheit, it is unfit for many of the 

 purposes for which a thermometer is required. For instruments 

 capable of measuring very high temperatures, see PVROMKTHH. 



In the construction of a thermometer, the air should be carefully 

 expelled from the tube, and even from the mercury or spirit within 

 it : the variations in the density of the atmosphere cannot, under 

 proper precaution, affect the instrument, since the tube is hermetically 

 sealed. This precaution consists in not graduating the tube for some 

 months after it has been sealed, or until the glass has accommodated 

 itself to the altered circumstances of a vacuum within and atmospheric 

 pressure without. The most carefully constructed thermometers will 

 shift their fixed points if graduated too soon after filling. It must 

 also be observed that the indications of temperature are not precisely 

 expressed in terms of the dilatation of the mercury or spirit only, but 

 in terms of the excess of that dilatation above the dilatation of glass. 

 The apparent dilatation of mercury in a glass tube is equal to ^ of its 

 volume, between the temperatures of freezing and boiling water ; and 

 its true dilatation between the raine limits is g^ of its volume. 



A perfect thermometer would be one in which the expansions of the 

 fluid in the tube were exactly proportional to the increments of heat 

 which it might receive from the substance whose temperature is to be 

 determined ; but it cannot be said that any of the fluids which as yet 

 have been employed in the construction of thermometers strictly pos- 

 sess this property. Mercury is the fluid in which it exists in the 

 greatest degree ; but from the experiments of Deluc it has been ascer- 

 tained that, between the points of freezing and boiling-water, the tem- 

 perature indicated by the mercurial thermometer is lower than the 

 true temperature, the greatest difference (which, however, is only equal 

 to l-4 of Reaumur's scale, or 3-15 Fahrenheit), being in the middle 

 between those two points on the scale. From the same experiments 

 it is also found that when thermometers are regulated so as to agree at 

 the points of freezing and boiling-water, whether the liquid be oil, 

 spirit, or water, the indications are always below those of mercury, 

 the difference being the greatest at the middle between those point"-. 

 With oil of olives the difference is 1 of Reaumur's scale (2-25 Fahr.) ; 

 with highly-rectified alcohol, 4'9 Reaumur (11-02 Fahr.) ; with half 

 alcohol and half water, 6'7 Reaumur (lfi-07 Fahr.) ; and with water, 

 19-5 Reaumur (43'87 Fahr.). It must be observed that great irregu- 

 larities take place in the expansion of all fluids when near their boiling 

 state, and that mercury contracts very suddenly when at the point of 

 its congelation. The deviations of the spiritrthennometer from the 

 true indications of heat are known to be rather greater than those of 

 the mercurial thermometer. It may be added, that the alcohol in a 

 thcrmometer-tuta lose*, iu time, part of iU strength ; and that, in con- 

 sequence, the degree of expansion by a given increment of heat is not 

 the same as when the instrument was made. The expansion of alcohol 

 for temperatures greater than about 173 Fahr., at which the spirit 

 boils, cannot be ascertained practically, because the spirit at that 

 temperature passe* into a state of vapour ; and the comparison between 

 the mercurial and thn spirit thermometer ought not to be carried 

 higher than that temperature ; or the scales for mercury and spirit 

 ought to be regulated so as to agree with one another at the freezing- 

 point of water and at the temperature of 173 Fahr. ; if this were 

 attended to. the differences between the indications of the mercurial 

 and spirit thermometers, above that point, would be less than they 

 appear to be by the tables of Deluc. 



Later measurement* by Regnault of the total expansion of mercury 

 for three progressive intervals of 180 Fahr., give the following results : 

 Between 32 and 212 it is 1 part in 55-08 ; between 212 and 392 it 

 is 1 in 64-61 ; and between 392 and 672 it is 1 in 64-01. In the 

 mercurial thermometer it may be assumed, without sensible error, that 



between 32 and 212 equal increments of heat raise the thermometer 

 through an equal number of degrees. The increase in the capacity of 

 the glass bulb (especially for crown glass) almost exactly compensates 

 for the increasing rate of the expansion of mercury, ultlio. 

 temperatures above 212 the compensation is not so exact. It has 

 been found, also, that the temperature of 672* Fahr., as measured by 

 an air thermometer, is 686* by a mercurial thermometer, on account of 

 the increasing dilatation of mercury, with an increased temperature. 

 Mr. Dixon, in his ' Treatise on Heat,' says : " Different glasses have 

 different co-efficients of expansion, and abo vary in the law of their 

 dilatation at high temperatures, and as the amount of absolute dilata- 

 tion of mercury is small, this variation in the expansion of the glass 

 envelope produces irregularities of considerable magnitude in the 

 apparent dilatation of mercury. As the real expansibility of air ia 

 much greater, its apparent expansion in glass is not affected to the 

 same extent by these variations in the rate of expansion of the latter, 

 and accordingly in an air-thermometer the rate of eipaum. >u of the 

 glass may be considered as sensibly uniform. When corrected, there- 

 fore, for the expansion of its envelope, such an instrument forms the 

 most perfect thermometer with which we are acquainted in the 

 present state of science." An air-thermometer, corrected for the 

 expansion of its envelope, being compared with a mercurial thi-i-iiin- 

 meter made with the peculiar description of gloss employed by M. 

 Regnault, the agreement between the two instruments was perfect 

 up to 200 C. ; whereas, in a mercurial thermometer made of ordi- 

 nary tube, compared with one of crystal glass, although they agreed 

 from to 100 C., yet at higher temperatures the discrepancies were : 



1st Thermometer. 

 190-91 

 246-68 

 251-87 

 279-08 

 810-69 

 333-72 



2nd Thermometer. 

 191-66 

 249-36 

 254-S7 

 282-50 

 819-28 

 310-07 



Difference. 



I'll 



2-68 

 2-70 

 8-42 

 4-59 

 6-39 



Water, like other substances, suffers a diminution of volume by the 

 abstraction of heat, but when it is cooled to a temperature between 

 89* and 40 of Fahrenheit's scale, it seems to have attained the 

 maximum of density ; and if the process of cooling be continued, it 

 then increases in volume till it is converted into ice. Therefore, if a 

 thermometer were made with water, and the top of the column were 

 at 50 Fahr., it would be impossible to know whether the temper, it ure 

 were 50 or 30, the expansion being nearly equal at equal distances 

 within 10 above and below 40 of the scale. The cause is uncertain, 

 but it is probably owing to a partial crystallisation, which may begin 

 to take place in water when at a temperature expressed by about 

 8 above its freezing-point. 



The mercurial and spirit thermometers formerly differed considerably 

 at temperatures below that of freezing water. By observations made 

 during Sir Edward Parry's second voyage, the differences between the 

 indications of the spirit and mercurial thermometers varied from 3 -05 

 to 8 Fahr. between the temperatures +58 and -30, the alcohol 

 being always too low. At very low temperatures alcohol thermometers, 

 unless very carefully prepared, differ greatly among themselves. 

 Dr. Kane in his Arctic voyage records temperatures as low as from 

 60 to 75 ; but he admits that " it was not uncommon for ther- 

 mometers which had given us correct and agreeing temperatures aa 

 low as 40 to show at 60 differences of from fifteen to twenty 

 degrees." In remarkable contrast with this are the results obtained 

 by Sir E. Belcher in his Arctic voyage with the thermometers furnished 

 to him by Mr. Welsh of the Kew Observatory. While wintering in 

 Northumberland Sound, 76" 62' N., 97 W., he made a comparison of 

 sewn thermometers, marked 2, 6, 8, 20, 8, 4, 6, with a standard instru- 

 ment, with the following results. The temperatures were all natural 



The lowest temperature observed was on 12th January, 1853, 

 namely 62 - 5. The indices of the minimum thermometers read next 

 day at 8 A.M., gave 62, -61"-6, - 66*. -63--'. ' The night was bright 

 and calm ; no sensation of cold. 63'2 is the external exposed thermo- 

 meter, but was never read excepting by its index of that graduation." 

 Parry's coldest was 54 for 154 hours. In Belcher's Expedition, the 

 following temperatures were observed : 



40' and below, for 156 continuous hours. 



98to62 :> -4 



14 



In the preparation of spirit thermometers the alcohol should be 

 , and the stem should be deprived of air by boiling. When the 



