THE 



349 



THE 



degrees at which the water will boil under the pressure 

 expressed by the height of the mercurial column. The 

 committee observe that in trying the heat of liquors, the 

 quicksilver in the tube of the 'thermometer should be 

 heated to the same degree as that in the ball ; or if this 

 cannot be done, a correction should be applied on that 

 account. (Phil. Trans., vol. Ixvii.) 



Thermometer-tubes should have their bo^es very slen- 

 der, and, if possible, perfectly equable in the whole of 

 their length. When thee is any inequality in the trans- 

 verse sections, the best artists make the graduations of the 

 scale vary so that they may correspond to the equal divi- 

 sions of a cylindrical tube ; and in order to ascertain the 

 relative dimensions of the sections, they cause a small 

 quantity of mercury, about an inch in length, to slide 

 along the interior of the tube, measuring its length in dif- 

 ferent places ; then, since the lengths are inversely pro- 

 portional to the areas of the sections, the variations of the 

 former will immediately show the corresponding variations 

 of the latter. It is usual to give to the bore an oval form 

 with the broader side towards the front, in order that the 

 mercury or spirit may be easily distinguished at a certain 

 distance, as by approaching very near the instrument, the 

 heat of the observer's person may affect the length of the 

 column. 



It is of course essential that the extent of the thermo- 

 meter-scale should be great enough to comprehend all 

 the temperatures at which the substances generally re- 

 quired to be examined exist in a state of fluidity ; and this 

 extent may be obtained when mercury is employed. Ac- 

 cording 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 39 divisions below the 

 zero of that scale, or 71 below the freezing-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 is expressed by 91 below the zero of Fahren- 

 heit ; and therefore a spirit-thermometer is to be preferred 

 to one of mercury when it is intended to ascertain the 

 temperature of the air in high northern or southern lati- 

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

 heat expressed by 175 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 PYROMETER. 



In the construction of a thermometer, the air should be 

 carefully expelled from the tube, and even from the mer- 

 cury or spirit within it : the variations in the density of 

 the atmosphere cannot of course affect the instrument, 

 since the tube is hermetically sealed. It must be ob- 

 served however 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 di- 

 latation of mercury in a glass tube is equal to j^ of its 



volume, between the temperatures of freezing and boiling 

 water ; and its true dilatation between the same limits is 



of its volume. 



6a'5 



A perfect thermometer would be one in which the ex- 

 pansions 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 

 possess this property. Mercury is the fluid in which it 

 exists in the greatest degree ; but from the accurate ex- 

 periments of Deluc it has been ascertained that, between 

 the points of freezing and boiling-water, the temperature 

 indicated by the mercurial thermometer is lower than the 

 true temperature, the greatest difference, which however 

 it only equal to 1.4 of Reaumur's scale or 3.15 Fahren- 

 heit, 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 points. With oil 6f olives the dif- 

 ference 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, ft" .7 Reaumur <15.07 Fahr.) ; and 

 with water, 19.5 Reaumur (43.87 Fahr.). It must be ob- 

 served that great irregularities take place in the expansion 

 of all fluids when near their boiling state, and that mer- 

 cury contracts very suddenly when at the point of its con- 

 gelation. The deviations of the spirit-thermometer 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 thermometer-tube loses, in time, part 

 of its strength ; and that in consequence, 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 175 Fahr., at which 

 the spirit boils, cannot be ascertained practically, because 

 the spirit at that temperature passes into a state of vapour ; 

 and the comparison between the mercurial and the 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 tempeiature of 175 Fahrenheit : 

 if this were attended to, the differences between the indi- 

 cations of the mercurial and spirit thermometers, above 

 that point, would be less than they appear to be by the 

 tables of Deluc. 



Water, like other substances, suffers a diminution of 

 volume by the abstraction of calorie, but when it is cooled 

 to a temperature between 39 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 ther- 

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

 were at 50 Fahr., it would be impossible to know whether 

 the temperature were 50" or 30, the expansion being 

 nearly equal at equal distances within ten degrees above 

 and below 40 of the scale. The cause is uncertain, but. it 

 is probably owing to a partial crystallization, which may 

 begin to take place in water when at a temperature ex- 

 pressed by about 8 degrees above its freezing-point. 



The mercurial and spirit, thermometers, regulated as be- 

 fore said, differ very considerably at temperatures below 

 that of freezing water ; and at 39 below the zero of Fah- 

 renheit, or 32" below the zero of Reaumur, when the mer- 

 cury is frozen, the difference has been computed to be 

 about 10 Reaumur (22.5 Fahr.), by which the spirit 

 stands too low. By observations made during Sir Edward 

 Parry's second voyage, the differences between the indica- 

 tions 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. 



Register Thermometers. It is of 

 great importance in meteorology that 

 the observer shonld be able to ascertain 

 the highest or lowest point of a ther- 

 mometer scale at which the column 

 of mercury may have stood during his 

 absence ; and several contrivances have 

 been adopted by artists in order to ob- 

 tain this end. Of these, one, which is 

 still preferred, was invented by Mr. 

 Six, whose name the instrument bears, 

 and is described in the ' Philosophical 

 Transactions' for 1782. It consists of a 

 long tube bent so as to form three 

 parallel branches, A, B, and C : the part 

 A is an elongated bulb, and the rest of 

 the tube has a capillary bore. The lower 

 portion, b, contains mercury, which rises 

 in B and C to certain points, as a and c, 

 and the bulb is filled with spirit of wine, 

 which passing over the bend at d, de- 

 scends to the upper extremity of the 

 mercury in B : above c the branch C is 

 also filled with spirit to near the upper 

 extremity, which is hermetically sealed. 



Two small indices of steel coated with 

 glass, which are represented at m and n, 

 are introduced in the branches B and 

 C : these are capable of being forced 

 upwards by the rising of the column of 

 mercury in either tube, and they have 

 about them a fine wire or a thread of 

 glass ; so that they will remain stationary where they happen 

 to be when the heads a and c of the columns recede from 



