HEAT. 



and the heavier to descend. As spirit 

 of wine is considerably lighter than 

 water, an admixture of it, in any pro- 

 portion, produces a fluid lighter than 

 water; but the quantity of spirit re- 

 quired in this experiment is very small, 

 and therefore the difference of specific 

 gravity must be very small also. It 

 is very remarkable, that so minute a 

 difference should not only enable the 

 one portion to ascend, and the other to 

 descend ; but also to pass each other 

 in the very narrow neck of the bulb 

 without mingling. 



CHAPTER IV. 



Application of the Doctrine to 

 Thermometers. 



THE expansion of bodies by heat and 

 their contraction by cold, afford the 

 means of measuring degrees of tempe- 

 rature. The instrument used for this 

 purpose, and which acts upon these 

 principles, is called a thermometer, and 

 was originally invented by an Italian 

 physician of the seventeenth century, 

 named Santorio ; who used air as a 

 measure of the variations of tempera- 

 ture. This instrument is now called 

 an air -thermometer, and consists of a 

 glass vessel or bottle, with a broad base, 

 and nairow neck, containing a coloured 

 liquid a (Jig. 5) ; a long tube having a 

 glass bulb blown at one fig. 5. 

 end b ; the other extremity 

 of the tube being open, and 

 plunged in the coloured 

 liquid ; and a scale of equal 

 parts c. Heat is applied 

 to the bulb to expel a part 

 of the air, which permits a 

 portion of the coloured fluid 

 to rise in the tube: this 

 fluid column indicates the 

 slightest changes of tem- 

 perature by rising wjien it 

 is diminished, and falling 

 when it is increased ; be- 

 cause every increase of 

 temperature expands the 

 ah- in the bulb, occasion- 

 ing it to press upon the 

 fluid column and force it' 

 down, while any decrease of temperature 

 contracts the bulk of the air, and per- 

 mits the fluid to rise. This instrument 

 is not capable of measuring variations 

 of temperature through any very consi- 

 derable range, and it is liable t-j other 

 objections j yet, notwithstanding, it is 



o 



still applicable, from its'great delicacy, 

 to many purposes. 



The members of the Academia del 

 Cimento substituted spirit of wine, 

 coloured, instead of air ; and, to pre- 

 vent any effect from being produced by 

 the variations of atmospheric pressure, 

 the tube was hermetically sealed. Dr. 

 Halley and Sir Isaac Newton used 

 mercury as the thermometric fluid, 

 which is now most generally employed ; 

 but spirit-thermometers are also in 

 use. The mercurial is better adapted 

 for high, the spirit for low tempera- 

 tures. 



Whichever fluid is used, the shape 

 of the instrument is the same, consist- 

 ing of a tube; having a bulb blown at 

 one end of a globular or cylindrical 

 shape, the latter being considered the 

 best. Fig. 6 represents a mercurial 

 thermometer, with part of . g. 

 its scale moveable. The 

 tube ought to have an equal 

 bore throughout ; but as 

 tubes;of this kind can rarely 

 if ever be obtained, it is ne- 

 'cessary to ascertain the ine- 

 qualities of the tube, that 

 they may afterwards be 

 adapted to the scale. To 

 divide the tube into spaces 

 of equal capacity, it is dip- 

 ped into quicksilver, until a 

 column about half an inch 

 in length enters' the bore ; 

 this is removed to about two 

 inches from the end, at 

 which the bulb is to be made ; 

 then marking the tube with 

 a file or diamond, it is laid 

 upon the brass, ivory, or wood, 

 which is to form the scale intended 

 to be applied, so that the lower end 

 of the mercurial column may cor- 

 respond with the lower end of the 

 scale, the upper end of the column 

 being marked with a fine point upon 

 the scale ; the tube is to be a little in- 

 clined and gently shaken, until the 

 lower part of the mercurial column 

 stands exactly where the upper part 

 was before; the place of the upper 

 part must then be marked upon the 

 scale, and the process must be con- 

 tinued until the tube is divided to a 

 sufficient height. 



A bulb having been blown upon the 

 instrument in the usual way, with the 

 aid of a blow-pipe, in order to fill it 

 with quicksilver, a piece of paper is 

 tied over the open end of the tube in 



