THE EARTH. . . 75 



of other bodies : for if put into a glass tube, by its swelling and rising 

 it shows the quantity of heat in the body to which it is applied ; by its 

 contracting and sinking, it shows the absence of the same.' Instead 

 of using water in this instrument, which is called a thermometer, they 

 now make use of spirit of wine, which is not apt to freeze, and which 

 is endued even with a greater expansion by heat than water. The 

 instrument consists of nothing more than a hollow ball of glass, with 

 a long tube growing out of it. This being partly filled with spirits of 

 wine tinctured red, so as to be seen when it rises, the ball is plunged 

 into boiling water, which making the spirit witmn expand and rise in 

 the tube, the water marks the greatest height to which it ascends ; at 

 this point the tube is to be broken off, and then hermetically sec Jed, 

 by melting the glass with a blow-pipe : a scale being placed by the 

 side, completes the thermometer. Now as the fluid expands or con- 

 denses with heat or cold, it will rise and fall in the tube in proportion ; 

 and the degree or quantity of ascent or descent will be seen in the scale. 



No fire, as was said, can make water hotter, after it begins to boil. 

 We can, therefore, at any time be sure of an equable certain heat ; 

 which is that of boiling water, which is invariably the same. The 

 certainty of such a heat is not less useful than the instrument that 

 measures it. It affords a standard, fixed, degree of heat over the 

 whole world ; boiling water being as hot hi Greenland, as upon the 

 coasts of Guinea. One fire is more intense than another ; of heat 

 there are various degrees ; but boiling water is a heat every where 

 the same, and easily procurable. 



As heat thus expands water, so cold, when it is violent enough to 

 freeze the same, produces exactly the same effect, and expands it like- 

 wise. Thus water is acted upon in the same manner by two opposite 

 qualities ; being dilated by both. As a proof that it is dilated by 

 cold, we have only to observe the ice floating on the surface of a 

 pond, which it would not do were it not dilated, and grown more 

 bulky, by freezing, than the water, which remains unfroze. Mr. 

 Boyle, however, put the matter past a doubt, by a variety of experi- 

 ments.* Having poured a proper quantity of water into a strong 

 earthen vessel, he exposed it, uncovered, to the open air, in frosty 

 nights ; and observed, that continually the ice reached higher than the 

 water, before it was frozen. He filled also a tube with water, and 

 stopped both ends with wax : the water, wht,n frozen, was found to 

 push out the stopples from both ends ; and a rod of ice appeared at 

 each end of the tube, which showed how much it was swollen by the 

 cold within. 



From hence, therefore, we may be very certain ^/ the cold's dilat- 

 ing of the water ; and experience also shows, that the force of this 

 expansion has been found as great as any which heat has been found 

 to produce. The touch-hole of a strong gun-barrel being stopped, 

 and a plug of iron forcibly driven into the muzzle, after the barrel 

 had been filled with water, it was placed in a mixture of ice and salt ; 

 the plug, though soldered to the barrel, at first gave way, but being 

 fixed in more firmly, within a quarter of an hour the gun-barrei burs! 



* Boyle, vol. i. p. 610. 



