OUTLINES OF NATURAL PHILOSOPHY. 



316. At the heat of 40 mercury freezes, and 

 at 600 it boils, and is in both cases useless as a 

 measure of heat. For measuring great cold, a 

 thermometer of alcohol or spirit of wine is employ- 

 ed ; for measuring great heat, the expansion of a 

 solid is used, and the instrument is called a Pyro- 

 meter. 



The best pyrometer hitherto in use, is that invented 

 by Mr WEDGWOOD, consisting of a small cylinder 

 of Cornish clay, which contracts continually by heat. 

 The scale of this instrument begins at a red heat, of 

 such intensity as to be visible in day-light, which, 

 on FAHRENHEIT'S scale, could it be continued so 

 far, would be marked 1077. From this it ascends, 

 (by degrees that are each of them equal to about 

 130 of FAHRENHEIT,) as far as 240, or 32277 of 

 FAHRENHEIT, which is its utmost Kmit; 28 mark- 

 ing the melting heat of silver, 32 of gold, 125 the 



^greatest heat of a glass-house furnace, and 150 the 

 temperature at which pig-iron melts. 



317. The fact that all bodies expand by heat, 

 admits also of another exception. Water is most 

 dense at a temperature between 37 and 39, and 

 itsv specific gravity diminishes both by the addition 

 and subtraction of heat. 



When water is converted into ice a new arrangement 

 takes place, and the specific gravity is considerably 

 diminished. 



318. Heat 



