HEAT OR CALORIC. H7 



13. Large Cryophorus. 



" This figure represents a very large Cryophorus, the blowing of 

 which 1 superintended ; and by means of which I have successfully 

 repeated Wollaston's experiment." 



" This instrument is about four feet long ; and its bulbs are about 

 five inches in diameter." 



VI. IGNITION OR INCANDESCENCE. 



(a.) Bodies become luminous in consequence of the accumulation 

 of heat in them.* In common language, this is expressed by saying 

 that bodies become red hot, as a bar of iron does among burning 

 coals. 



Some bodies melt during their ignition ; this is the fact with stones 

 and most metals, and the melted stone or metal is as truly red hot as 

 the bar of ignited iron. Some bodies evaporate during ignition ; 

 such are antimony, bismuth, lead and tin ; some evaporate before ig- 

 nition, as water and most fluids, not excepting the most fixed fluids, 

 as quicksilver, and sulphuric acid, and dense oils ; the latter are de- 

 composed before ignition. 



Gases do not become luminous at any temperature, although they 

 may cause solid bodies, as gold, &tc. immersed in them, to become 

 luminous, the reason appears to be, that there is not matter enough 

 in any one point to project the light to the eye, although from their 

 communicating ignition to solid bodies, it is certain that they have 

 the requisite heat. 



Mr. Perkins' high steam, it would appear, is capable of igniting 

 other bodies, (as already stated under steam and vapor ;) it kindled 

 tow and ropes, and it even ignited the bored orifice in the generator 

 from which it was issuing ; still it does not appear certain that it was 

 itself luminous, nor is it certain that it was not, because we cannot 

 inspect the steam formed in opake vessels, like those of metal, and 

 when the steam issues into the air, it is no longer high steam ; just 

 at the orifice of emission, it is elastic and invisible, but a little way 

 from it, it forms a cloud of mist. 



(b.) Bodies become luminous by friction. Glass, or agate, or 

 quartz, held against a revolving gritstone or grindstone, become hot 

 and luminous. Metals are affected in the same manner. The parts 

 of gun locks and other pieces of steel emit sparks when held firmly 

 against grindstones or revolving wheels, covered with emery powder 



* They are not supposed to undergo decomposition during their ignition. 



