464 KOVUM oiiGAJflTM. [book II. 



taiu degree of Wtiymtli when separated from its external atmos« 

 phere. Let an experiment be made, therefore with fibrous gub* 

 stances of linen, and not of wool, feathers, or silk, which are 

 animal excretions. For it is to be observed that all powders 

 (where air is manifestly enclosed) are less cold than the sub* 

 stances when whole, just as we imagine froth (which contains air) 

 to be less cold than the liquid itself. 



We have here no exactly nejrative instance, for we are not 

 acquainted with any body tangible or spirituous which does not 

 admit of heat when exposed to the fire. There is, however, this 

 diflference, that some admit it more rapidly, as air, oil, and water, 

 others more slowly, as stone and mctals-^ This, however, beh ngs 

 to the table of degrees. 



No nci^ative is here subjoined, except the remark that sparks 

 are not kindled by flint and steel, or any other hard substance, 

 unless some small particles of the stone or metal are struck off, 

 and that the air never forms them by friction, as is commonly 

 supposed ; besides, the sparks from the weight of the ignited 

 substance, have a tendency to descend rather than to rise, and 

 when extinguished becomes a sort of dark ash. 



We are of opinion that here again there is no neoative; for 

 we are not acquainted with any tangible body which does not 

 become decidedly warm by friction, so that the ancienl* feigned 

 that the gods had no other means or power of creating heat 

 than the friction of air, by rapid and violent rotation. On tins 

 point, however, further inquiry must be made, whether bodies 

 ])rqjected by machines (as balls from cannon) do not derive some 

 degree of heat i'rom meeting the air, which renders them some- 

 what warm when they fall. The air in motion rather cools than 

 heats, as in the winds, the bellows, or breath when the mouth is 

 contracted. The motion, however, in such instances is not suffi- 

 ciently rapid to excite heat, and is applied to a body of air, and 

 not to its component parts, so that it is not surprising that heat 

 should not bo generated. 



AV^c must make a more diligent inquiry into this instance ; for 

 herbs and green and moist vegetables appear to possess a latent 

 heat, so small, however, as not to be perceived by the touch in 

 sin<;le specimens, but Avhen they are united and confined, so that, 

 their spirit cannot exhale into the air, and they rather warm each 

 other, their heat is at once manifested, and even flame occasion- 

 ally in suitable substances. 



ductor, and therefore not allowing the escape of heat. The confined 

 air is disengaged when these substances are placed under an exhausted 

 receiver. 



y This is erroneous. Air, in fact, is one ci the worst, and metaif 

 %re the b<>8t couductom of heal. 



