380 BACON 



be enclosed in a vessel of such materials as would not imbue it 

 with heat or cold of itself, nor easily admit the influence of 

 the external atmosphere. The experiment should be made 

 therefore with an earthen jar, covered with folds of leather to 

 protect it from the external air, and the air should be kept three 

 or four days in this vessel well closed. On opening the jar, the 

 degree of heat may be ascertained either by the hand or a gradu- 

 ated glass tube. 



There is a similar doubt as to whether the warmth of wool, 

 skins, feathers, and the like, is derived from a slight inherent 

 heat, since they are animal excretions, or from their being of a 

 certain fat and oily nature that accords with heat, or merely 

 from the confinement and separation of air which we spoke of in 

 the preceding paragraph ; for all air appears to possess a certain 

 degree of warmth when separated from the external atmosphere. 

 Let an experiment be made, therefore, with fibrous substances 

 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 substances 

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

 be less cold than the liquid itself. 



We have here no exactly negative 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 

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

 others more slowly, as stone and metals. This, however, be- 

 longs to the table of degrees. 



No negative 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 become a sort of dark ash. 



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

 we are not acquainted with any tangible body which does not 

 become decidedly warm by friction, so that the ancients feigned 

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

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

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

 projected by machines (as balls from cannon) do not derive 

 some degree of heat from meeting the air, which renders them 

 somewhat 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 sufficiently 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 be generated. 



