NOVUM OROANUM 131 



inclosed in a vessel of such materials as would not imbue 

 it with heat or cold of themselves, 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 graduated 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; 21 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 mani 

 festly inclosed) 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. 32 This, however, belongs to the table of degrees. 



21 This last is found to be the real reason, air not being a good conductor, 

 and therefore not allowing the escape of heat. The confined air is disengaged 

 when these substances are placed under an exhausted receiver. 



22 This is erroneous. Air, in fact, is one of the worst, and metals are the 

 best conductors of heat. 



