BOOK U.J APHORISMS. 463 



like, and whicli speedily burn, are yet cold at first to the toucn, 

 and tbe water of natural baths, poured into any vessel and sepa- 

 rated from its source, cools down like Nvater heated by the fire. 

 It is, however, true that oily substances are rather less cold to 

 the touch than those that are aqueous, oil for instance than 

 water, silk than linen ; but this belongs to the table of degrees 

 of cold. 



In like manner we may subjoin a negative instance to that of 

 warm vapour, derived from the nature of vapour itself, as far as 

 we are acquainted with it. For exhalations from oily substances, 

 though easily inflammable, are yet never warm unless recently 

 exhaled fi'om some warm substance. 



The same may be said of the instance of air ; for we never per- 

 ceive that air is warm unless confined or pressed, or manifestly 

 heated by the sun, by fire, or some other warm body. 



A negative instance is exhibited in weather by its coldness 

 with an east or north wind, beyond what the season would lead 

 us to expect, just as the contrary takes place with the south or 

 west winds. An inclination to rain (especially in winter) attcnda 

 warms weather, and to frost cold weather. 



A negative instance as to air confined in caverns may be ob- 

 eervcd in summer. Indeed, wc should make a more diligent 

 inquiry into the nature of confined air. For in the first place the 

 qualities of air in its own nature with regard to heat and cold 

 may reasonably be the subject of doubt ; for air evidently derives 

 its heat from the effects of celestial bodies, and possibly its cold 

 from the exhalation of the earth, and in the mid region of air 

 (as it is termed) from cold vapours and snow, so that no judg- 

 ment can be formed of the nature of air by that which is out of 

 doors and exposed, but a more correct one might be derived from 

 confined air. It is necessary, however, that the air should be 

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

 with heat or cold of themselves, nor easily admire 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 gra- 

 duated glass tube. 



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

 dkins, 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 cpoke of ia 

 the preceding paragraph j*^ for all air appears to possess a cer- 



* This last is found to be the real reason, air not being a good con* 



