ioo General Properties of Fire or Caloric,. [Book II. 



univerfally allowed to be a mere negative quality, and 

 to mean nothing more than the abfence of heat or 

 fire. 



It appears the moil' convenient form of treating this 

 important fubject, firft to confider caloric or the mat- 

 ter of fire in its capacity of exciting heat and produc- 

 ing expanfion ; and fecondly, to direct our attention to 

 the various phenomena which it exhibits in its latent 

 or combined ftate, as the efficient caufe of fluidity 

 both in the incomprefllble and elaftic or aerial fluids. 

 I fhall firft enter into a brief detail of the principal 

 and known properties of caloric; and fhall afterwards 

 illuftrate thefe properties by its effects in different in- 

 ftances. 



Firft. The particles of fire appear to be more mi- 

 ffx/tf.than thole of any other fubftance whatever. It 

 penetrates all bodies with the utmoit eaie. If the 

 pores of a body are difpofed in right lines, fo as to 

 admit the paffage of fire without impeding its velo- 

 city, it will be tranfmitted in the ftate of light as well 

 as in its ordinary ftate, when it excites the fcnfation of 

 heat j as is the cafe with all tranfparent or diaphanous 

 bodies. But there is no body, however denfe, which 

 will not admit this elemenl to circulate through its 

 pores with the utmoil rapidity. A. .piece of charcoal 

 fcrewed up faft in a vefTel of iron will be ignited as 

 effectually as in the naked fire. Thole bodies which 

 mod completely exclude the air, are utterly unable to 

 refift the entrance of caloric : for a thermometer will 

 rile equally in the moft complete vacuum that can 

 be produced, as in the open ak *. 



idly. The matter of fire is attracted more or lefs 

 by all bodies. When any heate^i body comes in con- 



* See Jones's Pbyf. Difq. p. 38. 



tact 



