150 ACTION 



and in other kinds there is little ; but there is none 

 in which there is not some resistance ; and there 

 is perhaps no substance that becomes sensibly hot 

 to the full extent of the heat applied to it, but 

 shifts its bulk, of course insensibly, by the very 

 slightest variations of temperature; when, however, 

 the resistance of the substance is overcome, and 

 there is no other opposition to the motion pro- 

 duced by the heat, no more sensible heat is shown ; 

 though it continues to drive off the particles of the 

 substance until, if it be in the free air, they are dis- 

 sipated through that, and the object is lost to the 

 senses, except indeed the viewless and touchless 

 particles remain to bid adieu to the sense of smell- 

 ing ; and it is not a little curious that that sense, 

 which has much less apparent connexion with 

 external things than some of the other senses, 

 should yet be, in many instances, the first to find 

 things, and the last to lose them. 



After the power of heat has overcome that of 

 cohesion in the heated substance, so as that sub- 

 stance would spread in vapour through the thin 

 air, the heat instantly commences its attack upon 

 the vessel, or whatever else confines the matter 

 which it has overcome, and subdued to its pur- 

 pose. The general principle in this has been al- 

 ready noticed and illustrated at some length in the 

 case of water, but there are still more magnificent 

 displays of the triumph of heat over matter, which 

 take place on the great scale. Volcanoes, whether 

 under the dry land, or the sea, are instances of 

 that kind of action, and so also are earthquakes ; 

 and the chief difference between these is, that in 

 the volcano the heat drives the expanding matters 

 through one aperture; while in the case of the 



