ACTION OF HEAT. 153 



cloth-yard shaft" is drawn to the head. But if the 

 bow is " made of a trusty-tree" not a jot of its elas- 

 ticity is destroyed, but the more vigorously it is 

 drawn the more it accumulates ; and if the bowman 

 slips his fingers at the very instant of his utmost 

 stretch, the returning bow sends the arrow in per- 

 fect silence through the air fleeter than an eagle. 

 If, however, the bow were too small for the man, he 

 could draw it either till it broke or till its substance 

 were so much injured that it would not spring; and 

 if it were made of brittle wood, or of a pliant osier 

 twig, it might be overcome by the strength of ^ 

 child. 



It is the same with matter in resisting heat : in 

 some kinds of matter there is much resistance, 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 va^ 

 nations of temperature ; when, however, the resist^ 

 ance of the substance is overcome, and there is no 

 other opposition to the motion produced 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 dissipated 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 smelling; and it is not a little 

 curious that that sense, which has much less appa- 

 rent 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 purpose. The 



