630 LECTURE L. 



a single grain of gold has been drawn into a wire 500 yai'ds in length, and 

 consequently little more than-j-^Wof an inch in diameter. The ductility 

 or tenacity of a spider's web is of a different kind, it is particularly shown 

 by its capability of being twisted, almost without limit, and of accommodat- 

 ing itself to its new position without any effort to untwist. 



With respect to the ultimate agent by which the effects of cohesion arc 

 produced, if it is allowable to seek for any other agent than a fundamental 

 property of matter, it has already been observed, that appearances extremely 

 similar might be derived from the pressure of a universal medium of great 

 elasticity; and we see some effects, so nearly resembling them, Avhich are 

 unquestionably produced by the pressure of the atmosphere, that one can 

 scarcely avoid suspecting that there must be some analogy in the causes. 

 Two plates of metal, which cohere enough to support each other in the 

 open air, will often separate in a vacuum.:; when a boy draws along a stone by a 

 piece of wet leather, the pressure of the atmosphere, appears to be materially 

 concerned. The well known experiment, of the two exhausted hemispheres 

 of Magdeburg, affords a still more striking instance of apparent cohesion 

 derived from atmospherical pressure; and if wq place betweea them ift. thick 

 ring of elastic gum, we may represent the natural equilibrium between the 

 forces of cohesion and of repulsion ; for the ring would resist any small 

 additional pressure with the same force as, would be required for separating 

 the hemis|»heres so far, as to allow it to, expand in an .equal degree: and at 

 a certain point the ring would expand no more; the air, would be admitted, 

 and the cohesion destroyed, in the same manner as when a solid of any 

 kind is torn asunder. But all suppositions founded on these analogies must 

 be considered as merely conjectural ; and our knowledge of every thing 

 which relates to the intimate constitution of matter, partly from the in- 

 tricacy of the subject, and partly for want of sufficient experiments, is at 

 present in a state of great uncertainty and imperfection. One of the most 

 powerful agents, in changing and modifying the forms of matter, is the 

 operation of heat, by which the states of solidity, liquidity, and elastic 

 fluidity are often produced in succession; and the investigation of the nature 

 and effects of heat will constitute the subject of the two next lectures. 



