ESSAYS 



505 



It is at present almost impossible to deduce how far the cohesive attraction 

 between two isolated molecules differs from that between a similar pair forming 

 part of an appreciable volume, but since the effective range within which it can 

 preponderate over gravity or other external force is small, it seems improbable 

 that the gross tractation between a contacting pair in a mass can be much greater 

 than ten times that between an isolated pair. For the purpose of this inquiry no 



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distinction has been drawn, although it will doubtless be ultimately desirable to 

 do so. 



Tensile strengths of 20 kg. per sq. cm., or say 2 x io 7 dynes per sq. cm., 

 are common (e.g. cement), and since this represents the net value after deducting 

 the pellation (of which more later) and corresponds to a decreased intensity due 

 to separation, it is a conservative figure. Assuming unitary linkage between 

 molecular pairs separated by the imaginary section plane and reckoning only 

 io 15 pairs, this corresponds to a tractation of 2 x io -8 dynes. 



Maximum tensile strength is in the neighbourhood of 10,000 kg. per sq. cm. 

 (hard steel), or say io 10 dynes. Assuming again io 1 * molecular pairs (which is 

 probably too few) we get a tractation of 10 " 8 dynes. 



33 



