22 PHYSIOLOGY <>F Mrsu. K- AND NF.KVES. 



one square cent imet r<\ we may a nine that this 



ta of a Inimln-il n>dh 'ts of equal length, lying >ide by 

 si !<. tin- Cross-Section of cadi of \\hich mea-mvs ex- 

 actly one square millimetre. On attaching a weight ,,f 

 one kilogramme ( = 1000 gr.) to this r<><l, each one of the 

 hunili-c;! thin r< ll ( s would have to bear a weight of 

 but ten graiiiincs. ('tunj)aring with tin's the tension of 

 another steel rod of the same length, but of which the 

 cross-sect ion measures twice as much, we may assume 

 that this second rod is composed of two hundred minute 

 n>d let <, the cross-section of each of which niea.-uivs unc 

 millimetre. The weight being now distributed between 

 two hundred of these rodlcts, each has to support a 

 weight of onU live grammes. This explains why the 

 tension by the same \\eight is only half as great in a 

 rod of double thickness. That the extension is pro- 

 portionate to the length of the extended rod can be 

 explained in the following way. According to the \iews 

 of modern ph\>iei-ls every body consists of a number 

 of -mall molecules or ] (articles which are held at definite 

 distances from each other by attractive and repulsive 

 forces. On fastening a rod by its upper end and at- 

 taching a weight- to its lower end, the molecules are 

 bv these means slightly separated from each other. 

 The sum of all these small separations represents that 

 whole extension mea-nrable at the end. The longer 

 any given bodv is the greater is the number of these 

 small particles which occur in its whole length, and 



con^eiplelltly tin- greater lllll-t itse\tellMoH be, pl'e- 



vided all otlyr circumstance- are eijiial. 



From the-e .ih-e|-\:ii ions may be deduced a la\\ fa 

 to ela-lic teii-i"ii, which is fnrt her continued by accurate 

 Sj and this law is that tin tension i* <lin'clh/ 



