22 PHYSIOLOGY OF MUSCLES AND NERVES. 



one square centimetre, we may assume that this con- 

 sists of a hundred rodlets of equal length, lying side by 

 side, the cross-section of each of which measures ex- 

 actly one square millimetre. On attaching a weight of 

 one kilogramme ( = 1000 gr.) to this rod, each one of the 

 hundred thin rodlets would have to bear a weight of 

 but ten grammes. Comparing with this the tension of 

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

 cross-section measures twice as much, we may assume 

 that this second rod is composed of two hundred minute 

 rodlets, the cross-section of each of which measures one 

 millimetre. The weight being now distributed between 

 two hundred of these rodlets, each has to support a 

 weight of only five grammes. This explains why the 

 tension by the same weight 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 views 

 of modern physicists every body consists of a number 

 of small molecules or particles 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 

 by these means slightly separated from each other. 

 The sum of all these small separations represents that 

 whole extension measurable at the end. The long-er 

 any given body is, the greater is the number of these 

 small particles which occur in its whole length, and 

 consequently the greater must its extension be, pro- 

 vided all other circumstances are equal. 



P'rom these observations may be deduced a law as 

 to elastic tension, which is further confirmed by accurate 

 researches, and this law is that the tension is directly 



