1909.] Elasticity of Rubber Balloons and Hollow Viscera. 489 



investigations on elasticity are generally confined to substances where the 

 maximum extension is always a small fraction of the initial length, and as 

 Frank's experiments did not follow rubber further than linear extensions 

 to double the initial, it would be almost idle to expect that laws deduced 

 from these experiments could be applicable to the large and two dimensional 

 stretchings of an inflated balloon. 



The difficulty in explaining the rise of pressure and the subsequent partial 

 fall on inflation is, I believe, more apparent than real. This crest is due, 

 I take it, to a disturbing factor which, for lack of a better name, may be 

 called initial rigidity. This view is supported by the following facts : — 



1. If s fresh balloon is inflated, so that the pressure is anywhere on the 

 rise or fall of the crest, it will be found that the pressure does not remain 

 at a constant value, but tends to fall. In fact, to obtain a graph such as 

 fig. 2, the convention had to be adopted of reading the pressure after a given 

 interval of time — 3 minutes. But the fall had by no means stopped when 

 the reading was taken, and could be detected even some hours after infla- 

 tion. An attempt to register the pressure after a long interval of time 

 when no further fall might be expected, failed owing to the fact that some 

 of the air diffused out, as was proved by deflating the balloon in measured 

 decrements. 



2. If a balloon is inflated a second time (care being taken that the elastic 

 limit has not been reached in the first inflation) the crest is always less 

 pointed than in the first inflation. A third inflation gives a more obtuse 

 convexity than the second, and so on. The longer a balloon remains 

 collapsed the steeper is the rise and fall of pressure on inflation. This is 

 particularly marked if the collapsed balloon is exposed to light. 



3. When an inflated balloon is deflated in measured decrements and the 

 corresponding pressures recorded, in the vast majority of cases the pressure 

 falls to zero without any rise being manifested. I obtained this pronounced 

 hysteresis constantly in my earlier experiments, and was inclined to look 

 upon it as the invariable behaviour of a balloon during deflation. Fig. 3 

 gives graphs for two typical instances. 



But a rise of pressure may be obtained on deflation if certain conditions 

 are fulfilled. The rubber must be in good condition, the inflation should not 

 be taken far past the maximum pressure, and the return by deflation should 

 be carried out at once. The rise of pressure, however, is never more than a 

 few millimetres of water. The better condition the rubber is in the blunter 

 is the inflation crest and the less abrupt is the deflation fall of pressure. 

 Conversely, the more the rubber has been exposed in a deflated state to light 

 the sharper is the crest and the more abrupt is the deflation fall. 



