v TENSILE STRENGTH OF SAP OF TREES 105 



If the vessel was then cautiously heated, the water ex- 

 panded more than its glass envelope, and the air bubble 

 was compressed. The bubble became smaller and smaller 

 as the temperature rose and the contained gas was forced 

 into solution. When the bubble had reached very small 

 dimensions and was about to disappear, great care had to 

 be exercised in the further application of heat ; for, if the 

 water expanded too much and strained the glass beyond 

 its elastic limit, the whole experiment was rendered abortive 

 by the breaking of the glass. But, if the heating process 

 had been carried out successfully, all the air had been 

 dissolved so that the water had been made to fill the 

 vessel completely without breaking it. 



At this moment the water in the vessel was either in 

 compression, being constrained by a tension in the glass- 

 walls, or it was quite unconstrained, just exactly filling the 

 envelope, and neither suffering compression nor causing 

 tension in the walls. As soon as cooling began, the water 

 and the glass commenced to contract. The coefficient of 

 thermal expansion of water being greater than that of 

 glass, the water tended to contract more. This contrac- 

 tion, however, was resisted by its adhesion to the glass 

 and its own cohesion, and consequently a tension, which 

 kept it sufficiently dilated to fill the glass, was set up. 

 As cooling proceeded the tension grew greater and 

 greater, till at last either the adhesion or cohesion 

 was overcome and a break appeared between the water 

 and the glass or in the substance of the water itself. 

 This rupture was signalised by a sharp click, and a 

 bubble sprang into existence, which rapidly augmented in 

 size as the water, now relieved from the stretching forces, 

 assumed a volume corresponding to its temperature at the 

 moment. Bubbles appeared round the original bubble and 

 passed into it. 



By estimating the amount of deformation of the glass 

 envelope when strained by the contracting water, and by 



