34 Henry H. Dixon. 



heard. Donny explained that in the first case the blow causes 

 minute bubbles to be opened against the forces of surface tension 

 and of atmospheric pressure, therefore in the second case where 

 bubbles are not formed the cohesion must be greater than these two 

 forces together. Donny believed that even a little air in solution 

 suffices to reduce the cohesion of a liquid to an insensible figure. 

 This error, soon to be corrected, has been frequently copied by writers 

 on this subject. Donny also points out that the boiling of liquids 

 is retarded when air is removed, owing to their increased cohesion 

 and it is the cohesion being suddenly overcome which causes ex- 

 plosive boiling. 



As will immediately appear, the removal of dissolved air by 

 boiling from the water is by no means necessary for its cohesion, and 

 in these experiments on the tensile state of liquids it is only 

 necessary in order to secure perfect contact with the glass and 

 complete wetting of the dust particles suspended in the liquid. 



In his memoir Donny ^) points out that when one withdraws a 

 plane disc from contact with a surface of water the tensile strength 

 of the latter does not come into play. As the disc is raised water 

 adheres to its lower surface, but the column of water connecting the 

 disc with the liquid below grows gradually thinner, until a moment 

 when the disc is removed a certain distance above the general level 

 of the lower liquid, then the column spontaneously draws in from the 

 edges of the disc, and, when its diameter becomes extremely small, 

 breaks in two. He shows also, that in a tensile liquid column a bubble, 

 sufficiently small to have surface-tension forces capable of supporting 

 the hydrostatic head of the liquid below, will not destroy the tensile 

 state. 



Berthelot ^) a few j^ears afterwards succeeded in showing 

 directly that water has a very considerable cohesive strength and, 

 under proper conditions, can sustain a very great tensile stress. His 

 proceedure was as follows: He filled a strong capillary tube, which 

 was sealed at one end and drawn to a fine point at the other, with 

 water at a temperature of 28 ^ or 30 " C. He allowed it to cool to 

 18^, and, as it cooled, to draw in air. Then the fine-drawn end was 

 sealed. The tube was now heated to 28 ", or over, and the air forced 

 into solution in the water which now occupied the whole of the 

 internal space of the tube. On cooling to 18 ^ or lower, it was found 

 that the liquid continued to occupy the entire space enclosed by the 



^) The presence of undissolved air, or unwetted surfaces, or both, probably 

 prevented J an se from obtaining considerable tensions in the experiment quoted by 

 him. Der aufsteigende Strom in der Pflanze. Jahrb. f. wiss. Bot., 45, 3 (1908) p. 314. 



^) M. Berthelot, Sur quelques phénomènes de dilatation forcée des liquide?. 

 Ann. Chim. et de Phys , XXX, 1850, pp. 232 et seq. 



