144 



Mr. W. N. Hartley on Attraction and [Apr. 12, 



pressed gas ; actual experiment, however, has proved that the bubbles are 

 spaces left by the contraction of the water on cooling from a high tempe- 

 rature, and therefore contain aqueous vapour and only such gas as may 

 be dissolved in the water. Some obstruction, probably friction or 

 adhesion of the liquid, caused by the flatness of the cavity, prevents 

 the bubble in A' from moving freely ; but it is actually repelled, or there 

 is a tendency to repel it, if a wire very strongly heated be brought 

 near. It is not attracted, however, at any temperature. When repelled 

 it returns as if squeezed back. Capillarity makes the bubble assume a 

 spherular form whenever possible ; therefore it returns to such a position 

 as is most compatible with this shape. Sometimes the motion is not a 

 transference of the bubble from one point to another ; it seems to be 

 fixed, but flattened at one side, and shaken as if something were pushing 

 and trying to move it. The bubble in A, a deeper cavity, moves very 

 freely and is repelled by heat. The cavities B, C, D contain bubbles 

 which, curiously enough, are attracted by heat. 



Another cavity of irregular shape, and at least four times the size of 

 the largest of these, behaved exactly in the same manner. As in the 

 other experiments, the objects were frequently turned about in different 

 directions to prevent mistakes, 



A series of experiments were made on these cavities to ascertain the 

 precise difference, if any, between them. 



The bubble in A was found to have disappeared at 105° C, and it 

 returned immediately on cooling with a sort of jump, which carried it 

 the whole length of the cavity, and made it rebound from the further 

 end. At 104° C. the bubble had not disappeared. These numbers are 

 the result of sixteen experiments. 



The bubbles in the cavities B, C, D did not all behave in the same way. 

 Thus, from ten experiments at different temperatures, it was found that 

 at 85° C. the liquid in C had expanded so as to fill the entire space, at 

 83° C. it had not done so, while D required a temperature of 123° C. 

 The liquid in B was apparently unaffected by so slight a rise of tempera- 

 ture, bat it was made to fill the cavity at 138° C. "When heat had been 

 applied so that the bubbles in all the cavities had disappeared, the one in 

 B returned first, that in C generally appeared next, and that in D last. 



Sometimes, after very strongly heating the specimen, the bubbles in C 

 and D did not return for half an hour, though two or three minutes was 

 a period quite sufficient for the specimen to become cooled down. 



Sometimes the appearance in the cavity on cooling somewhat resem- 

 bled the sort of ebullition which occurs when carbonic acid is cooled 

 when at a temperature above its critical point ; the motion, however, of 

 the bubbles was much slower, and occurred in one direction only, except 

 when the bubbles rebounded from the lowest point of the cavities. 



The bubble in B does not roll about when the microscope-stage is 

 rotated ; in this respect it differs from those in C and D. Careful 



