146 



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



heated spatula approached them both from the same side ; they then 

 instantly darted in opposite directions. The movements were unaffected 

 by raising the temperature above the critical point of carbonic acid. It 

 is certainly very perplexing to find two cavities in the same section 

 closely adjacent to each other, and nearly of the same size, the bubbles 

 in which are moved in opposite directions by the same source of heat 

 applied from the same side. 



Bubbles containing gas at high tension, under different conditions of tempera- 

 ture, are first repelled and then attracted by a heated body. 



My work was discontinued for a period of some months ; but on 

 being able to look over my specimens once more, I verified all 

 my former observations, and became surprised by the following dis- 

 covery. A bubble which was repelled by a gentle heat was attracted 

 after it had been heated more . strongly, and then on cooling it was 

 again repelled. It appeared to contain some liquid carbonic acid 

 floating on water with the gas. Searching for such other specimens, 

 the cavity, fig. 6, A, was met with ; it contained a large proportion of 

 liquid carbonic acid, with a little in the gaseous state floating on water, 

 and the bubble is so movable as to act like a spirit-level. On cautiously 

 applying a warm spatula the bubble was repelled ; on heating it a little 

 more, the liquid carbonic acid became gas, and the bubble was again 

 repelled. The spatula was then made almost red-hot and applied ; the 

 bubble was then strongly attracted ; after cooling somewhat it was again 

 repelled. It was noticed that after the critical point of the carbonic 

 acid had been reached, the bubble sank through the water. It has been 

 shown by the various experiments already related that at only moderate 

 temperatures both repulsion and attraction can occur. 



Temperature, then, does not directly cause these opposite effects ; it 

 can only be some alteration in the conditions of experiment caused by rise 

 of temperature. Increase of tension or pressure within the cavity is appa- 

 rently the only condition which has varied ; and probability that this is the 

 cause of this contradictory attraction is afforded by the following experi- 

 ment. The specimen was placed in a water-oven (the temperature it 

 would there acquire would be about 94° C. to 96° C.) ; the platinum 

 spatula was heated in a beaker of oil to 130° C. The warm specimen 

 insulated by india-rubber was placed on' the microscope-stage, and the 

 warm spatula presented to it, when instant attraction was seen. The 

 same proceeding was repeated many times, always with the same result. 

 As the spatula cooled it ceased to affect the bubble at all ; of course at 

 the same time the specimen was also cooling. The spatula at the tem- 

 perature of 130° C. was. applied to the cooled specimen, which was, how- 

 ever, still at a temperature above 30° C. ; the effect was repulsion as 

 at first. 



This seems to show that the temperature which the rock-crystal attained 



