1877.] Constant Vibration of Minute Bubbles. 151 



The particle was moving rapidly at 13*5 C. 



Well seen at 15-5 C. 



Moving particle very small, motion very rapid at 17-5 C. 



f 20-5 c. 



Particle not visible { 19*5 C. 



I 18-7 C. 



Just visible 17' 7 C. 



Invisible 18-0 0. 



At —14° C. the moving particle appeared to occupy nearly half the 

 cavity ; its motion was very slight ; had it not been previously seen in 

 rapid motion, it would have been impossible to say with certainty that it 

 moved at all. At —15° C. it was as nearly as possible motionless. It 

 was now necessary to watch the effect of gravitation upon the particle ; 

 for if it be gaseous it will have a tendency towards the upper part of the 

 cavity when the liquid condenses, while if it be liquid it will more fre- 

 quently be observed towards the lower end. In eight out of ten experi- 

 ments in which the liquid was condensed from the gaseous state, the 

 particle took up a position at the top of the cavity. The microscope was 

 placed at an angle of 45°, and the stage was rotated partially before each 

 experiment in order to avoid examining the specimen in one position 

 only. As, therefore, the spherule is not the fluid with greatest density, it 

 is most probably a gas-bubble, and the contents of the cavity are in such 

 a state of compression that the liquid expands so as to entirely fill the 

 cavity at 18° C. The liquid is doubtless carbon dioxide. Several other 

 cavities of the same kind were noted. 



This shows that in examining rock-sections it is necessary to cool as 

 well as warm the specimens. 



The following test gave indisputable evidence that the moving particle 

 was the bubble. While in rapid motion the bubble was strongly attrac- 

 ted to the side of the cavity when a body of a slightly higher tempera- 

 ture was made to approach it. That the liquid which exhibits such mo- 

 tion need not be a condensed gas is shown by the following facts. A piece 

 of quartz from the summit of Snowdon abounded in water- cavities of dif- 

 ferent sizes. Some of these contained a bubble in constant vibratory motion 

 at 18° C. The size of the cavities was about ^wo X yo^nro- of an inch, and 

 the size of the bubbles ^zhou °^ an ^ ncn ^ n diameter (see fig. 7, A and B). 

 The movements were chiefly vibrations up and down, and gentle alterations 

 in position from right to ieft. A platinum spatula was made red-hot 

 and brought near to -the object-glass; the motion instantly ceased, and 

 the bubble was seen clinging to the side nearest the heated spatula. On 

 changing the position of the spatula the bubble did not immediately move ; 

 it seemed to require some time for a rise of temperature to overbalance 

 that which the stone had previously acquired. From right to left and 

 from top to bottom the bubble was attracted and made to come to rest. 



