138 



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



acid will always take place when the proportion of liquid to gas is so 

 small that evaporation is readily effected. If the liquid at 15° C. occu- 

 pies one half of the space of the cavity, this will occur only under special 

 conditions, because the liquid in such proportions expands by increase of 

 temperature. Thus in the case of a cavity in a topaz, shown in fig. 1, 

 a number of experiments have invariably failed to cause any transference 

 of the bubble from place to place. The approach of a warm substance 

 causes immediate expansion of the liquid and decrease in size of the bubble. 



The gas-bubble in a cavity of rock-crystal, shown in fig. 2, behaves 

 quite differently ; the proportion of liquid to gas is such that heat causes 

 evaporation instead of expansion ; and accordingly the liquid is repelled 

 apparently, and the gas-bubble attracted by a heated body. With the 

 cavity in tourmaline (fig. 3), when the heat is applied in a particular 

 manner, the same movement takes place ; ordinarily the liquid ex- 

 pands. To cause distillation and not expansion, the source of heat must 

 be small and the rise of temperature slight, in order that only one end 

 of the cavity may be heated. I have always failed to get this effect with 

 the topaz cavity, probably because the thickness of the section causes 

 the heat to be diffused over the liquid. The tourmaline section is thin 

 and the cavity long and narrow, and is therefore an easy one to experi- 

 ment with ; so likewise is the cavity shown in fig. 5, E ; it contains car- 

 bonic acid only, and the bubble is easily attracted when the source of 

 heat is properly applied. In the course of some thousands of observa- 

 tions, made within the last two years, I have noticed other movements 

 than such as may be compared with the experiments of Professors Tait 

 and Swan on tubes of liquid sulphurous acid. The circumstances in- 

 fluencing these movements, and the various conditions under which they 

 take place, render it necessary that I should disregard the order in which 

 I observed and originally recorded them ; for I have on more than one 

 occasion been bewildered by noticing what appeared to be diametrically 

 opposite facts in experimenting on the same specimen, and even on the 

 contents of the same cavity. I therefore consider it expedient to 

 classify my experiments, in order to make the account of them intel- 

 ligible. 



The attraction of bubbles by heat, water being the only liquid present. 



With regard to the attraction of bubbles by heat, I have noticed this 

 take place in some water-cavities when the bubbles were free to move 

 and no carbonic acid was present. In order that no mistake might 

 possibly occur as to the relative positions of the source of heat and 

 the moving bubble, a hot platinum wire was used and always 

 brought into the field of the microscope. Some thousands of cavities 

 have been noticed occurring in sections of rock-crystal and in the 

 quartz of various kinds of granite. The rise in temperature required 

 to cause this movement was measured at first by blowing warm air 



