THE MINING DISTRICTS OF CORNWALL. 333 



spring* collar, The bath, thus constructed, rested on supports 

 attached to the microscope-stand, and could be heated either by 

 a gas-burner or spirit-lamp. A condenser directed the light from 

 a lamp upon the section, which was fixed against the inside of the 

 thin glass cover. 



After placing the section in its proper position the bath was filled 

 with melted paraffin ; and as soon as a group of suitable cavities was 

 brought approximately into focus, a thermometer was lowered into 

 the paraffin, and heat gradually applied. The observer now watched 

 the filling of the cavities, while an assistant read off the indications 

 of the thermometer. 



Some years since, I made use of this arrangement for determin- 

 ing the temperatures at which the bubbles in various Californian 

 veinstones disappeared, and found that very different degrees of heat 

 were required to effect this result— some disappearing at a tempe- 

 rature as low as 80° C, while others were still distinctly visible at 

 180° C * 



On applying this method to Gornish rocks the results were less 

 satisfactory, since the temperature required to cause the disappear- 

 ance of the bubbles is higher than can be obtained with melted 

 paraffin ; the vapours which were given off also imparted a consider- 

 able degree of cloudiness to the object-glass. It was however found 

 that at a temperature of 185° C. some of the bubbles in the granites, 

 elvans, and veinstones had sensibly decreased in size, while the 

 dimensions of others remained apparently unchanged ; this would 

 indicate that the bubbles in the latter case could not have been 

 vacuities. 



In addition to the usual fluid-cavities enclosing bubbles, the quartz 

 of granites, quartz-porphyries, and veinstones would appear to con- 

 tain others entirely filled with liquid. These, of course, have no 

 vacuity, and after having been strongly heated assume the character- 

 istics of ordinary gas- or vapour-cavities. It is to be regretted that 

 it has been found impossible to expel a portion only of this water 

 by heat, thus leaving a vacuity in further confirmation of their 

 character ; but in all attempts of this nature I have, as might have 

 been anticipated, invariably failed. With regard to such cavities 

 Zirkel remarks that, " as in artificially formed crystals from solu- 

 tion, so in natural crystals there are liquid-cavities which contain 

 no bubble." 



After a series of experiments on this subject I have arrived at the 

 conclusion that any method of determining the relation between the 

 cavity and bubbles by measurement must necessarily be beset with 

 grave difficulties, and that the disappearance of the latter, on the 

 application of heat, must take place at very varying temperatures. 

 Such being the case, any calculations based thereon can be reliable 

 only within certain somewhat wide limits ; and if the presence of 

 full fluid- cavities be admitted, for which there is the same evi- 

 dence as for the occurrence of vapour- and gas-cavities, all such 

 calculations must necessarily be fallacious. 



* Phil. Mag. 1.871, vol. xxxvi. p. 333. 



