546 A. ASHE ON PHOTOGRAPHY OF CAVITIES IX MINERALS, AND 



An attempt was therefore made to obtain some instantaneous 

 photographs of the liquid in this effervescing condition, and the 

 photographs Nos. 3, 4, and 5 were secured by employing lime- 

 light as an illuminant, together with an instantaneous shutter 

 attached to the camera. 



In order that the shutter might be released at the right 

 moment, a " vertical illuminator," of the cover-glass pattern, 

 was placed over the ocular, wdiich thereby deflected a small 

 portion of the light to the side of the microscope, whilst the 

 greater portion of it passed on through the glass into the 

 camera. By this simple means the field of view could be observed 

 and the photograph taken at any desired moment, 



No. 3 shows the appearance produced when the slide was 

 quickly cooled and the liquid was in a state of violent 

 effervescence. 



No. 4 is the same, but was taken when the action had nearly 

 ceased ; and as the cavity was placed in a different position, 

 the form of the vacuum bubble is seen to greater advantage. 

 The effects of surface tension may be noticed in this photograph 

 by the little drops of liquid which linger for an appreciable 

 time on the surface of the condensed portion. 



No. 5 is similar to No. 3, but is slightly enlarged. In a few 

 cavities the condensation took place more gradually, and gave 

 the appearance of falling raindrops rather than of ebullition. 

 This difference in their behaviour seemed to be due entirely 

 to the ratio of the volume of the enclosed liquid when cool to 

 the volume of the cavity it occupied. 



When this ratio was high, the liquid contracted from a large 

 bulk to a small one with great violence ; but wdien, on the 

 contrary, the ratio was low, the contraction being less, it 

 proceeded more evenly, and produced the appearance of falling 

 rain. 



In order to determine w^th accuracy the precise temperature 

 at which these effects took place, an apparatus was employed 

 of which the following description, together with the photographs 

 given, will render its use apparent in similar work. 



The first requisite in an apparatus of this kind is that the 

 temperature may be made to fall at a very low and even rate, 

 and this is best attained by enclosing the mineral in a rela- 

 tively enormous mass of water and allowing the whole to cool 



