Constants of Igneous Rock. 



181 



Finally K k is prolonged by a straight cylindrical tube of 

 platinum d d, which may be fixed to the steel rod k by the 

 clamp e in any position along the vertical. The tube fits the 

 rod k singly, and a special steel rod is provided by which the 

 tube may be straightened, should it become warped. 



The lower end of d is clearly visible in the telescope of the 

 cathetometer, through the sight-tube A. On being screwed 

 down, d enters the fusion-tube axially, supposing both tubes 

 (d and Z Z) to have been properly fixed in position. In how 

 far such adjustment has been made, can be seen by tempo- 

 rarily removing one of the efflux tubes, E (fig. 4), when the 

 parts concerned are visible to the eye. 



11. Telephonic Registration. — Since it is necessary to find 

 the depth of the meniscus of the molten rock Z Z below the 

 plane of the top of the fusion-tube, the moment of contact of 

 Z Z and d is registered electrically. Above, say, 500° C, rock 

 is a good conductor. Hence, if a current be passed into the 

 screw K k, through the clamp-screw/ (fig. 3), it will issue at 

 the bottom at the clamp-screw c (fig. 1), provided c be electric- 

 ally connected (platinum wires) with the bottom of the fusion- 

 tube, Z Z. 



A telephone actuated by Kohlrausch's small inductor is 

 more convenient for the present purpose than a galvanometer. 

 Contact is then indicated by a loud roar, and the observer's 

 attention is not further distracted. If the glass Z Z be sticky, 

 the drawing out of a thread corresponds to a more or less 

 gradual cessation of the noise, so that the character of the 

 fusion can be pretty well indicated in this way. Care should 

 be taken to insure a small sparking-distance. 



If the expansion of the rock be known, and the tube d d 

 be sunk deeply into the mass, it is clear that the present 

 method admits of a measurement of the relation of the electric 

 resistance of the glass and temperature. I mention this, 

 believing that not only will a suitable method of temperature 

 measurement be thus available, but that the volume diagrams, 

 constructed below, may also be derived solely from measure- 

 ments of electrolytic resistance *. 



Method of Measurement. 



12. Consecutive Adjustments. — Thus far I have only studied 

 rock-contraction. For this purpose, the graduated faucet, 



* Cf. Am. Journ. Sci. xlii. pp. 134-3-5 (1891), where I have already 

 speculated on the character of electrolytic resistance referred to tem- 

 perature. 



