Constants of Igneous Rock, 5 



in the following pages I have proved that bulbs not glazed 

 internally are quite available for the exact air-thermometry 

 of high temperatures, one of the serious difficulties which the 

 work entails will have been removed. 



6. Metallic Capillary Tubes*. — The bulb and the mano- 

 meter are joined by seamless metallic capillary tubes, about 

 200 centim. or more long, and '05 centim. in diameter within. 

 These tubes are to be cut apart near the middle, and joined 

 by a suitable three-way glass stopcock, through which the 

 bulb may be exhausted and thoroughly dried at red heat, and 

 both bulb and manometer finally charged with the necessary 

 amount of dry air or other gas. 



Capillary tubes may be made of either copper, silver, or 

 platinum. The latter are preferable because they do not 

 amalgamate in case of an accidental contact with mercury. 

 I succeeded in making copper capillaries myself, by filling 

 a thick copper tube with fusible metal, rolling and drawing 

 the tube down to a wire, and finally melting out the fusible 

 core under slight pressure applied at one end of the tube. 

 Tubes of the kind necessary, however, are now in the market, 

 and may be obtained in lengths not exceeding 5 metres. As 

 employed in the work below, an external diameter of *1 centim. 

 to a bore of *056 centim., or a volume of '0025 cub. centim. 

 per centimetre of length, was found to insure a sufficiently 

 rapid flow of gas from bulb to manometer, while the air con- 

 tained in the capillary is nearly negligible in amount. 



7. Manometer. — This is practically a U-tube, one branch 

 of which is made of glass, 150 centim. long, accurately 

 graduated in cubic centimetres, with a total capacity of about 

 300 cub. centim. Into the top of this tube, the available end 

 of the platinum capillary, tapering to a fine conical point, is 

 suitably soldered with cement. The point so adjusted is a 

 convenient fiducial mark, the registration being made either 

 by optical or by electric contact with the mercury meniscus 

 of the manometer. The other branch of the U-tube is flexible, 

 and communicates with a sufficiently large mercury cistern, 

 which may be raised or lowered at pleasure. The whole is 

 attached to a prismatic stand, 2 metres high, consisting of 

 four uprights rigidly joined at the top and the bottom. 

 Observations are made for equal heights of the mercury in 

 both branches, by the cathetometer. A barometer is attached 

 to the stand. Sensitive thermometers at the lower and the 

 upper end of the glass measuring-tube show its temperature. 

 Cf. § 18. 



* First used by Kegnault (Relation des Experiences, Paris, 1847, p. 264). 



