54 Chemical and Physical Notes 



1 10 mm. long, and 15 mm. wide; and the exit tube (d) is 



50 mm. long, and 10 mm. wide. It is connected with the 



flask by means of a well-fitting cork, which is 



d r~ preferable to indiarubber as it does not adhere 



to the hot metal. From the above measurements 

 it results that the internal volume of the whole 

 tube is 239 c.c., and its internal surface is 

 291*5 sq. cm. The weight of the tube is 

 109 grms. ; and if its specific heat be O'2, the 

 weight of water thermally equivalent to it is 

 2 1 - 8 grms. 



These dimensions are suitable for the thermo- 

 meter which was used. The length of the tube 

 has to be suited 'to each thermometer. For 

 mountain work the thermometer should be 

 graduated from 85 C. to 101 C. and should 

 be divided into tenths of a degree, the space 

 between each division being about a millimetre, 

 so that the length of the scale would be 16 cm., 

 and that of the whole thermometer about 20 cm. 

 It is convenient also to have a thermometer divided into 

 fiftieths of a degree. It is essential that the thermometer 

 be read entirely in the steam. If any part of the mercury 

 extends outside of the apparatus, it introduces an uncertainty 

 which stultifies the use of very delicate instruments. What- 

 ever the size of the tube may be, it is essential to see that all 

 the parts are sufficiently wide and properly proportioned ; 

 there must be no resistance anywhere to the passage of the 

 steam. The lamp should be regulated to keep up a brisk 

 flow of steam of good volume through d. The thermometer 

 then finds itself immersed in saturated steam of atmospheric 

 pressure contained in a vessel, the outer surface of which is 

 glass, but the inner surface is a film of water continually 

 renewed by steam condensing on it. So long as the full 

 supply of steam is kept up, this continually renewed film of 

 water, in contact with saturated steam, is a perfect protection 

 against variation of temperature in the interior of the steam 

 tube. When the steam first passes through the tube there 



