40 Uapt. A. C. Egerton on the Vapour Pressure 



were made later in the course of the work ; the purpose was- 

 to make certain of the " resistance "" of the tube when an 

 internal cooled tube was used on which to deposit the metal. 

 The mercury was condensed out by keeping the inner 

 cooling tube filled with liquid air. It is noticeable that the 

 mercury condenses as an opaque dark film on the cooled 

 tube ; opaque, although when warmed and melted there is 

 often no trace of mercury on the tube visible to the naked 



Although this way of calibrating the tube is not so 

 accurate as the former method, it is very useful in obtaining 

 the necessary correction for w 2 the " resistance " of the tube. 

 w 1 can be measured directly in the case of the iron tube. 

 It is found that the resistance of the tube is increased some- 

 what by the presence of the inner tube, as one would be 

 led to expect. The lower value w 1 + iv 2 = 8'3 m 6 for the iron 

 tube gives vapour pressures which are in good agreement 

 with the values for the vapour pressure obtained in the 

 experiments with cadmium using the glass tube C ; this is 

 additional evidence of the validity of the figure for the 

 resistance of the tube F. 



It would have been quite sufficient to have only considered 

 the values obtained with tube C in the cadmium experi- 

 ments. Tube F was employed partly to check and add 

 additional weight to the results with tube C, and partly to 

 further investigate the use of the iron-plate type of tube, so 

 that plates with many holes could be constructed and utilized 

 for later experiments. 



In an experiment with the large tube (3 cm. diameter) H 

 with iron plate of 9 holes, the holes were first measured and 

 the tube also standardized in the oil-bath at 200° and found 

 to agree with the measured values, if the requisite measure- 

 ment of length of tube is made for calculating w. Considering 

 the smallness of the vapour pressure, measurements down to 

 150° C. were made fairly satisfactorily. But two experi- 

 ments at 140° C. and 128° C. gave no measurable result. 

 It is noteworthy that Krafft *, working with cathode-ray 

 vacuum, and also Demarcay t mention that at about 150° the 

 vapour pressure of cadmium suddenly becomes indetectable. 



The experiments with zinc were first of all carried out 

 with tube 0, in order to investigate roughly the lie of the 

 curve. In the meantime, however, Messrs. Johnson and 



* Krafft, Ber. Deut. Ges. 1903 A, vol. xxxvi. p. 1690; 1905 A,, 

 vol. xxxviii. p. 254. 



f Demarcay, Comptes liendus, vol. xcv. p. 183 (1882). 



