530 Messrs. Owen and Hughes on Condensation Nuclei 



apparatus * which, together with its accompanying gauge and 

 mercury reservoir for adjusting the expansion, is omitted 

 from the figure. The cooling was produced by immersing 

 the bend of the glass tube C always to the same depth in 

 petroleum ether previously cooled by stirring with a test-tube 

 containing liquid air. We find it convenient to give the 

 name "tester" to the part of the apparatus immersed in the 

 cooling liquid. The internal volume of the " tester " was 

 about 3 cub. centims. The volume of the space ABODE 

 with the mercury in B at a fixed mark P was about 

 80 cub. cms. At the commencement of a test experiment 

 this space was filled with the gas at a pressure of 80 cms. By 

 raising or lowering the mercury in B, the pressure of the gas 

 in the " tester " could be increased to nearly two atmospheres 

 or reduced to less than half an atmosphere. The gas was 

 admitted through A and could be withdrawn through E or D 

 by means of an aspirator. The tap E was found useful in 

 rendering it possible to exhaust the apparatus to a low pressure 

 in order to remove the gas lodging in the gauge G. Also by 

 means of it a slow stream of gas would be drawn through 

 the apparatus without there being any danger of water- vapour 

 diffusing back into the "tester," as might conceivably happen 

 when the gas was drawn through the cloud chamber via D. 

 As a matter of fact, this fear proved to be groundless, the 

 results obtained being independent of the path along which 

 the gas was swept out of the " tester." 



The method of making a test was as follows : — With the 

 mercury at the top of the tube B, the gas was allowed to 

 stream through the apparatus for several minutes in order to 

 remove any nuclei produced by the previous test. The taps 

 D and E were then shut and the mercury lowered in B. By 

 shutting A oft' at a suitable moment, it could be so arranged 

 that the pressure was 80 cms. when the mercury was at 

 the fixed mark P. In this way the same volume of gas was 

 always experimented upon. The level of the mercury in B 

 was now adjusted until the pressure was that proposed to 

 use. One observer then surrounded the " tester " with the 

 cooling liquid for 40 seconds, noting (by means of a pentane 

 thermometer reading to —200° 0.) the temperature of the 

 liquid at the beginning and end of the cooling process. The 

 mean of these two readings was taken to be temperature which 

 the gas in the "tester" attained. During the cooling the 

 pressure of the gas was maintained constant by the second 

 observer. The tester was now surrounded by water at a 

 temperature of about 15° C. for one and a half minutes. At 

 * 0. T. R; Wilson, Phil. Trans. A. vol. clxxxix. p. 265 (1897). 



