274 Mr. J. H. T. Roberts on the 



The expansion apparatus was of the ordinary Wilson 

 type *. It was, however, fired by means of a trigger which 

 was worked electromagnetically, so that all expansions could 

 be relied upon as being of the same degree of suddenness. 



The tap B enabled the apparatus to be put into com- 

 munication with a Gaede rotatory mercury pump, or with a 

 source of nuclei-free air. The simple device A is worth 

 bringing to the notice of persons using expansion apparatus. 

 It consists of a bottle of 2 litres capacity, very carefully 

 sealed round the stopper, and having a tube 2 cm. diameter, 

 fairly loosely packed with cotton-wool soaked with glycerine, 

 passing through and dipping into glycerine at the bottom. 

 As will be seen later, in the description of the nuclei 

 obtained in a vacuum, it was necessary, after the nuclei- 

 chamber C was evacuated, to have it filled instantly with 

 nuclei-free air ; this was impossible with the ordinary cotton- 

 wool plug. After the reservoir A has been in use a few 

 days, the air drawn from it can be subjected to an expansion 

 of 1'20 without a single drop being seen ; the writer has 

 never found this to be possible with a cotton-wool plug, 

 except at great waste of time. 



The method of making a test of the nuclei was as 

 follows : — The tap F being closed, the expansion chamber Gr 

 was pumped down to a pressure much lower than that 

 required for the pressure drop of the expansion ; H was 

 then closed, and the pressure in having been adjusted, the 

 wire was heated. When the heating current was cut off, 

 was put into communication with A, B was closed, and 

 F was opened, with the result that a puff of gas went from 



c to a. 



Experiments. — When the experiments upon the nuclei 

 from hot wires were first begun, it was found impossible to 

 obtain consistent results. The temperature required to give 

 nuclei of a definite size, for example, or the minimum 

 temperature at which the emission of nuclei could be 

 detected, was extremely variable, and the behaviour of the 

 wire after treatment in various gases, for various intervals 

 of time, and at various temperatures, seemed to be entirely 

 capricious. After a considerable amount of preliminary 

 work, it was decided to keep the wire in air, and to try the 

 variation of the effects with time, no variation being made 

 in the conditions, other than that of heating up the wire to 

 make the tests. 



In the earlier stages of the work, when the conditions 

 under which the wire was placed were varied somewhat at 

 * C. T. E, Wilson, Phil. Trans. A. 1897 ; A. 1899. 



