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



After some little experience, the difference between the 

 two kinds o£ nuclei is very easily observable : the nuclei 

 produced from 350° C. onwards are very easily got rid of; 

 they diminish in size very rapidly after their production, and 

 will entirely disappear in less than five minutes ; they can 

 be preserved to some extent, however, by causing con- 

 densation of water-vapour to take place upon them. The 

 nuclei which begin to be produced at about 500° C, however, 

 are very persistent, and they do not alter in size. 



The nuclei represented by the portion ACD, fig. 3, are 

 the ones which are got rid of after many weeks of experi- 

 menting ; thus a wire whose condition was represented by 

 the part E of curve, fig. 2, would give the curve BDE, fig. 3, 

 the portion ACD being absent. 



If a wire which has been got into the condition represented 

 by BDE, fig. 3, and by the part E, fig. 2, is heated and 

 cooled in hydrogen, the wire goes back to its original con- 

 dition, represented by the whole of fig. 3 and by F, fig. 2 r 

 and on repeated heatings all the variations there represented 

 are gone through as before. No treatment with any other 

 gas, or in any other way, has the effect of bringing back the 

 variations represented by fig. 2. 



Relation betiveen Pressure of Air and Size of Nuclei. 



Fig. 4 shows the results of experiments upon the relation 

 between the pressure of the air round the wire, and the size 

 of the nuclei produced, for different temperatures. The 

 curve for a temperature of 320° C. is AB, the curve for 600° C. 

 is FGrE together with two points practically on AB, while 

 the curve for 1650° C. is GDH, together with one point 

 on AB. By comparing fig. 4 with fig. 3 it will be seen that 

 the first set of nuclei are obtained at 320° C. and expansion 

 of 1*041 as against 340° C. and expansion 1*119 at atmo- 

 spheric pressure, and that the expansion required increases, 

 that is, the size of the nuclei decreases, as the pressure of the 

 surrounding gas increases. At 600° C. none of the second 

 set of nuclei are produced when the air is at a pressure below 

 5 cm. ; two points corresponding to the first set of nuclei 

 lie on AB, showing that the temperature is of less importance 

 than the pressure in determining the size of these first 

 nuclei. At 5 cm. pressure the second set begin at expansion 

 1*188 and rapidly increase in size with increase of pressure. 



At 1650° C. there is one point on AB at a pressure of 

 1 cm. : the second set of nuclei are produced at a pressure 

 of 1*5 cm., and require an expansion of 1*188 to catch them ; 



