44 



THE STEUCTUKE OF TIIE NtTCLEUS. 



containiiii,' results with tlie double druiu of figure 7, justifies the conclusion that 

 more saturated colors and finely graded effects are obtaiuahle with the longer col- 

 umns of nuclei (180 cm.). Still, the colors of the second order beyond the violet 

 (8th exhaustion), are very faint. The tendency to complementary contrasts between 

 axial color and coronal fit-Id is again in evidence, but has not been ultimately 

 determined. 



TABLE i8.— PRELIx\IINARY EXPERIMENTS WITH THE DOUBLE DRUM. 



HAUSTION, 76-66 cm. Time, 11" to i" ^o" 



EX- 



' Color of the central field of the corona. 



23. Definite observations for axial color. — A final series of investigations was 

 now made to find the distributions of axial color in its dependence on the number 

 of nuclei. The double drum, with its vapor column of 180 cm. in length, was used, 

 the white mantel of the Welsbach burner just beyond the further window being 

 the source of light. The exhaustions were necessarily kept low to avoid a collapse 

 of the drum ; but the low supersatu ration is rather an advantage since a more finely 

 graded sequence of color is obtained. The usual limits of exhaustion were from 

 76 to 67.5 cm. of mercury. The question whethei- an isothermal or an adiabatic 

 chan<'e of state is to be assumed must again be answered and it seems best to 

 compute both. 



Two series of virtually coincident experiments were made. In the second, the 

 nucleation was repeated or doubled at the outset, but without appreciable difference. 

 The colors are, as usual, very fleeting ; and though they tend to pass through the 

 preceding colors, as the cloud pai'ticles evaporate, they soon become too faint for 

 recognition. The first blues apparently fade through the yellows of the first order, 

 which I have not in these exhaustion experiments been able to produce with cer- 

 tainty in any case whatever. They are characteristic of the steam-jet. 



In table 18, the two series are given in parallel columns and the second series 



