Lecture-Eaperiments on Spectrum Analysis. 365 
in Thomson and Tait, Part II. Appendix F. Ina lecture on 
the size of atoms delivered at the Royal Institution in February 
1883 he restates these estimates with slight modifications 
thus :— 
If atomic dimensions are comparable with 10-® centim., 
brass would rise 62° C. at the instant of formation; while if 
atoms are so small as 2°5x10-9, it would rise 1000° C. 
Hence 10-8 is to be regarded as a limit of smallness. 
A soap-film so thin as 10-* centim. would raise itself 280° 
by collapsing ; therefore there are not several molecules in this 
thickness. 
The theory of gaseous collision, combined with the density 
of liquids, suggests a range lying between 7 x 10-9 and 
Ax 10-9, 
The dispersion of light seems to require atomic dimensions 
to lie between 10-7 and 107°. 
The final estimate made by Sir Wiiliam is something between 
2% 10-7 and 10;-%, 
But if the reasoning in the present paper be admitted as 
correct, it would seem possible to reduce this range of uncer- 
tainty, and to make an even more precise estimate. 
XXXIX. Lecture-Experiments on Spectrum Analysis. 
By H. Cieminsuaw, IA., £.C.S.* 
 § PROPOSE in the following paper to explain some pro- 
cesses by which all the phenomena in spectrum analysis, 
usually shown upon the screen in class-demonsirations, may 
be exhibited without the use of the electric light, which for 
various reasons is frequently not available. The methods 
employed depend upon the use of the limelight and the oxy- 
hydrogen flame : these of course do not give the same brilliant 
effects as the electric light, but all the phenomena may be 
shown upon the screen with much simpler apparatus ; and for 
some of my methods | think I may claim advantages over the 
usual method of demonstration. 
i. Spectra of the Alkalis and Alkaline Earths. 
Debray (Ann. Ch. Phys. [3] lxv. p. 831) proposed the use 
of the oxy-hydrogen flame for the volatilization of the alkalis 
and alkaline earths. By a modification of the ingenious method 
of Bunsen for procuring a monochromatic flame, the sub- 
* Communicated by the Physical Society : read March 14, 1885, 
Piul. Mag. 8. 5. Vol. 19. No. 120. May 1885.  2C 
