Oil Molecular Attraction. 901 



Previous work on C0 2 is as follows. Julius finds bands at 

 2'S and 4*4. Rubens and Aschkinass find, in addition, a 

 weak at 14*1. Water vapour has a band at 5*4. The small 

 maximum which we find at 2*84 is undoubtedly a first order 

 maximum on the concentration side which Julius located 

 at 2*8. The two large maxima at 4*41 and 4*51 are first 

 order spectra of the Julius band, found at 4*4, which he failed 

 to resolve but which was distinctly resolved in the present 

 case. The small band at 5*37 is probably first order for the 

 water-vapour band which Rubens and Aschkinass found, 

 while the other small band may be a first order band at 5*77 

 (due to ?), or more probably a second order of the band 

 at 2*84. If this be so, its wave-length in second order figures 

 ought to be 2*89. 



No very great effort was made to get the highest resolution 

 possible, and the results given are to be regarded rather as a 

 by-product of the other investigation. 



It is interesting to compare the dispersion of the gratings 

 with that of rock-salt and fluorite prisms : — 



In the interval 4/jl to 5/jl in the spectrum furnished by a 

 rock-salt prism, the difference in angles of minimum deviation 

 is 0° 15' of arc. This means 7'*5 of arc change in the setting 

 of the "Wadsworth prism-mirror combination. In the grating- 

 spectrum grating of No. 8, 13 / of arc corresponds to O'l^u, or 

 lfjb would correspond to about 130 / of arc. Our dispersion 

 therefore near the C0 2 band is nearly 17 times that of a 60° 

 rock-salt prism in the same region. This region is about the 

 worst part of the rock-salt spectrum, on account of the 

 flatness of the dispersion curve at this point. The dispersion 

 of a 60° fluorite prism is apparently 4 times that of a salt 

 prism between 4/4 and 5/z; therefore our dispersion is about 

 4 times that of fluorite in this region. Near the quartz bands 

 with grating No. 5 we have about 150' of arc to 1//,, while with 

 a 60° rock-salt prism there are 27' of arc to Ifju. Here then 

 we have 5*4 times the dispersion of a standard 60° salt prism. 



Further investigations with the gratings will be made in 

 the near future. 



C. On Molecular Attraction. 

 To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



IN a paper published in the October number of the 

 Philosophical Magazine, Mr. Mills gives a resume of 

 some of his work on molecular attraction. May I be per- 

 mitted to call attention to some points in connexion with the 

 subject. 



