OF THE VAPOURS OF BENZENE AND ITS HOMOLOGUES. 
481 
At t 
A strong band. 
A faint band. 
A faint band. 
A faint band. 
43° C. 
/2669 
/2660 
/2629 
2625 
| 2672 
| 2663 
| 2631 
— 
At t 
Four strong and well-defined bands. 
53 °C. 
/ 2669 
/2661 
2631 
72625 
| 2672 
| 2663 
— 
| 2626 
There are likewise four feeble bands between wave-lengths 2260 and 2300, of which 
only one or two are visible at temperatures below 53° C. 
The spectrum at 11°;5 C. and 21 mm, pressure is comparable with that at 12°'7 G 
and 759‘5 mm. bar. As the pressures increase, the temperature being constantly 
the same, gradual changes occur, as are seen, at constant pressure and increased 
temperatures up to 53° C. (Figs. 3 and 4, from 591 mm. to 88 mm., and from 
67'5 mm. to 28‘5 mm. pressure.) 
Grebe observed a regularity in the differences between the wave-length of the 
bands, and he arranged them in twelve groups, seven of which are complete, and 
similarly constituted in that they contain in each group six bands. Of the five other 
groups, one contains five bands, and the others four in each group. An arrangement 
similar to that of Grebe fails to show such a relationship between the more numerous 
bands which I photographed ; nevertheless, the very appearance of the photographs 
affords evidence of great regularity in their arrangement in similarly constituted 
groups of bands, which are separated by similar intervals. The reason of this is 
quite intelligible when the changes in each band and in the constitution of the groups 
brought about by changes of temperature are closely examined. The first group lies, 
for instance, between XX 2589 and 2650 ; then there is an isolated band—or possibly it 
is a feeble narrow group—about A. 2670. The next group lies between XX 2528 and 
2560, the third between XX 2469 and 2500, the fourth between XX 2416 and 2430, 
the fifth between XX 2365 and 2380, or thereabouts, but the termination is difficult to 
see, because of the general absorption which renders the last band less distinct. 
These groups, which were measured at 12° - 7 G, are seen to be composed principally 
of a very strong band and one narrow and less strong, which order is repeated 
generally four times in the same group, then a weak band is seen lying close to the 
strongest, but on the more refrangible side. (See fig. 1, about X 2540, at temperatures 
about 12°'7 G to 53° C.) The bands diminish in intensity as the refrangibility 
VOL. CCVIII.—A. 3 Q 
