488 
DR. WALTER NOEL HARTLEY ON THE ABSORPTION SPECTRA 
lengths 2700 and 2250. At 100° C. the positions of the different groups are still well 
defined, but the individual bands of the groups cannot be clearly distinguished, by 
reason of a general absorption overlying that portion of the photograph where the 
bands are seen to be the strongest at the lower temperature of 12°'7 C. 
In addition to this change, a very strong and well-defined group of bands comes out 
clearly at the less refrangible end below wave-length 2575. These are seen in fig. 4 
at 67'5 mm. and 22 ‘5 mm. pressure and 100° C. Only the strongest band of this 
group appears at 12 0- 7 C. The principal differences observed between the benzene 
spectra at low and high temperatures with varying pressures may be summed up as 
follows :—At 100° C. the same quantity of benzene vapour produces greatly increased 
general absorption, extending over that part of the spectrum in which the groups of 
bands occur. At 100° C. the characteristic absorption is greatly increased at the less 
refrangible end of the spectrum, giving rise to a strong and well-defined series of 
bands, of which previously there had been only rudimentary indications. At 100° C. 
the bands have a peculiar sharpness, particularly those of the series less refrangible 
than wave-lenuth 2575. 
o 
Benzene. (Fig. 1.) 
The numbers of bands observed in the spectra at 759'5 mm. bar. and different 
temperatures, in a glass tube :— 
° C. 
At 12-7 
„ 25-0 
„ 43-0 
„ 53-0 
Bands. 
55 
84 
82 
56 
| The increased general absorption decreases the number of bands. 
The numbers of bands observed at 1 L 0, 5 C. 
tube :— 
and different pressures, in a brass 
mm. Bands. 
At 778 36 
„ 483 38 
,, 253 46 
„ 21 44 
The numbers of bands 
pressures :— 
Photographic 
plate I., 
No. 13. 
mm. 
Bands. 
At 683 
9 
„ 589 
13 
„ 478 
14 
„ 381 
16 
„ 279 
17 
„ 172 
18 
2 
•) 
observed in the spectra at 100° G. and at different 
(Not reproduced.') 
A general absorption extinguishes many of the bands. There is a 
r gradual increase, principally at the less refrangible end, in the 
number of bands on reduction of pressure. 
