OF ORGANIC SUBSTANCES ON THE RAYS OF THE SPECTRUM. 
271 
benzoic and phthalic acids. This strengthens the view already expressed, that the 
introduction of the carboxyl group into a molecule increases the actinic absorption, 
phthalic acid standing to benzoic as oxalic to acetic acid, while benzoic acid and 
phenol are related in the same manner as acetic acid and alcohol. 
Hippuric Acid, C 9 H 9 N0 3 , (Diagram No. 15).—This being a benzamido-acetic acid 
it was interesting to ascertain whether the benzoic acid residue was traceable; its 
peculiarities, however, do not appear. Solutions examined contained -^o, ToW, 5 ~ o oo> 
10 ooo , and 40 Foo °f their weight of the substance. 
Aniline, C 6 H 5 .NH 3 , BP. 182° (Diagram No. 16).—Solutions of aniline in alcohol 
were made which were afterwards diluted with water. Solutions containing the fol¬ 
lowing proportions by volume of aniline were examined : xwoo; Too oo ~> TT 00 IO TTooo> 
2oooo5 Xoooo> Tooooo. TT 0 V 00 ) 1 2 o\) o 0 ) T 80 V 005 and tycmToo'- fr first absorption band 
lies immediately below 17 Cd; it first makes a definite appearance when the rays 
just beyond have been transmitted after a dilution with 12,000 parts of water. The 
absorption is not destroyed by carrying the dilution 3 - 0000 - No rays are transmitted 
further than midway between 18 Cd and 23 Cd until a dilution of TTmToo is attained, 
and all absorption of the more refrangible rays is not effected until after dilution with 
220,000 parts of water. 
One of the chief objects in view in commencing a study of the benzene derivatives 
was to ascertain the difference in the absorption spectra of strictly isomeric substances. 
We have accordingly examined the three oxybenzoic acids, two nitro-plienols, and two 
nitranilines. 
Salicylic Acid, CyH^.OH.COOH, or P2 Oxybenzoic Acid (Diagram No. 17).—This 
substance, prepared some years ago by one of us from the oil of winter-green, was 
in the form of large crystals. We recrystallised it from alcohol, and took the choicest 
crystals for preparing our solutions. Solutions were made containing ^ 
__1_ 1 1 __ _1__1__ _1_ _1_ _i_ 
10? 10005 1 7 005 20005 3000> 3200) 4000) 4400) 450 
r 5^0 05 and 2 -oti oo °f their weight of the substance. 
the spectrum until after a dilution of yoVo 5 from this point, however, until - 1 
_ _i__ 1 
00) 300) 400) 
1 _1_ 1 1 __ _1__1___1__1_ _1__ 1_ _ 1__1__ 1__ 1 
800? 1000) 1700) 2000) 3000) 3200? 4000) 4400) 4500) 4700) 5000? 6000) 8000) 10000) 
There is nothing remarkable in 
5 0 0 0 
is 
reached,' the curves obtained are very curious. 
Oxybenzoic Acid, P3 (Diagram No. 18).—This compound is remarkable for its 
adiactinic quality ; it transmits no rays beyond 12 Cd until after a dilution of anc ^ 
there is still a distinct absorption between 12 Cd and 17 Cd until -200005 when even 
the rays beyond 18 Cd are only feebly transmitted. 
Paroxybenzoic Acid, 1'4 (Diagram No. 19).—This acid shows an increase in adiac¬ 
tinic quality exceeding that of the preceding substances. Solutions containing 
13000? 
1 X 40 oo) 2 0 5000 ) aiJ fr TroVoo their weight ol 
26000) 39000) 5 2000) 7 7000) 103000) 
the substance were examined, and the diagrams constructed from these two other sets 
of observations were, however, taken with various proportions of solid to solution ; 
they gave, however, the same result. No rays beyond 17 Cd are transmitted until 
after a dilution of 
3 0 0 0 0 
has been reached. There is a broad absorption band between 
