ABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. Sak 
(4) Primary and secondary aromatic bases or amido-derivatives and 
salts thereof : ortho- and para-toluidine and their hydrochlorides. 
(5) Isomeric bodies : the three xylenes. 
The Preparation of Solutions and Method of Examination.—In dealing 
with a variety of nearly related substances from which similar solutions 
have to be prepared, it is necessary that the solution of the least soluble 
substance (largest solution) shall, as far as possible, serve as a standard 
for the preparation of the other solutions. It was found most convenient 
to take a molecular weight in milligrams and dissolve it in 20 c.c. of 
absolute alcohol or any other menstruum better suited to the particular 
substance. In this way molecular weights were distributed through—that 
is to say, made to occupy equal volumes. The solutions, instead of being 
repeatedly diluted and examined in cells of the same thickness, were 
placed in a series of cells varying in thickness from 25 to 1 mm. ; if with 
a thickness of 1 mm, absorption bands were still visible, the liquid was 
diluted to five times its original volume, and another series of photographs 
taken ranging from 5 mm. downwards. 
The wave-lengths in tenth-metres have been converted into reciprocal 
numbers, which have the advantage of representing oscillation frequencies 
per unit of time. 
When a series of photographs had been secured which gave sufficient 
information from which a curve could be drawn indicating both the 
general and the selective absorption, the oscillation frequencies of the 
absorbed rays were taken as abscissz and the proportional thicknesses in 
mm. of the weakest solution as ordinates. The curves are made con- 
tinuous, and a careful description of the spectra obtained by transmitting 
rays through varying thicknesses of the solutions is intended to supple- 
ment the curves and serve the purpose of the shaded diagrams which had 
hitherto been employed.! 
It will be convenient here to introduce some recent measurements of 
the bands in the spectrum of benzene. 
(1) Aromatic hydrocarbons.—The absorption spectrum of benzene shows 
six absorption bands, while that of naphthalene shows four. 
(2) Aromatic tertiary bases and their salis—In the case of the 
bases and their hydrochlorides, the method of examination consisted 
in photographing the spectra, transmitted by the base contained in 
one series, and the molecule of hydrochloric acid contained in another, 
the rays from the spark passing through both. The contents of the 
two series of cells being then mixed and returned to their original 
vessels, a second series of photographs was taken. As the hydro- 
chloric acid proved perfectly diactinic, the first spectra represent the 
absorption caused by the base alone, the second that caused by the salt. 
The difference in the mode of vibration of the base, the acid, and the 
salt, is very striking ; the amplitude of the vibrations within the molecule 
of the salt being much less, as one would imagine, than in that of the 
base. The absorption spectra of pyridine and its hydrochloride, of dipyri- 
dine, and picoline, show one absorption band each. Two specimens of 
quinoline were examined ; one prepared from coal-tar, and the other 
synthetically by Skraup’s reaction. The absorption spectra of the two 
specimens were identical, and showed three absorption bands. 
» Trans. Chem. Soc. vol, xlyii, p, 687, 1885. 
