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MESSRS. W. N. HARTLEY AND A. K. HUNTINGTON ON THE ACTION 
able to trace any special connexion between the chemical complexity of a substance 
and its diactinic power.” (‘Journal of the Chemical Society,’ vol. ii., p. 68.) Thus 
water was found to be perfectly diactinic, and etliylic alcohol more than methylic 
alcohol, though less so than water. Since all the physical properties of organic sub¬ 
stances are dependent on their molecular constitution, it is very probable that the 
photographic transparency of a substance is a delicate test of its purity, and that not¬ 
withstanding the care taken in the purification of specimens, yet some of the experi¬ 
ments of Dr. Miller were made with impure substances. In the face of this difficulty 
it was thought that by observing a large number of bodies of similar constitution, 
many of which would be metameric substances, such as the ethereal salts of the organic 
acids, and homologous series of the normal alcohols and acids, evidence might be 
forthcoming of the influence of impurities, and the variations in the absorption of the 
invisible rays caused by each increment of CH 3 in the molecule. As according to 
the researches of Miller the physical state of a substance does not generally affect 
its absorptive power, we may avoid the difficulty of making observations on equal 
volumes of organic substances in a state of vapour, and easily arrive at the maximum 
absorption due to a molecule of a substance, by taking into account its specific volume 
in the liquid state, and by making the thickness of the layer of liquid experimented 
on proportional to its specific volume, or by dissolving molecular weights of substances 
in solvents of known transparency. 
The Method of Experimenting. 
After considerable time spent in giving a trial to the various methods of studying 
the ultra-violet rays, the photographic method appeared to be decidedly the most 
satisfactory. 
Bays which are very indistinct, or even quite invisible on any fluorescent screen, 
may be brought out distinctly on a properly prepared photographic plate. It is useful, 
however, to have a focussing screen of uranium glass in the camera, or what answers 
the purpose equally well, a glass plate coated with gelatine in the solution of which 
some sesculin has been dissolved. To observe the spectrum by reflected light, and 
view the visible and invisible rays simultaneously, a piece of paper steeped in a 
solution of sesculin, to which a little ammonia has been added, may be employed, and 
it answers the purpose even when dry. 
The instruments employed were the following :—1st. An induction coil and Leyden 
jar for producing an unbroken stream of sparks between metal points. 2nd. A 
collimator tube 3 feet long, carrying a pair of clips at one end for holding the points 
close to the slit, and a quartz lens at the other. 3rd. A quartz prism placed at the 
angle of minimum deviation for the sodium line D. 4th. Another quartz lens and the 
body of a photographic camera which could be extended to 36 inches. The separate 
portions of this lengthy apparatus were firmly screwed down to two heavy tables to 
prevent the shifting of any part, since the proper adjustment of the whole takes some 
