OF PHOTOGEAPHIC TEANSPAEENCY. 
863 
absorptive effect upon the chemical rays, preserve their absorptive power with greater or 
less intensity both in the liquid and the gaseous state (21). 
Whether the compound be dissolved in water or be liquefied by heat, these conclu” 
sions are equally true as regards liquids. Water is perfectly permeable to the chemical 
rays ; and this circumstance, conjoined with the fact that in no instance does the process 
of solution seem to interfere with the special action of the substance dissolved upon the 
incident rays, renders it practicable to submit to trial a great number of bodies which 
it would otherwise be impossible to subject to experiments of this nature, owing to the 
extreme difficulty of obtaining them in crystals of sufficient size and limpidity. 
§ 1. ABSOEPTION OF THE CHEMICAL EATS, 
a. By transmission through Solids. 
6. The general arrangement of the apparatus employed in this inquiry is represented 
in fig. 1, in which the observer is supposed to be looking down upon the instrument, 
c, c is a camera which allows of a considerable range of adjustment, and is attached to 
a cylindrical box, within which is a prism 5, of rock-crystal. At Z is a quartz lens of 
1-j-inch aperture, and VI \ inches focal length. At one end of the tube t, which can be 
lengthened or shortened by a sliding joint, is a slits, provided with a screw for regulating 
the width of the opening. This slit is arranged parallel to the axis of the prism, and in 
these experiments was adjusted to a distance of 37 inches from the lens 1. The prism is 
placed at about its angle of minimum deviation for the mean ray, and, for facility of 
manipulation, can be turned round upon its own axis by means of the lever a. The angle 
formed between the camera and the tube t also admits of variation as circumstances may 
require. At d is placed the substance the transparency of which is to be tested ; and 
at e are the metallic electrodes, which are connected with secondary wires /y of a 10-inch 
induction-coil, not shown in the figure. The wires of the coil terminate in electrodes 
composed of fine silver. The coil was excited by means of a battery consisting of five 
6 B 2 
