clear, as the little pimpernel, the index of fair weather; others, as the sunflower, turn to the sun in his course from east to west; the eyes, or germinating stalk of the potato, though they shoot vigorously in the dark, are blanched; so are the covered leaves of endive, and the covered stalks of celery and sea-kale. Light is a chemical agent; light, as the apothecary well knows, greatly impairs the activity of the dried and powdered leaves of foxglove, and blanches castor oil. Light, therefore, is found to effect both chemical combination and chemical decomposition. In nothing, perhaps, is the chemical agency of light more clearly shown than in the photogenic drawings, which are the fac-simile representations of objects delineated by the action of light on a thin sensitive layer of ioduret of silver. The following is the mode of preparing the plates for the reception of photogenic drawings, given in the Penny Cyclopædia:--These drawings "are produced on plates of copper, coated over with silver, which are found to answer better than such as are entirely of the last-mentioned metal. After being washed with a solution of nitric acid, the plate is put into a well-closed box, where it is exposed to the action of iodine, a small quantity of the latter being placed at the bottom of the box, with a thin gauze between it and the plate. A layer of ioduret of silver is thus formed on the surface of the plate, and manifests itself by the yellow hue produced on the silver, which shows that the process of giving the plate the sensitive coating on which the action of the light delineates objects is completed. Thus prepared, the plate is placed within a camera obscura, of particular construction, and the delineation of the object is then effected in a very short space of time; but has to be afterwards brought out and rendered distinct by another operation, namely, submitting the plate to the action of vapour of mercury. Even then the process is not completed, for the plate has to be plunged into a solution of hyposulphate of soda, and afterwards washed in distilled water, which being done, the impression is fixed, and the plate may be exposed to light with perfect safety."[4] With reflection, refraction, and polarization of light, we have here no concern; therefore, interesting as the subjects are, we must pass them by. [4] For further details, see the Handbook of Heliography, London, 1840; and (for preparing-paper) the Visitor, 1839, p. 290.