Caseine, the animal principle of milk, is coagulated by Nitrate of Silver, and the red substance formed on exposing the curds to light may be viewed as analogous in composition to the corresponding compounds with Albumen and Gelatine. Many other organic salts of Silver are darkened by light. The white Citrate of Protoxide of Silver changes to a red substance, reacting with chemical tests in the same manner as Wöhler's Citrate of Suboxide of Silver, which he obtained by reducing the ordinary Citrate in Hydrogen Gas. Glycyrrhizin, the Sugar of Liquorice, also forms a white compound with Oxide of Silver which becomes brown or red in the sun's rays.[6] [6] For further particulars on the action of light upon the Salts of Silver associated with organic matter, see the Author's paper on the composition of the photographic image, in the eighth Chapter. SIMPLE EXPERIMENTS ILLUSTRATING THE ACTION OF LIGHT UPON A SENSITIVE LAYER OF CHLORIDE OF SILVER ON PAPER. In the performance of the most simple experiments on the decomposition of Silver Salts by Light, the student may employ ordinary test-tubes, in which small quantities of the two liquids required for the double decomposition may be mixed together. When however concentrated solutions are used in this way, the insoluble Silver Salt falls in dense and clotted masses, which, exposed to the sun's rays, quickly blacken on the exterior, but the inside is protected, and remains white. It is of importance therefore in Photography that the sensitive material should exist in the form of _a surface_, in order that the various particles of which it is composed may each one individually be brought into relation with the disturbing force. Full directions for the preparation of sensitive Photographic paper are given in the second division of this work. The following is the theory of the process:--A sheet of paper is treated with solution of Chloride of Sodium or Ammonium, and subsequently with Nitrate of Silver; hence results a formation of Chloride of Silver in a fine state of division, with an excess of Nitrate of Silver, the Silver bath having been purposely made stronger in proportion than the salting solution. _Illustrative Experiment No. I._--Place a square of sensitive paper (prepared according to the directions given in the Second Part of the work) in the direct rays of the sun, and observe the gradual process of darkening which takes place; the surface passes through a variety of changes in colour until it becomes of a deep chocolate-brown. If the Light is tolerably intense, the brown shades are probably reached in from three to five minutes; but the sensibility of the paper, and also the nature of the tints, will vary much with the character of the organic matter present.