Page:Popular Science Monthly Volume 1.djvu/391

Rh curious and interesting article by a German oculist, showing how the colors of paintings are affected by changes in the eye of the painter. M. Pouchet matches this discovery with the curious fact that the lower animals change their own colors through the action of the eye. In a recent work on change of color in Crustacea, this author shows that in fishes, especially the turbot, notable changes of color take place, due to the quantity of luminous rays which fall upon the eyes of these animals. The eye is the point of departure for a nervous shock which is transmitted to the skin, and which finally results in a change, more or less complete, of the color of the animal. This shock, starting from the eyes, is transmitted by nerves to the skin; so that, if certain of these nerves be severed, the animal will become striped with clearly-defined bands, the shock being intercepted by the severed nerves, and transmitted by the others. If the animal is deprived of sight, it ceases to change color, and preserves the same tint, whether it be placed on a black or on a white ground. In the course of his researches at Concarneau, M. Pouchet confirmed these results in the case of Crustacea.

The skin of the frog becomes clearer when the cutaneous nerves are severed. Prof. Goltz, of Halle, shows that primarily the action of the nerves affects the vessels, and that the change in the pigment-cells is a secondary result, and due to a modification of the circulation. In the active state, these cells are star-shaped, or branching; in repose, they are round. Having removed the spinal cord and brain, if then you sever the nerves leading out to one portion of the skin, that portion will become clearer, the pigment-cells then first assuming the condition of repose. Dr. Witlich thinks this change of color is owing to the decay of the color-bearing cellules, and says it would take place equally in shreds of skin separated from the body. Dr. Mendel, of Berlin, has observed one-sided pigmentation frequently in the insane.

Observations on the Hydrogen-Flame.—Besides the phenomena common to ordinary burning, such as luminosity, the disappearance of oxygen, and of the substance burned, the production of water and carbonic acid, or some other of the various oxides, there are also certain other phenomena that differ with different substances, and that, when known, often become an important means of distinguishing these substances. Thus, burning sodium gives a yellow flame; burning potassium, a flame of a purple color; burning arsenic, a strong odor of garlic; burning sulphur, suffocating fumes of sulphurous acid, and so on. Burning hydrogen also has its peculiar phenomena, which have lately been made the subject of further investigation by Mr. W. F. Barrett, who contributes an interesting account of his researches to a recent number of Nature.

To study these phenomena to the best advantage, three things, he says, are requisite: 1. That the gas be purified and stored in the ordinary way; 2. That it be led through red or black India-rubber tubing to a platinum, or, better, a steatite jet; 3. That it be burnt in a perfectly dark room, and amid calm and dustless air. In this way, the flame gives a faint, reddish-brown color, invisible in bright daylight. Issuing from a narrow jet in a dark room, a stream of luminosity more than six times the length of the flame is seen to stretch upward from the burning hydrogen. This weird appearance is probably caused by the swifter flow of the particles of gas in the centre of the tube. The central particles, as they shoot upward, are protected awhile by their neighbors; metaphorically, they are hindered from entering the fiery ordeal which dooms them finally to a watery grave.

Brought in contact with certain solid bodies, the flame gives rise to phosphorescent effects. Thus, allowing it to play for a moment on sand-paper, and then promptly extinguishing the gas, a vivid-green phosphorescence remains for some seconds. A similar appearance follows when the flame is directed upon white writing-paper, marble, chalk, granite, or gypsum.

A much more general effect than the one last mentioned is, the production of a magnificent blue image of the flame, that starts up on almost every substance with which the flame is brought into contact. When directed either vertically or sideways, say upon a white plate, or block of marble, there instantly appears a deep-blue and