Page:Popular Science Monthly Volume 9.djvu/794

766 corner will dip into water soon becomes saturated. This is capillary attraction, and has a place in root-absorption. From an extended study of the properties of liquids, the law of diffusion has been established, viz., that when two or more miscible liquids of different densities are placed in contact, interchange will take place till the whole liquid is homogeneous. This property of liquids will account for the movement of the absorbed sap to any part of the same cell—from the tip of the hair to its base. But there is another kind of diffusion—osmose, or membrane diffusion. When liquids of different densities are separated by a thin membrane, diffusion takes place through this partition with a rapidity depending on the nature of the liquids and membrane, the greater flow being toward the denser fluid. The cell-wall of a root-hair is such a membrane, separating the denser liquid within the cell from the thinner one without; and, as this membrane is a living, growing one, it may be specially effective for osmotic action. From the function, position, and delicate structure of the root-hairs, at least one important practical conclusion can be drawn, viz., the importance of preserving them when a plant is to be potted or transplanted.

The Philosophy of Dreams.—Prof. Ferrier recently delivered, at the London Institution, a lecture on "Dreaming," explaining its phenomena by the results of his famous experiments on the localization of faculties in the brain. For each class of impressions there are, he said, special regions of consciousness in the brain. The impressions received are photographed on the brain, and are capable of being revived. But for this power of recalling them no knowledge would be possible. Memory, or the registration of sense-impressions, is the ultimate basis of all our mental furniture. Bach piece of that furniture has its function, like the litters in a compositor's case. We have a sight-memory, a hearing-memory, etc. When thinking, or engaged in ideation, we ire but recalling, as shown by Herbert Spencer and Bain, our original sensations ami acts of cognition. Commonly the reproduction is very faint, but in some instances it is nearly or quite as vivid as the original sensation. This is especially true of poets, painters, religious enthusiasts, and others. Those portions of the brain which are most continuously in action during waking-hours require the longest rest during the hours of sleep. Hence the centres of attention would sleep while the functions allied to reflex actions would more easily waken.

The brain in sleep Prof. Ferrier compared to a calm pool, in which a stone causes ripples, liable to interruption by other ripples similarly caused. So the ripples of ideation get confused. But, again, the circle on the pool may not be interrupted, and then the ideation will be regular. The current of ideation may be coherent or incoherent. The most vivid association, which is commonly the latest, dominates over the rest. Dr. Reid, the metaphysician, once dreamed of being scalped—there was a blister upon his head. Dr. Gregory, from having a bottle of hot water at his feet, dreamed of walking up the crater of Etna. Visceral conditions are the most frequent sources of dreams; the hungry dream of feasts, the thirsty of water, the dropsical of drowning. Dr. Ferrier happily compares incoherent dreaming to the changes in a kaleidoscope. There is nothing new in dreams; the blind do not dream that they see, nor the deaf of music. In such cases there is a letter missing from the font of type. Our fancy is awake during dreams, and the faculties which should check it are asleep. Hence it is that nothing surprises us in dreaming.

Locusts in Africa.—In his work, "The Victoria Falls of the Zambezi," Eduard Mohr gives an impressive description of a flight of locusts witnessed by him in the region of the Vaal River. "I noticed," he writes, "on the western horizon what I took to be columns of smoke, rising higher and higher until they reached the zenith. I thought the bush must have been set on fire, for the whole of the horizon from the northwest to the southeast was already apparently enveloped in clouds of smoke. This, however, was caused by no fire, but by locusts. Presently a few, then dozens, then hundreds, then thousands, of locusts fell upon us, coming down in such heavy