7 66 



THE POPULAR SCIENCE MONTHLY. 



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 with- 

 out ; 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 con- 

 clusion can be drawn, viz., the importance 

 of preserving them when a plant is to be 

 potted or transplanted. 



The Philosophy of Dreams. Prof. Fer- 

 rier recently delivered, at the London In- 

 stitution, a lecture on "Dreaming," explain- 

 ing 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 re- 

 gions 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 re- 

 production is very faint, but in some in- 

 stances 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 dur- 

 ing 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 com- 

 pared to a calm pool, in which a stone 

 causes ripples, liable to interruption by 

 other ripples similarly caused. So the rip- 

 ples of ideation get confused. But, again, 

 the circle on the pool may not be interrupt- 

 ed, 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 ap- 

 parently 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 



