640 THE POPULAR SCIENCE MONTHLY. 



This new material is absorbed by the cells from the blood, through 

 the thin walls of the minute blood-vessels in their vicinity. Through 

 the walls of the same vessels the cast-off matters pass in the opposite 

 direction into the circulation and are washed away by its current. 

 While the tissue is hard at work, the process of disintegration is at 

 a maximum, and that of repair at the opposite extreme — consequently 

 the waste is produced more rapidly than it can be carried away, and 

 accumulates. As ashes in a stove interfere with combustion, it im- 

 pedes the current of thought, and lessens its intensity. But, during 

 repose, the opposite conditions obtain — repair is at its maximum, and 

 waste almost or entirely suspended. The blood has been busy all 

 night ridding itself and the tissues of all impurities, and is richly 

 charged with oxygen. The brain, and consequently the mind, is fresh 

 and vigorous after the night's repose ; the damages have been all 

 repaired, and the d'ehris cleared away. It is a matter even of common 

 observation that at no other time is the mind so sharp, clear, and 

 strong, as in the morning. 



Concrete ideas tax the mind but lightly. The more abstract ideas 

 become, the more difficult is their comprehension, and the greater the 

 nervous strain involved in their contemplation. For this reason, the 

 abstruse studies should be taken up during the forenoon session, as the 

 faculties of the mind are then in the most favorable condition to grap- 

 ple with their difficulties. 



Of all school-studies, mathematics requires the strongest grasp of 

 mind, and the closest exercise of the reasoning powers and the judg- 

 ment. In abstruseness and difficulty of comprehension, geometry, 

 algebra, and arithmetic, rank in the order enumerated. Rhetoric, in- 

 cluding grammar and composition, comes next. In every school and 

 college, therefore, these subjects should be taken up during the morn- 

 ing session. 



The mind learns by means of impressions made upon the gray 

 nerve-cells, through the senses, of which sight is the most vivid and 

 durable in its effects. Hearing ranks next, but its impressions are less 

 vivid and more fleeting. Further, they are recalled to the memory 

 less readily and distinctly. We all remember what we see longer than 

 what we hear. Hence the most reliance should be placed upon the 

 eye as an avenue of instruction, and the teacher should make use of it 

 whenever practicable. When an impression is made upon a nerve-cell, 

 it is said to retain it " in potency " — that is, it is capable of renewing 

 it by an exercise of the memory. Now, the clearness and permanence 

 of a mental impression depend — («) upon its vividness ; (h) upon 

 the frequency of its repetition ; and (c) upon the inherent vigor of 

 the nerve-cell. 



To obtain vividness of impression, the teacher's language should be 



clear and simple ; his descriptions of processes and objects sharp and 



• vivid ; he must present the same ideas again and again, in order to fix 



