24 General Biology 



Nearly every parent desirous of his child's welfare, wishes he could 

 leave the child the benefit of his own experience, so that the child might 

 profit by his parent's mistakes and not make the same blunders. The 

 value of this desire may be appreciated when it is remembered that if 

 experience cannot be handed down, there is absolutely no progress. For, 

 each individual, instead of starting where his parents left off and con- 

 tinuing onward, would necessarily have to begin where they began, and, 

 consequently, when life came to a close, the children would be prac- 

 tically where their parents had been at the close of their careers. 



Men of the past have, therefore, written their experience in books, 

 and we of to-day can profit not only by the experience of our immediate 

 parents but by that of our forefathers. 



The laboratory has gone even a little further than this. 



As no one man can work out every detail in the study of a single 

 plant or animal without having to take the work of all those into con- 

 sideration who have gone before and who have contributed something 

 to the knowledge of the particular plant or animal under discussion, so, 

 men have gathered into a single grouping the important physical experi- 

 ences which have been found convincing to their minds and have called 

 such grouping a textbook. 



The study of the subject-matter of a textbook, plus the actual work- 

 ing out of these same convincing experiences (now called experiments) 

 in the laboratory, cause the student to see the way in which proof is 

 obtained for the conclusions men hold. 



In fact, laboratory work, plus a study of the text, is the fulfillment 

 of the parent's wish that his child inherit the parent's experience. 



From the experiments which give us conclusive evidence of the 

 way some physical process works, we draw our principles. 



Principles are mental tools without which no mental progress is 

 possible. 



In fact, a principle is a law of nature, proved by physical experi- 

 ment, to which no exception has been found. Physics presents an excel- 

 lent illustration of the value of principles. 1 



Everyone knows that, if a substance, such as iron, which is heavier 

 than water, is placed in water, it will sink. Yet iron ships do not sink. 

 Why? Because when we speak of anything as "heavier than water" 

 we mean that the same quantity of a given substance is heavier than the 

 same quantity of water. 



After Archimedes discovered his principle, we knew immediately 

 that, if we could bend iron so that it would occupy more cubic feet of 

 space than that same number of cubic feet of water would weigh, it 

 would be "lighter than water" and would float. 



Heavy iron ships could not, however, be of practical use until some 

 one again discovered the principles of steam or electricity, and so they 



x An old Greek named Archimedes, while taking a bath, discovered that when he immersed his 

 body in a tub filled with water, his body lost considerable weight. Later he was able to prove 

 experimentally that the weight of the water that ran over the top of the tub was exactly the same 

 as the weight his body lost while immersed. It was this discovery which made iron ships possible. 



