us cautious how we suddenly change our condition, either of motion or rest. I will show by experiment the inertia of a brass ball. I place a stiff card on a pillar, and then, by means of a spring, I drive the card from under the ball which was resting upon it. The ball will be found on the pillar, from which the card has been removed. Whatever tends to change the state of matter, whether of rest or motion, is called force. There is also a property of matter called figure. Can you tell me what is meant by it?

Fig. 1.

5. Frank. I understand it to mean form, as all matter must have some form or shape.

Mr. M. True; and many bodies have forms peculiar to themselves, such as crystals. You perhaps recollect having seen crystals of common salt; and they are always cubical, or, as you might call them, square blocks. Liquids have no peculiar form, but assume1 that of the vessel containing them. Attraction is another general property of matter. Sometimes weight, which is one kind of attraction, is called a distinct property; but we refer it to its proper place. Who will define attraction?

6. Ida. It is that quality in the particles of bodies which makes them tend toward each other.

Mr. M. I would prefer the words atoms and masses to particles, as they will include all kinds of attraction, from that called chemical to that called gravitation or weight. Attraction has received different names from the different circumstances under which it manifests itself. Attraction between atoms is called chemical affinity; between molecules, it is termed cohesion; and between masses, gravitation. There is also a modification of attraction called adhesion, and another known as capillarity. Besides, there is electrical attraction, which will be hereafter explained. George, can you tell me why this pencil falls to the floor?

.4

7. George. The pencil is a mass and the floor is a mass; and the pencil must fall toward the greater mass by the force of gravitation.

N

Mr. M. Very well; but you must remember that there are two whys, or two causes-a primary and a secondary, or an intelligent and a physical cause. The secondary or physical cause is gravitation, or a property to which we give this name; but the intelligent cause is nothing less than the "volitions of Deity." Besides these there are specific properties of matter, as I mentioned in our last conversation. Can any one of you tell what they are?

8

8. Ella. I have learned from the books in the library that there are such properties as hardness, elasticity, flexibility," brittleness, malleability, ductility, 10 and tenacity;11 but I do not exactly understand why they are called specific properties. 9. Mr. M. If you will reflect a moment that specific means that which distinguishes one kind from another, I think you will perceive that the term is a very appropriate one; for those properties which you mentioned do not belong in the same degree to all kinds of matter. Inertia belongs as much to one kind of matter as to another, and it is therefore a general property of matter; but does hardness belong, in the same degree, to all kinds of matter?

10. Ella. Oh no. I see the difference now. Some bodies are much harder than others, and therefore hardness is a specific property of matter; but inertia is a general property, belonging alike to all.

Mr. M. This illustrates the importance of understanding the precise meaning of terms. Will some one tell me the

name of the hardest body known?

Frank. The diamond.

Mr. M. What bodies are elastic?

Ella. India-rubber, and steel springs.

11. Mr. M. And all other bodies to a certain extent, though some possess so little elasticity that they are called non-elastic. Air is perhaps the most perfectly elastic substance; but we have been using a term before defining it. What do you understand by elasticity?

12. Ella. The property by which bodies, when their form has been changed, endeavor to recover their original shape. Mr. M. What is the difference between flexibility and brit tleness?

George. When a body readily yields or bends under a force applied to it, it is said to be flexible; but if, instead of bending, it breaks, it is brittle.

13. John. I have seen knives and other tools very brittle when new, but very flexible when they had been heated, and have heard people say they had lost their temper.

Mr. M. Yes; when a piece of steel is heated, and then slowly cooled, it is flexible; but if cooled suddenly, it is brittle. You can try this experiment by heating and cooling a needle.

14. John. I recollect my father had a quantity of iron wire which he could not use because it was so brittle, and that he heated it red-hot, and cooled it slowly. I think he called it annealing.12

15. Mr. M. That is the name of the process. Glass can be annealed; and the value of glassware depends much on the manner in which it is cooled. Malleability is the property which allows bodies to be hammered or rolled into thin sheets. Do you know what is the most malleable substance?

16. George. Gold is considered the most malleable. Mr. M. And what is most ductile, or can be drawn to the finest wire?

Frank. Platinum13 has been drawn into finer wire than any other substance I ever heard of.

17. Mr. M. What is tenacity?

Frank. I believe the word is derived from the Latin word teneo, I hold; and if so, it must mean the property of holding together.

John. I think I have heard that iron is the most tenacious of the metals.

18. Mr. M. Yes; iron wire is so tenacious that when only the sixteenth of an inch in diameter it will support 540 pounds, while a similar wire of lead will only sustain 27 pounds. The tenacity of iron makes it valuable in the construction of suspension bridges and other structures.

19. Here Mr. Maynard remarked that, although their hour had not quite expired, he would close this conversation, as he had promised to accompany a number of the younger pupils in a long ramble immediately after dinner, for the purpose of aiding them in making a map of the stream which flowed

through the valley. He informed the class that the subject of their next conversation would be MOTION AND ITS Laws.

20. The means which Mr. Maynard adopted for interesting his pupils in a great variety of subjects of study were wisely contrived for combining amusement and instruction. With a view to such results, the excursion referred to had been planned for his younger pupils; and by such means they were early grounded in the principles of geographical knowledge, and interested in learning more of a subject whose very rudiments had proved to them a delightful recreation.

21. Thus, before his youthful pupils were aware that they were studying geography, they could tell the direction by the compass, and the distance from the old mansion, of every grove, fountain, and hillock for two miles around, the windings of the stream which flowed through the valley, the various ravines which entered the glen below, and could accurately trace on a slate or paper a map of the whole, the boundaries of the estate on which they resided, and a profile outline of the hills which separated their little world of tranquil beauty from the great and noisy world around them.

22. But they had visited many of the neighboring villages beyond the hills, and could locate11 them also; and as they inquired about distant cities and countries, and read, or heard related, interesting accounts of them and their people, they not only learned more of geography, but began the acquisition of historical knowledge. Thus, though secluded 15 for a while from the noisy scenes of life, they were taught that, if their lives should be spared, they were soon to mingle with the moving throng, and that the duties of life required of them a knowledge of that world on whose stage of action they were soon to enter.

10 DUE-TIL-I-TY, that property of bodies
which renders them capable of being ex-
tended by drawing without breaking.
11 TE-NAC-I-TY, that property of bodies
which keeps them from being parted with-
out considerable force.

12 AN-NEAL'-ING, the process of applying
heat for removing brittleness.

13 PLAT-I-NUM, the heaviest of the metals. 14 Lo'-CATE, determine the place of; arrange on a map.

15 SE-CLU'-DED, shut out; living in retire

ment.

LESSON V.

MOTION AND ITS LAWS.

1. Mr. M. WHEN Plato, an ancient philosopher, was asked for a definition of motion, he is said to have arisen, and to have walked back and forth in the presence of his interrogators,1 as much as to say, "You see it, but I can not tell you." Who of you will define motion?

Ida. Motion is a continued change of place with regard to a fixed point.

2. Mr. M. I would prefer to call motion simply the act of changing place, and consider it the condition of matter opposed to rest. It is enough for our purpose, however, that we understand the usual meaning of the term. Can you define uniform motion?

John. When a body passes over equal spaces in equal successive portions of time, its motion is called uniform.

3. Mr. M. Very well; now tell me when it is accelerated,2 and when retarded.3

John. When the spaces passed over in equal times continually increase, the motion is called accelerated; and when such spaces decrease, it is called retarded; as a stone thrown up in the air is retarded in ascending, and accelerated in descending.

4. Mr. M. A good definition, and I think you understand it. Can you define velocity?

George. It is the swiftness of the motion, and is measured by the space passed over in a given time.

Mr. M. Now tell me what momentum1 is.

5. George. Momentum is the quantity of motion, and is the products of the quantity of matter by the velocity.

Mr. M. If that is so, a small body moving swiftly may have as much momentum as a large one moving slowly.

George. Yes, sir. A 32-pound ball, moving 1000 feet in a second, will have as much momentum as a battering-ram weighing 2000 pounds, and moving 16 feet in a second.

6. Mr. M. I am pleased to see you so promptly make the practical calculations illustrative of the principles of motion.