12. Mr. M. I am glad you have asked the question; for the most important thing in starting is to know what we are talking about, and I urge you always to obtain clear definitions of terms. The word atom means any thing that can not be cut or divided. It is the smallest part of a body. A number of atoms form a molecule, and a number of molecules form a particle. Please remember this. I now desire to hear from each of you the most surprising instance of division of matter that you have been able to find.

13. Ida. A single grain of musk will perfume a room for many years and still be a grain of musk. I suppose, wherever the musk is perceived, there must be particles enough to affect the sense of smell.

Ella. The thread of a spider's web, which I can scarcely see, is composed of 6000 single threads, according to the books, though I do not know how any body can see and. count them.

14. Frank. One grain of carmine13 will tinge ten pounds of water so that the color may be seen. I have also read that two grains of silk have been spun into a thread 300 yards in length.

John. A single grain of gold has been divided into three million six hundred thousand parts, each visible through a microscope magnifying 500 times.

George. Dogs pursue their game by means of odors imperceptible11 by man, and I suppose these odors must consist of very minute atoms or particles of matter diffused through the air.

Frank. There is another interesting example of practical divisibility which I had forgotten to mention. I have seen it stated that artists, aided by the microscope, have ruled parallel lines upon glass, with a diamond point, so close to each other that ten thousand are contained in a single inch.

15. Mr. M. Your examples are well chosen. I will name but a single one. Ehrenberg has found the shells of animals so small that the one hundred and forty-fourth part of a cubic inch contained twenty-three millions of them! These animals must have had limbs, and blood circulating through their tiny veins, and this blood was doubtless itself compounded of various kinds of molecules, each itself composed of atoms.

16. John. It seems scarcely possible that real live animals so small can exist!

Mr. M. When we take up the study of the "Microscopic World" we shall find stranger things than these to excite our wonder. The next property of matter is porosity. What do you understand by it?

John. It is the quality or state of having pores or openings in it. It means that the particles of matter are not close together, and the farther apart they are, the more porous is the substance. It is the opposite of density.15

17. Mr. M. Very well; and now each of you may give an example of porosity.

Ida. I have seen mercury forced through the pores of oak, and fall in a fine shower. That showed that the oak was

porous or full of small holes.

Ella. We can put a large quantity of cotton into a tumbler completely filled with spirits of wine, and none of the liquid will overflow.

John. Salt can be put into water without increasing its bulk. This shows that the particles of water are not close together, and that there is room between them for the salt.

18. Frank. A pint of alcohol and a pint of water will not make a quart of mixture. I think this must be due to porosity also.

George. I have read that there are innumerable small pores in the skin, through which insensible perspiration passes.

Mr. M. It is evident that if there were no porosity, and the particles of matter were already close together, they could not be brought any closer together by pressure. Matter is therefore said to have the property of compressibility.16

Frank. Can matter in all of its three forms, solid, liquid, and gaseous, be compressed?

19. Mr. M. It can, though water is compressible only in a slight degree compared with air. Matter in every form can also be expanded by heat. But I perceive we shall not have time to finish the subject of the properties of matter during the hour, and we will therefore leave the remaining topics for our next conversation. In the mean time you will recollect that on next Saturday we are to talk about the remain

ing general or accidental properties of matter, which are inertia, figure, and attraction; and if we have time we shall also call your attention to the specific properties of matter. I will not mention now what these properties are, but hope each of you will find out, and learn what you can about them.

1 COM-POS'ED, formed; made of.

2 FREIGHT (frate), to load with goods.

9 DI-VIS-I-BIL'-I-TY, the quality of being di-
10 PO-ROS'-I-TY.

3 Sci'-ENCE, knowledge; a collection of the 11 IN-ER'-TIA (in-er'-shä).
general principles or leading truths relat-12
ing to any subject.

4 IN-VIS'-I-BLE, that can not be seen.
5 IM-PEN-E-TRA-BIL'-I-TY, that quality of
matter by which it excludes all other mat-
ter from the space it occupies.

6 IN-VERT'-ED, turned upside down.

7 FUN'-NEL, a tunnel.

8 IL-LUS-TRA'-TION, that which explains or renders any thing more clear.

[visible.

MOL'-E-CULE (or MōLE'-CULE), a name given to the minute particles of bodies. 13 CAR-MINE, a beautiful red color.

14 IM-PER-CEP'-TI-BLE, that can not be per. ceived by the senses.

15 DENS'-I-TY, closeness or compactness of parts.

16 COM-PRESS-I-BIL'-I-TY, the quality of being brought into a smaller compass by pressure.

LESSON IV.

THE PROPERTIES OF MATTER-Continued.

1. THE class having assembled in the library as usual, Mr. Maynard asked John if he could describe that property of matter called inertia.

John. It is that property by which matter tends to retain its present state, whether of motion or rest.

Mr. M. True, that is the scientific definition; but can you show me that you understand the subject by giving illustra tive examples of inertia?

2. John. Our mill never starts till some one lets the water on the wheel, and thus sets it in motion.

George. It is always harder for the horses to start a loaded wagon than to draw it when in motion.

Frank. I have seen a boy standing up in a cart fall backward when the cart was suddenly put in motion, and I think this must be due to the inertia of the boy.

3. Ida. I was once thrown from a horse by his starting suddenly forward.

Ella. Inertia is certainly a very accidental property, for I was once thrown over a horse's head, from his suddenly stopping when I had been riding rapidly.

4. Mr. M. A knowledge of the inertia of matter should make

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 matter, whether of rest or motion, is called force. also a property of matter called figure. Can you tell me what is meant by it?

Fig. 1.

state of

There is

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 assume 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 mole cules, it is termed cohesion; and between masses, gravita. tion. 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?

3

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 you tell what they are?

one of

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 gen eral 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.?