作者：（美）文森特·默奇
出版社：天津人民出版社
格式: AZW3, DOCX, EPUB, MOBI, PDF, TXT
科学读本（英文原版）（第4册）试读：
Lesson 01 Cohesion
“Well,how did you like your science lesson today,boys？” asked Mr.Wilson,as he overtook our two young friends,Fred and Willie,on their way home.The boys had been promoted to a higher class,and this was the first lesson of the new course.Mr.Wilson,their teacher,was a rare man for his boys,especially those boys who showed that they took an interest in their work.He had long been struck with the earnest attention these two boys paid,and the trouble they took to follow him intelligently,and he made up his mind to help them.
“I think,sir,” said Fred,“we shall soon begin to feel at home,for I am sure,from what I saw today,our lessons in the lower classes will help us very much.We are going to try hard,for father has promised to send us to the Institute by and by,if we learn all we can now.”
“Suppose you tell me something about today’s lesson,as we walk along,” said Mr.Wilson.“Well,” said Willie,“the first thing we learned was that new word matter.We know now that the name matter means every substance that exists.”
“I think I understand,sir,” said Fred,“what you mean by molecules of matter,although it seems difficult to imagine particles so small that they cannot be seen even with the help of a powerful microscope.We learned from our lesson that matter of every kind—solid,liquid,and gas—is made up of extremely small particles,and these particles are called molecules.A molecule is the name for the smallest particle of matter that can possibly exist.”
“Quite right,Fred,” said Mr.Wilson.“If you will keep two things in your mind,you will be pretty clear about these molecules of matter.
“First think of the dissolved particles of a soluble substance.They are all in the liquid,but they have been divided up into such minute particles that they are invisible.
“Then think of our little experiment with mercury.We boiled the mercury in the tube,and as it boiled it passed away in vapor.But we could not see the vapor,because the particles had been divided up too small to be seen.These were molecules of mercury.
“We knew they were there,and we found them when we held the cold slate over the tube.The tiny drops,as they condensed,ran together again and again,till at last they were large enough for us to see.”
“We learned,too,” said Fred,“that,as all matter is composed of molecules,there must be a force of some kind,which holds them together,or else everything in the world would at once fall away to the finest dust or powder.This force which holds the molecules of matter together is called cohesion.It is so named because the word cohesion means holding together.”
“That’s very good,” said Mr.Wilson.“Try and think of our experiments with the poker,and the pieces of lead,wood,glass,and chalk,and tell me what they teach us.”
“Oh yes,I remember,sir,” said Willie.“We can’t break or twist the poker with all our trying.This means that the force of cohesion is so strong that we cannot separate the molecules from one another.We can bend the piece of cane and the lead,but they do not easily break.We say they are tough.The glass and the chalk snap quickly.There is less cohesion between their molecules than there is between the molecules of either lead,wood,or iron.We say the glass and the chalk are brittle.”
Lesson 02 Solids,Liquids,Gases
“May we walk home with you this afternoon,sir？” asked Fred.
“Certainly,my boys,” replied Mr.Wilson.“I was thinking of putting my museum cupboard into your charge,” he continued.“You could keep the things in order,dust them regularly,and help me with the experiments during the lessons.Would you like that？”
“Oh,thank you very,very much,sir,” said both the boys at once.“We’ll be very careful with everything.”
“Now,what have you got to chat about as we walk along？” asked Mr.Wilson.
“Our lesson on the force of cohesion,sir,” said Fred,“helps us to understand,better than we have ever done,why there should be three distinct states of matter—the solid,the liquid,and the gas.
“A solid is a body whose molecules are held strongly together.It is a solid simply because the force of cohesion is very strong in it.Even when we break a soft solid,like a piece of chalk,it does not fall to powder.It breaks into pieces,and these pieces still hold together.”
“It was so easy,sir,” said Will,“to compare the cohesive force in a liquid and a solid,when you set me to take the water out of the basin,a spoonful at a time.It was no trouble to separate the molecules of water from one another with the spoon,because they are not held together firmly,as the molecules of a solid are.”
“Quite right,boys,” said Mr.Wilson.“Now what have you to say about the gases？”
“The molecules of a gas soon spread themselves out till they seem to fill the room,” said Fred.“This proves that gases are quite different from either solids or liquids.Their molecules have no cohesion at all;they actually repel each other.They are always trying to get as far away from each other as possible.”
“Now I want you to try and tell me how the force of cohesion acts,” said Mr.Wilson.
“The force of cohesion can act only when the particles are in close contact,” said Fred.
“Do you remember,” asked Mr.Wilson,“how I proved that？”
“Oh,I remember,” said Fred.“It is useless to try and join the two edges of a broken plate or saucer,or any other solid body,by pressing them together,because we cannot bring all the particles into actual contact,and without actual contact there can be no cohesion.But it is possible to join two perfectly smooth and level sheets of glass by pressing them together.All their particles are in actual contact,and cohesion acts and joins them.”
“Are we to understand,sir,” asked Willie,“that cohesion acts only between the molecules of the same kind of substance？”
“Yes,” said Mr.Wilson,“the molecules of solid bodies are held together by cohesion,and the molecules of liquids are also held together by cohesion.When we mix two glasses of water,they immediately mingle and form a compact whole,because the molecules are brought into actual contact with each other on all sides.”
Lesson 03 Our Bodies
We can feel,in every part of our body,portions of the hard,solid framework on which the body is built.This strong framework is called the skeleton,and consists of upwards of two hundred distinct and separate bones of various shapes and sizes,intended to give strength and solidity,and to support the softer fleshy parts.If you call to mind the various skeletons of animals you have seen,you will at once understand that in every animal it is the skeleton which determines the shape of the body.The skeleton itself suggests at a glance the well-known shape of the individual animal.
Let us examine our own skeleton,and see how it is made.We will commence with the head,which comprises two parts—the skull and the face.
The skull is a hollow box intended to hold and protect the brain.It is built of eight separate bones,most of them broad flat plates.They are joined together firmly at their edges,because,as they simply form a box,there is no need for these bones to move.The face comprises all the rest of the head that is not included in the skull.It is formed of no less than fourteen separate bones.The only thing calling for special notice in the arrangement of these bones is the provision,which is made by them for the protection of the eyes,by lodging those organs in great hollows formed in the bones themselves.These hollows are called the orbits or sockets of the eyes.Notice how providentially they are surrounded by the broad frontalbone of the forehead above,the nose-bone between them,and the cheekbones below.These effectually protect the delicate organs from injury.Only one of all these fourteen bones of the face is capable of movement.Which is it？The lower jawbone moves so as to open and close the mouth.It is attached on either side by a sort of hinge to the other bones of the skull.
Both jaws are armed with teeth for biting and chewing our food.We have during our lives two sets of teeth.The first,called the milk teeth,are twenty in number,and are shed while we are young.The others,known as the permanent teeth,last through the rest of our life.There are thirty-two permanent teeth in the complete set.
The trunk is that part of the body which would be left if the head and limbs were lopped off.The main pillar of the body is the backbone,which extends from the neck to the bottom of the trunk.It is not actually a single bone,but a string of separate bones called vertebra,and is called the vertebral column.The vertebrae are joined together by thick smooth pads of gristle,which form springy,yielding cushions between each bone and the one above it.The seven smallest and topmost vertebras form the neck;they support the head.Next to these come twelve vertebras,each of which supports a pair of ribs—one on either side.The ribs form a kind of hollow bony cage,which we call the chest.They are joined in front to the breastbone.It is worth noticing that the vertebras increase in size and solidity downwards.The bones at the base of the column are very thick,solid,and strong.
It is a curious fact,too,that the vertebral column of a child contains thirty-three distinct bones;but there are only twenty-six bones in the vertebral column of an adult.The explanation is this;as the child advances into adult life the four lowermost vertebras grow together and form one piece,corresponding to the tail of the lower animals—in fact,a sort of rudimentary tail;and at the same time the five next above these also become welded into one piece to form the sacrum.The sacrum and the great haunch or hip-bones form a sort of bony basin—the pelvis—at the base of the column.
Lesson 04 The Woody Stems of Plants
“It was a great surprise to me,sir,” said Fred,“to learn that the stems as well as the leaves of plants differ,according to the kind of seed from which they spring.Our last year’s lessons taught us to look for net-veined leaves on a plant which grows from a seed with double seed-leaves,and for parallel-veined leaves on one which comes from a seed with a single seed-leaf.We learn from our lesson today that the seed with the double seed-leaf produces one kind of stem,and the seed with a single seed-leaf a totally different stem.”
“Do you remember the other name for the seed-leaves,Fred？” asked Mr.Wilson.
“Yes,sir,they are called cotyledons.Those seeds which have only one seed-leaf are called mono-cotyledons;those with two seed-leaves are di-cotyledons.”
“Try and tell me all you can about the stem of the dicotyledon.” said Mr.Wilson.
“Most of the plants in every part of the world,sir,are di-cotyledons.They all have net-veined leaves;but some of them are herbaceous,and die down to the ground every season.These,of course,have no woody stem.The nature of the woody stem of the di-cotyledon can be easily understood from a piece of the stem or branch of one of our common trees.This always has a central pith,with the hard solid wood arranged round it in layers or circles.“You explained to us,too,sir,that when the tree was young this pith was soft,greenish,pulpy matter,and took up all the center of the stem.It was through this central pith that the dissolved earth-food absorbed by the roots rose upwards to the leaves.It had then only one woody layer round it.Next year,however,and each year after,a new layer of wood was formed on the outside,and these rings of new wood compressed the pith,till at last it became a mere thread.The woody stems of all di-cotyledons grow in this way,by the yearly addition of a new layer on the outside.They are called ex-ogens.The word exogen means growing outwards.”
“Very good,my boy,” said Mr.Wilson.“Now,Willie shall tell us all he can about the stem of the mono-cotyledon.”
“The woody stems of these plants are best understood,sir,” said Will,“by examining a piece of cane or bamboo.The wood is always arranged in parallel fibers through the stem from root to top.There is a pith,but it is mixed up with the parallel threads of wood.The parallel bundles of woody threads pass upwards through the pith itself.In these stems the oldest and hardest wood is on the outside—not in the center,as it is in the exogens.The stem grows from within;the new wood is in the center.Plants of this kind are called endogens,which means growing from within.These endogens or mono-cotyledons form a small class of plants.The palms,canes,and bamboos are the chief of them.”
Lesson 05 Capillary Attraction
“I can understand now,I think,sir,” said Fred,as they walked home,“why porous bodies have the power of absorbing liquids.Our former lessons merely taught us that this absorption actually takes place.We saw the liquids rise in the porous bodies.Today’s lesson shows us the reason why they rise.”
“Well,Fred,” said Mr.Wilson,“suppose you try and explain what really happens.”
“I think I should begin,sir,by trying to tell the difference between the force of cohesion and the force of adhesion.Our lesson on cohesion showed us that it is a very difficult matter to join the particles of a solid,because,smooth as their surfaces may appear to the naked eye,the magnifying glass proves them to be so rough and uneven that it is impossible to make them touch at all points,and without touching everywhere there can be no cohesion.
“On the other hand,it is quite easy to make a liquid and a solid join,because the liquid flows and fills up the inequalities in the surface of the solid,so that the two bodies touch everywhere,and the liquid holds to the solid.”
“Does the force of cohesion accomplish this too？” asked Mr.Wilson.
“No,sir,” said Fred.“We call the force which holds the liquid to the solid by another name—adhesion—and the liquid is said to adhere to the solid.The experiment which you showed us with the fine hair-tube,sir,is all due to this adhesive force between the liquid and the solid.The colored water rose in the tube by capillary attraction.The word capillary means like a hair.
“Capillary attraction takes place because of the adhesive force between the liquid and the sides of the glass tube.This adhesive force draws the particles of water upwards and makes them adhere to the sides of the tube.”
“But you have not told us yet,Fred,” said Mr.Wilson,“what all this has to do with the absorbing power of porous bodies.”
“I am coming to that now,sir,” said Fred.“Every pore in a porous body is really a little tube,and the liquids are absorbed into the pores by capillary attraction,just as the colored liquid rose in the capillary tube.”
“That’s capital,” said Mr.Wilson.“You’ve mastered this matter well,Fred.Remember,it is the same capillary attraction that raises the oil in the lamp wick for the supply of the flame;it is the same capillary attraction that emptied the tumbler of water when you hung the piece of loose wick over the edge of the glass;it is the same capillary attraction that allows the soil in the flowerpot to suck up water from the saucer in which it is placed.”
Lesson 06 More about the Skeleton
The human skeleton,and the skeletons of most of the animals we have examined,are built upon one general plan as regards the limbs.There are two pairs of limbs.In birds the upper limbs are modified to form wings,but the structure even here is essentially the same.
Let us examine the human limbs now.The arm has thirty-two distinct bones,and consists of three parts—an upper arm,a fore-arm,and a hand;the leg also consists of three corresponding parts—an upper leg (thigh),a lower leg,and a foot.It contains thirty separate bones.
The upper arm and the thigh are each formed by a single long bone.The fore-arm consists of two bones(one larger than the other),jointed at the elbow to the bone of the upper arm.The lower leg also consists of two bones (one larger than the other),jointed at the knee to the thigh-bone.The knee-joint,however,differs from the elbow-joint in having a small bone—the kneecap—placed over the joint.There is no corresponding bone in the arm.
The hand and the foot are built on very much the same plan.The wrist of the one corresponds to the ankle of the other.The wrist has eight small bones;the ankle seven.The palm of the hand corresponds to the sole of the foot,and each of them has five bones running through it,which carry the fingers and toes respectively.
The hand has four fingers and a thumb—each finger being formed of three bones,the thumb of two.The foot,in its turn,has four toes and a great toe;the great toe having two bones,each of the others three.
We have only to consider the purposes for which the hand and the foot are respectively designed,and we must easy rapid movement,delicate touch,and firm grasping powers have be at once struck with admiration for the manner in which each is adapted to its work.In the hand,flexib iration for the adapted to its ility and to be secured.Hence the wrist bones are small,those of the fingers very long,while the thumb is placed so as to be able to move in the opposite direction to the fingers.In the foot,we need strength and solidity of support,consequently the whole of the bones are short,thick,heavy,and clumsy in appearance.The great toe has no grasping power,corresponding to that of the thumb.
The bone of the upper arm is joined at the shoulder to a large flat bone—the shoulder-blade,which lies behind the ribs.The shoulder-blade is kept in position in a wonderful way by the collar-bone.This bone stretches from the top of the breast-bone to the shoulder joint,its outer end being firmly fixed to the shoulder-blade.It is this bone which forms the prominent part of the shoulder.It acts as a rigid bar to brace the shoulder-blade up.The two collar-bones in front and the two shoulder-blades behind form what is known as the shoulder-girdle.
The thigh-bone is joined to the great haunch or hip bone.
Lesson 07 The Oak Stem
“I had often wondered,sir,at the beautiful markings of the various ornamental woods used by the cabinet-maker,”said Fred,as they were putting the things away after the lesson.“I think I understand it clearly now.”
“I want you to be quite clear as to those medullary rays,” said Mr.Wilson.“In all these stems there are two kinds of matter.There is the hard,fibrous,woody tissue and the softer tissue of the pith.In the young tree you have seen that the stem is almost all pith,and that the pith is the channel for the upward passage of the sap.As the new layers of wood are formed round it year by year,the pith becomes more and more compressed,and it sends out thin walls of its own substance,radiating to the outside of the stem,so as to separate the woody tissue into wedge-like blocks.These radiating walls of soft matter are the medullary rays.They carry on the work of the pith,and form channels for the flow of the sap.”
“But the sap does not always rise through the medullary rays,does it,sir？” asked Willie.
“As the tree grows from the outside,” said Mr.Wilson,“the older part of the stem is subject to more and more pressure.The walls of soft matter between the wedges of wood become thinner and thinner,till they are the finest sheets,and the medullary tissue itself has ceased to be living matter.After this it no longer carries up the sap.When the stem is cut across,only the thin edges of these partition walls are seen,and they appear to be mere lines.”
“Thank you,sir,” said Fred.“These are the lines then that the cabinet-maker calls the silver-grain of the wood.”
“Yes,Fred,they are,” said Mr.Wilson.“The beauty of the wood depends upon the closeness of its texture,and the skill of the workman in cutting it so as to expose this silver grain.Which part of the woody stem did you say was first formed？”
“The central part of the stem,sir,is the oldest wood in the tree.”
“Tell me again what happens to this part as new layers are formed one by one around it？”
“The outer layers press upon it more and more,sir,and it becomes denser and denser.”
“Quite true,” said Mr.Wilson,“and it becomes the hardest wood in the tree.It is called heartwood.Heartwood is always selected where strength and durability are required.The heartwood becomes darker and more deeply coloured than the layers round it,and it is the graduation of coloring that makes our cabinet woods so beautiful.
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