英文科學(xué)讀本 第五冊(cè)·Lesson 34 Vegetable Foods

Lesson 34 Vegetable Foods

We use many varieties of vegetable substances as food. The principal are the grains—wheats, oats, barley, rice, and corn; the preparations known as sago, arrowroot, and tapioca; the seeds of beans and peas; fresh vegetables, such as are sold by the green-grocer; fruits, and sugar in various forms.

First among the vegetable foods stand those which yield flour or meal. We call them farinaceous foods, from farina, the Latin name for flour. These foods shall form the subject of our first inquiries.

Now, Fred, said Mr. Wilson, "you pleased me the other day very much by showing me that you had not forgotten the way to separate the starch grains from flour. You shall now come to the front and do it for me, as you have seen me do it in our earlier lessons. That will do; the water in the basin is white and milky with the starch. Now open the bag, and let us see what we have left behind in it. There is a white, sticky substance something like bird-lime left in the bag. We call it gluten. This gluten is very much the same kind of substance as the albumen, myosin, gelatine, and casein of animal food. It is a tissue-forming substance.

The flour, you see, consists mainly of these two constituents, gluten and starch. The starch is now in the water; the gluten in the muslin bag. Here are some grains of wheat. Let us cut them open and examine them. The middle of the grain, you observe, is white, and this white part is enclosed within an outer covering or skin. The white inner substance is starch; it is the outer covering of the grain which contains the tissue-forming gluten. The gluten is always found in this outer part of the grain, just beneath the skin.

The miller, after grinding the grain, passes the meal through a series of sieves. The first sieve separates the larger particles from the rest. These are really portions of this very outside skin, and are sold as bran. The second sifting separates some finer portions of the same skin, which are known as middling. The brown meal, which remains after this sifting, is really the best and most nourishing of all, for it contains all the gluten. It makes good, wholesome, brown bread. Many people, however, prefer white bread; hence the meal is sifted again and again, the result being that much of the gluten is separated from it, and the flour is white only because it consists very largely of starch.

You will now understand that although the miller may call this best wheaten flour and best whites, it is far from being equal in nutritive value to the browner, unsifted, whole-meal flour. This pure, unsifted whole-meal of wheat contains about 12 per cent of gluten, 60 per cent of starch, and 14 per cent of water. The remainder is made up of fat, sugar, and mineral matter. The proportion of gluten in the flour is lower after each sifting, so that fine wheaten flour does not contain usually more than 10 per cent.

The different qualities of wheaten flour in the market are known under various names. I have already told you that that called the best makes the whitest bread; but is it really the best? Wheaten flour is almost universally used in England, but in Scotland it gives place to oatmeal—that is, the meal or flour of ground oats. Oatmeal is even richer that wheaten flour in gluten, for it contains no less than 18 per cent of this tissue-forming substance.

Oatmeal will not make up into light spongy bread, such as we make with wheaten flour. Hence the Scotch people eat it in the form of porridge and oat cake, which form their principal food, and a very valuable and wholesome food too. In our land almost the only use we make of oatmeal is in a breakfast porridge. The grain itself is largely used for feeding cattle and horses. Nearly all the barley grown with us is used in the preparation of sugar for malt. Barley-meal, although much used for feeding cattle and pigs, is not made into bread. In a few places in the north of England, however, it is mixed with equal quantities of wheaten flour, and made into cakes called bannocks.

We usually see it as pearl barley and Scotch barley, and in this form it is used for making soups and broths. The reason why barley-flour is not used for bread is that the meal is coarse in flavor and color, and bread made from it, instead of being light and spongy, is heavy and close. Otherwise it contains about the same proportion of tissue-forming gluten as wheaten flour. The same remarks apply equally to rye-flour, which is used as a bread-stuff in this country, in making the rye-loaf and Boston brown bread. Lesson 35 Vegetable Secretions—India-rubber and Gutta-Percha Both these useful substances are vegetable secretions; they are in each case the solidified sap of the plant. India-rubber is obtained from a variety of trees, which grow in Central and South America and the East Indies. The best rubber and the largest quantities of the article come from Brazil, and are furnished by the great forests which cover the basin of the Amazons. The East Indian rubber is obtained from a kind of fig-tree, which grows to an enormous size. The mode of collecting the juice is almost identical everywhere. The trees are tapped by cutting a number of slanting holes in the bark with an axe, and tin cups are fixed, by means of clay, below the incisions, to catch the juice as it flows. From time to time, daring the day, the collector goes round from tree to tree, and empties the contents of the little cups into a pail. As he usually works about a hundred trees at one collection, he is kept pretty busy. The juice as it comes from the tree is a yellowish-white liquid, having something of the consistency of cream. It dries when it is spread out in thin layers and exposed to the sun. The East Indian rubber is always sun-dried, and in drying turns nearly pure white. The milky juice is dried in a very peculiar way. A number of variously-shaped molds made of clay are dipped into the vessel containing it, and, of course, when they are taken out, some of the juice adheres to the mold. This is laid in the sun, and as it dries it forms a thin coat or layer over the mold. The process is repeated again and again. Each dipping and drying adds to the thickness of the deposit, until the coat is thick enough, when the mold is broken up, and the solid rubber is left. The South American rubber is not sun-dried. The collector, in this case, dips his clay mold into the juice, and then holds it over the smoke of a wood fire for about half a minute. This quickly changes the milky juice into a thin layer of india-rubber, and he continues the dipping and smoking process until his deposit is thick enough on the mold. Then the mold is broken up, and the rubber removed, and set in the sun to further dry and harden. This smoker-drying changes the color from a creamy white to almost black. India-rubber is now put to an immense variety of uses, but before it is available for any of these purposes it has to undergo a process of mastication to remove dirt, and all solid impurities. It is first softened in boiling water, and in that state cut up into small pieces with sharp knives. These pieces are rolled by wooden rollers, to crush out all foreign substances, which are then washed away, and the purified rubber is kneaded, or masticated, into a lump while it is still warm and soft. Gutta-percha is prepared from the milky juice of a tree which grows in the forests of the Malay Peninsula and the islands near it. We have retained the native name, which is derived from gutta, meaning gum, and percha, the name of the tree. Gutta-percha simply means the gum of the percha-tree. The tree is very large; its trunk often measures 3 feet in diameter, but the wood is of a loose, spongy, fibrous nature and, of course, valueless as timber. The sap begins to flow immediately after the rainy season; this is the time to commence operations. The Malays have different modes of collecting the juice. Sometimes they merely bore a hole in the bark, and catch the sap in gourds as it exudes. More frequently, however, they cut the trees down, strip off the bark, piece by piece, and collect the sap as it flows. It dries and hardens rapidly into a solid lump on exposure to the air. Collected in this way, the gutta-percha is discolored with dirt and impurities, which have to be removed before it can be of use. This is done by first heating the substance in boiling water, till it is quite soft, and then tearing it to shreds by machinery. In this way all the impurities sink to the bottom of the water, and are washed away, leaving the shreds of soft gutta pure. All that remains is to knead them up, while they are still soft and pliable, into a solid lump. Lesson 36 Radiation of Heat When next the boys assembled for their lesson, Mr. Wilson began by calling upon them to describe the mode in which heat passes through solids and liquids respectively. I want you to think once more," said he, "of the poker in the fire, and tell me exactly by what means the handle becomes heated, although it is some distance from the fire."

The heat travels through the poker, and through all solid bodies, by conduction, sir, said Fred. "One molecule of matter receives the heat, and passes it on to the next."

Right. Now tell me what takes place when a liquid is heated?

In liquids the heat is carried, or conveyed, by the upward flow of the heated molecules themselves, which are not fixed and stationary as they are in solids. We call this convection.

Exactly, replied Mr. Wilson, "and now that we are sure of our ground as to solids and liquids, we will turn our attention to the passage of heat through the air, which, you know, is a gas.

One of you shall come to the front, and stand before the fire. You at once feel a sensation of warmth. The heat, of course, comes from the fire. We must find out how it travels through the air to your body. First, does it travel through the air by conduction? We will see. While you stand in the same position, I will hold this drawing-board between you and the fire. What do you observe?"

I no longer feel any heat from the fire, sir.

No, you get no heat now. But if the fire sent out its heat through the air by conduction, all the particles of the air would be heated in succession, and you would still feel the heat, in spite of the board. If we stood in the open air exposed to the most brilliant summer sun, we should experience exactly the same thing. Immediately the screen was put between us and the sun, the sensation of heat would disappear.

Hence we learn that, although heat does pass through the air from one body to another, it does not travel by conduction, nor does it heat, to any extent, the air through which it passes. Air is a non-conductor of heat. Heat, as you know, passes from the sun to the earth, but balloonists experience severe cold in the higher regions of the atmosphere, although they are then so much nearer the sun. The sun's heat travels through the atmosphere without raising its temperature to any extent.

Lower down, at the surface of the earth, the air is more or less warmed by the heat which is given out from the warm earth. We shall inquire further into this presently; we are now concerned only with the passage of heat through the air. Suppose we have an experiment. I have got something in the fire; it is a large iron ball. I daresay it is red-hot by this time. I want a dozen of you to come to the front, and stand in a circle in the body of the room. Now I will take the ball out of the fire, and hang it by its chain in the middle of the circle. Of course, each of you will tell me that he feels the glow of heat from the red-hot ball. Now sit on the floor below the ball, and you will still feel the heat, and so you would if I could place you above it.

You may take your places again; I have something else to show you now. Here is a large ball of wool, and you can see that I have stuck it full of pins, all pointing towards its center. I want you to imagine that, from every part of the surface of the red-hot ball, straight lines of heat are sent out through the air, much in the same way as the pins appear in the surface of the ball of wool. This is the universal way in which heat travels through the air from one body to another. We call these straight lines rays of heat. The drawing-board screen proved that the rays travel only in straight lines, for when it was placed in front of the heated body, it intercepted the rays, and no heat was felt. Heat, then, passes through the air in straight lines, which we call rays of heat, and we say that it travels by radiation. The body itself which sends out the heat we call a radiator.

Just one more thought before we leave the subject. You stood in front of the fire just now and became warm; we naturally stand before the fire to warm ourselves. We are warmed by taking in the heat which is radiated from the fire. We might stand there too long, or get too close to the fire, and we should be glad to move away. We should be getting too hot; we should burn ourselves if we remained there. Other bodies placed in front of the fire would be warmed too by taking in this radiated heat. We say they absorb the heat, and we call the bodies themselves absorbers of heat.

So then we see that one body radiates heat, and another absorbs it, and the double process would go on till both bodies were at the same temperature. I have a pretty experiment to prove this, but as it would take up too much time during the lesson, I will show it to you after school. I shall simply hang up our red-hot ball again, and place half a dozen thermometers in a circle round it. You will have nothing to do but watch what happens. You will see that, as the ball cools by radiating its heat, the mercury in each of the thermometers will rise by absorbing it, and after a time the ball and the thermometers will show the same temperature."