Part 6
If half a pint of water, coloured by putting a little ink into it, is added to the same quantity of clean water, the two will readily mingle; the total quantity of water will be a pint; and its colour will be just half as dark as that of the coloured half-pint. This is a case of simple =mixture=. The volume of the mixture equals the sum of the volumes of the things mixed, and there is no change in the properties of these things. So when water evaporates, the gaseous water or vapour mixes with the air in the same way, the molecules of the one body dispersing themselves between the molecules of the other until there is the same proportion of each everywhere. In like manner, sand and sugar may be (and unfortunately often are) mixed, without any change in the properties of either, or in the space which they primitively occupied.
On the other hand, oil and water will not mix, however much you may stir the two together; and the oil, being the lighter, rises to the top as soon as the fluid is quiet. Nor will quicksilver and water mix, but the quicksilver, being very much heavier than the water, rushes to the bottom of the vessel into which the two are put. Neither will sand nor iron filings mix with water; as heavier bodies, they also sink to the bottom. Nor does powdered ice, though it is water in another shape, mix with ice cold water; as a lighter body it floats at the top.
52. =Mixture followed by Increase of Density; Alcohol and Water.=
Strong spirit, or =alcohol=, is a clear transparent fluid which looks like water, but is a very different substance. For example, it boils at a much lower temperature, it burns with a blue flame, it has intoxicating properties, and, like oil, it is very much lighter than water. Hence, if coloured spirit is poured gently upon the surface of water the spirit rests upon the water. Suppose, now, that we take a tall measure graduated into ten equal parts. Fill the lower five with water, and then, very gently, pour in the strongest alcohol, coloured in some way, until the tenth mark is reached. We shall have five volumes of water below, and an equal quantity, or five volumes, of coloured alcohol above. Where the two are in contact, the colour will be diffused into the water for a short distance, but not far, showing that only a slight mixture is taking place. This, however, is not because the two fluids mingle with difficulty; for, with slight stirring, they mix completely, and you have a fluid the colour of which is about half as intense as that of the alcohol, and many of the other properties of which are intermediate between those of pure alcohol and those of pure water.
Thus far, nothing further than simple mixture, as when coloured water was added to pure water, seems to have occurred; but, in reality, something more has happened. In the first place, the mixture is a good deal warmer than either of its components; that is to say, =heat= has been =generated=. In the second place, if you measure the volume of the whole fluid after it has cooled, it no longer stands at the mark =ten=, but distinctly lower, or about =nine and three-quarters=. As the volume of the mixture is less than the sum of the volumes of its two components, it follows that the =density= of the mixture must be =greater= than a density midway between that of the water and that of the alcohol. In other words, the molecules in the mixture do not occupy the same space as they did when they were separate. The result is the same as if the ten volumes had been compressed until they occupied only nine and three-quarters; so that the effect is a contraction similar to that which would be brought about by taking away heat from the mixture. In fact, as we have seen, the mixture gives out a quantity of heat.
There is another respect in which the mixture is unlike both its constituents. It both =boils= and =freezes= at a much =lower= temperature than =water= does, and at a =higher= temperature than =alcohol= does. In fact pure alcohol has not yet been frozen. If the molecules of the alcohol were merely diffused among those of the water as water is diffused through wet sand, they ought to pass into the gaseous state at the same temperature as that at which alcohol boils; and it would then be very easy to separate alcohol from water by distillation. But the fact is not so; alcohol cannot be obtained free from water by distillation unless something which holds water very strongly, such as quicklime, is added, so as to keep all the water back when the fluid is heated.
Thus alcohol and water, mingled together, give rise to a fluid which is not a mere mixture, the properties of which are known if we know the properties of its components; it is, in strictness, a new body, in which the molecules of the water and those of the alcohol affect one another to a certain extent and modify the pre-existing properties of each.
This effect of different bodies upon one another becomes much more manifest when water is brought into contact with certain solids.
53. =Solution; Water Dissolves Salt.=
If a spoonful of salt is put into a tumbler of cold water and the water is stirred, the salt swiftly vanishes from view; and, after a time, so far as our sense of vision goes, the water appears to be just what it was before. But if the water in the tumbler at first weighed five ounces and the salt weighed two ounces, the water in the tumbler will now weigh seven ounces; the water will now taste salt, the salt is said to be =dissolved=, and the =solution= is called =brine=. Moreover, the solution is said to be =saturated=, for if you put more salt in it will remain unchanged. Water, in fact, will dissolve two-fifths of its weight of salt and no more. If the brine thus formed is put into a wide dish, so that the water may evaporate; or if it is heated and the water boiled away; as fast as the water diminishes, a quantity of salt, equal to two-fifths of the water which is converted into steam, returns to the solid state and falls to the bottom of the vessel. And when all the water is driven off, the salt which remains will have exactly the weight, and all the other properties which it had before it was dissolved by the water.
Thus, contact with water has had a very singular effect upon the salt. It appears to have changed one of the properties of the salt, namely, its =solidity=, but to have left all the rest unaltered. We saw just now that powdered ice does not mix with ice-cold water, but that the fragments of ice remain solid. The moment, however, that the temperature rises, the =cohesion=, or sticking together of the molecules, which is the characteristic of the solid state, comes to an end; they become loose and free to move, and they mingle with the surrounding water. Or we may say that the ties which held the molecules of the solid together are dissolved, so that the solid water becomes fluid.
The resemblance of this process to the dissolving of salt in water is so obvious that, in common language, it is often said that a lump of salt or of sugar =melts= away in water; but if you try to make salt fluid by heat, you will have to expose it to a very high temperature, so that the conversion of salt from the solid state into the liquid state by solution in cold water is obviously a very different process from liquefaction by heat. Nevertheless the result is the same so far as the condition of the salt is concerned. The cohesion between its molecules is destroyed, and they distribute themselves evenly among the molecules of the water, just as the molecules of steam distribute themselves among the molecules of air. And, when you study chemistry, you will learn how it may be proved that the smallest drop of the solution of salt contains exactly the same proportion of salt as the whole does.
If brine is allowed to evaporate slowly, the molecules of the salt arrange themselves, as the water leaves them, in beautifully regular cubical crystals. You may see them form easily enough if you watch a drop of brine gradually dry up under a microscope. The salt crystals contain nothing but salt. If they are heated till they become red-hot they pass into the fluid state; and when still further heated, the fluid salt becomes a vapour or gas and, as such, flies off into the air, or =volatilizes=.
Thus we see that when salt and water are brought into contact, the salt undergoes a certain amount of change, while the water does not remain wholly unchanged. For brine no longer boils at 212°, but requires a considerably higher temperature. The salt, as it were, holds the water back, and prevents it from assuming the gaseous state under the same conditions as if it were pure, just as, in the previous case, the water held the alcohol back; or we may say that the force of heat which drives the molecules of liquid water apart, when steam is formed, has a greater resistance to overcome when salt is dissolved in the water. And just as the presence of alcohol lowers the freezing point of the water with which it is mixed, so does the presence of salt lower the freezing point of water. Sea water, which is a weak brine, begins to freeze at about 27°; and the ice which is formed is quite pure, while the remainder of the sea water becomes richer in salt.
If we mean by attraction that which opposes any force which tends to separate bodies, then we may say that the molecules of salt and those of water attract one another. And such attraction between molecules of matter of different kinds is called =chemical attraction=.
54. =Quicklime and Water: Plaster of Paris and Water: Combination.=
Quicklime is a substance obtained by heating chalk or limestone to redness. When pure, it is a white hard solid which can be made to pass into the liquid and gaseous states only at enormously high temperatures. If a lump of fresh quicklime be placed in a saucer and about one-third of its weight of water poured upon it, there will be a great turmoil, heat will be evolved, the water will disappear, and the lime will crumble down into a soft white powder. This operation is what bricklayers call =slaking= lime. And if no more water has been added than the proportion mentioned, the pure white powder which results will be solid and dry, the water having, to all appearance, vanished.
In the solution of salt we saw a solid become fluid under the influence of water; in the slaking of lime the fluid water enters into the structure of a solid. If more water is added, this solid dissolves or becomes liquid, as the salt did, and the solution is called =limewater=. By carefully managed evaporation of the water the lime may be recovered in the form of crystals, just as the salt was recovered. But there is this difference, that the salt crystals contain no water, while the lime crystals not only contain water, but contain exactly the same proportion as exists in slaked lime, that is to say, 18 parts water to 56 parts lime.
The water thus bound up with the lime into a new solid holds on so firmly to the lime that it requires a red heat to separate the two. The lime and the water are said to be =chemically combined=; and as the proportion of lime and water in slaked lime, or lime crystals, is always the same, they are said to be combined in =definite proportions=; and the slaked lime receives the special name of =hydrate of lime=.
=Gypsum= or =Plaster of Paris= is a dry white powder. If mixed with a little water it does not slake after the fashion of quick lime, but the mixture soon =sets= or becomes hard; and, at the same time, the greater part of the water disappears. In fact, it has combined with the plaster of Paris and forms part of another hydrate, in which, when the superfluous moisture dries, not a trace of water is to be seen. It is this property which is taken advantage of when plaster of Paris is used in making casts and moulds. The fluid plaster is poured over and round the body to be cast; as a fluid, it applies itself conveniently to all the inequalities of its surface; and, when it sets, it retains the shape which it has thus acquired. Set plaster of Paris may be perfectly dry; but it nevertheless contains between ⅐ and ⅛ of its weight of water, fixed and forming an integral part of the solid hydrate. And if the set plaster is strongly heated, the combined water is driven off and it returns to its original state.
Gypsum is found abundantly in nature, in the shape of beautiful transparent crystals which are called =selenite=. These crystals have the same composition as set plaster, that is to say, they are hydrates. A thin flake of such a crystal viewed with the highest powers of the microscope appears perfectly homogeneous. Nevertheless, there is good reason for the conclusion that it consists of molecules of water and molecules of gypsum which hold together so strongly that they form a hard brittle glassy solid. Moreover, the molecules of the hydrate itself hold together more strongly in some directions than in others. It is very easy to split the crystals lengthwise; while much more force is needed to cut them crosswise and then they do not split, but break.
Glauber’s salts and Epsom salts are other examples of solids which dissolve in water and separate in the crystalline form as the water evaporates; and which, like lime and gypsum, combine with a definite proportion of water to form crystalline compounds. In fact, each of these glassy brittle solids contains more than half its weight of water.
Thus we see that two bodies, of which water is one, may combine together to give rise to something different from either. And we are thus led to the science of =chemistry=, which tells us exactly how bodies combine, what comes of their combination, and how compounds may be separated into their constituents.
55. =Mineral bodies may take on definite shapes and grow, or increase in size, by the addition of like parts.=
Water and all the other natural bodies which have hitherto been mentioned, are what are called =mineral bodies=, although, in common use, the term mineral is usually restricted to ores and metals. Now we have repeatedly had occasion to remark that, under certain circumstances, not only water, but many other mineral bodies, assume regular shapes. The most familiar example is that of the beautiful imitation of leaves and foliage which is presented by the ice which forms on a window in winter. But we have also seen that common salt, lime, gypsum, Glauber’s salts and Epsom salts, also assume the crystalline form as they or their compounds with water are deposited from their solutions. And if a drop of solution of Glauber’s salts or of Saltpetre, is allowed to evaporate under the microscope, a wonderful spectacle will be presented. As the salt assumes the solid state, the crystals suddenly appear in the field of view as needles and plates disposed in beautiful patterns, which rival those of hoar frost, though they are quite different from them. In fact, as you will learn if you study =crystallography=, every crystallizable substance has its proper crystalline forms and never departs from certain strictly related geometrical figures.
A crystal of any of these substances will =grow= if placed under proper conditions. Thus, if a crystal of common salt is hung by a thread in a saturated solution of salt, which is exposed to the air, so as to allow the water to evaporate slowly, the molecules of the salt which is left behind and can no longer be held in solution, deposit themselves on the crystal in regular order and increase its size without changing its form. And, in this way, the small crystal may =grow= to a great size. The large crystals of sugar candy, which consist of sugar and water deposited from a strong syrup or saturated solution of sugar, grow in the same fashion, upon threads suspended in the evaporating syrup. In this mode of growth you will observe that the enlargement is effected by addition to the outside of the growing body; and moreover the matter which is added, namely, the salt or the sugar, already exists as salt in the brine or as sugar in the syrup.
B. LIVING BODIES.
56. =The Wheat Plant and the substances of which it is composed.=
Every one has seen a cornfield. If you pluck up one of the innumerable =wheat plants= which are fixed in the soil of the field, about harvest time, you will find that it consists of a stem which ends in a =root= at one end and an =ear= at the other, and that blades or =leaves= are attached to the sides of the stem. The ear contains a multitude of oval grains which are the =seeds= of the wheat plant. You know that when these seeds are cleared from the =husk= or =bran= in which they are enveloped, they are ground into fine powder in mills, and that this powder is the =flour= of which bread is made. If a handful of flour mixed with a little cold water is tied up in a coarse cloth bag, and the bag is then put in a large vessel of water and well kneaded with the hands, it will become pasty, while the water will become white. If this water is poured away into another vessel, and the kneading process continued with some fresh water, the same thing will happen. But if the operation is repeated the paste will become more and more sticky, while the water will be rendered less and less white, and at last will remain colourless. The sticky substance which is thus obtained by itself is called =gluten=; in commerce it is the substance known as =maccaroni=.
If the water in which the flour has thus been washed is allowed to stand for a few hours, a white sediment will be found at the bottom of the vessel, while the fluid above will be clear and may be poured off. This white sediment consists of minute grains of =starch=, each of which, examined with the microscope, will be found to have a concentrically laminated structure. If the fluid from which the starch was deposited is now boiled it will become turbid, just as white of egg diluted with water does when it is boiled, and eventually a whitish lumpy substance will collect at the bottom of the vessel. This substance is called =vegetable albumin=.
Besides the albumin, the gluten, and the starch, other substances, about which this rough method of analysis gives us no information, are contained in the wheat grain. For example, there is woody matter or =cellulose=, and a certain quantity of =sugar= and =fat=. It would be possible to obtain a substance similar to albumin, starch, saccharine and fatty matters, and cellulose, by treating the stem, leaves, and root in a similar fashion, but the cellulose would be in far larger proportion. =Straw=, in fact, which consists of the dry stem and leaves of the wheat plant, is almost wholly made up of cellulose. Besides this, however, it contains a certain proportion of mineral bodies, among them, pure flint or =silica=; and, if you should ever see a wheat rick burnt, you will find more or less of this silica, in a glassy condition, in the embers. In the living plant, all these bodies are combined with a large proportion of water, or are dissolved, or suspended in that fluid. The relative quantity of water is much greater in the stem and leaves than in the seed.
57. =The Common Fowl and the Substances of which it is Composed.=
Everybody has seen a common fowl. It is an active creature which runs about and sometimes flies. It has a body covered with feathers, provided with two wings and two legs, and ending at one end in a neck terminated by a head with a beak, between the two parts of which the mouth is placed. The hen lays =eggs=, each of which is enclosed in a hard shell. If you break an egg the contents flow out and are seen to consist of the colourless glairy “white” and the yellow “yolk.” If the white is collected by itself in water and then heated it becomes turbid, forming a white solid, very similar to the vegetable albumin, which is called =animal albumin=.
If the yolk is beaten up with water, no starch nor cellulose is obtained from it, but there will be plenty of fatty and some saccharine matter, besides substances more or less similar to albumin and gluten.
The feathers of the fowl are chiefly composed of horn; if they are stripped off and the body is boiled for a long time, the water will be found to contain a quantity of =gelatin=, which sets into a jelly as it cools; and the body will fall to pieces, the bones and the flesh separating from one another. The bones consist almost entirely of a substance which yields gelatin when it is boiled in water, impregnated with a large quantity of salts of lime, just as the wood of the wheat stem is impregnated with silica. The flesh, on the other hand, will contain albumin, and some other substances which are very similar to albumin, termed =fibrin= and =syntonin=.
In the living bird, all these bodies are united with a great quantity of water, or dissolved, or suspended in water; and it must be remembered that there are sundry other constituents of the fowl’s body and of the egg, which are left unmentioned, as of no present importance.
58. =Certain Constituents of the Body are very similar in the Wheat Plant and in the Fowl.=
The wheat plant contains neither horn, nor gelatin, and the fowl contains neither starch, nor cellulose; but the albumin of the plant is very similar to that of the animal, and the fibrin and syntonin of the animal are bodies closely allied to both albumin and gluten.
That there is a close likeness between all these bodies is obvious from the fact that when any of them is strongly heated, or allowed to putrefy, it gives off the same sort of disagreeable smell; and careful chemical analysis has shown that they are, in fact, all composed of the elements =Carbon=, =Hydrogen=, =Oxygen=, and =Nitrogen=, combined in very nearly the same proportions. Indeed, =charcoal=, which is impure carbon, might be obtained by strongly heating either a handful of corn, or a piece of fowl’s flesh, in a vessel from which the air is excluded so as to keep the corn or the flesh from burning. And if the vessel were a still, so that the products of this =destructive distillation=, as it is called, could be condensed and collected, we should find water and ammonia, in some shape or other, in the receiver. Now ammonia is a compound of the elementary bodies, nitrogen and hydrogen; therefore (§ 50) both nitrogen and hydrogen must have been contained in the bodies from which it is derived.
It is certain, then, that very similar nitrogenous compounds form a large part of the bodies of both the wheat plant and the fowl, and these bodies are called =proteids=.
59. =Proteid Substances are met with in Nature only in Animals and Plants; and Animals and Plants always contain Proteids.=
It is a very remarkable fact that not only are such substances as albumin, gluten, fibrin and syntonin, known exclusively as products of animal and vegetable bodies; but that every animal and every plant, at all periods of its existence, contains one or other of them, though, in other respects, the composition of living bodies may vary indefinitely. Thus, some plants contain neither starch nor cellulose, while these substances are found in some animals; while many animals contain no horny matter and no gelatin-yielding substance. So that the matter which appears to be the =essential= foundation of both the animal and the plant is the =proteid= united with =water=; though it is probable that, in all animals and plants, these are associated with more or less =fatty= and =amyloid= (or starchy and saccharine) substances, and with very small quantities of certain mineral bodies, of which the most important appear to be =phosphorus=, =iron=, =lime=, =and potash=.
Thus there is a substance composed of water + proteids + fat + amyloids + mineral matters which is found in all animals and plants; and, when these are alive, this substance is termed =protoplasm=.
60. =What is meant by the word Living?=