Kitabı oku: «On the various forces of nature and their relations to each other», sayfa 3
LECTURE III.
COHESION – CHEMICAL AFFINITY
We will first return for a few minutes to one of the experiments made yesterday. You remember what we put together on that occasion – powdered alum and warm water; here is one of the basins then used. Nothing has been done to it since; but you will find on examining it, that it no longer contains any powder, but a multitude of beautiful crystals. Here also are the pieces of coke which I put into the other basin – they have a fine mass of crystals about them. That other basin I will leave as it is. I will not pour the water from it, because it will shew you that the particles of alum have done something more than merely crystallise together. They have pushed the dirty matter from them, laying it around the outside or outer edge of the lower crystals – squeezed out as it were by the strong attraction which the particles of alum have for each other.
And now for another experiment. We have already gained a knowledge of the manner in which the particles of bodies – of solid bodies – attract each other, and we have learnt that it makes calcareous spar, alum, and so forth, crystallise in these regular forms. Now, let me gradually lead your minds to a knowledge of the means we possess of making this attraction alter a little in its force; either of increasing, or diminishing, or apparently of destroying it altogether. I will take this piece of iron [a rod of iron about two feet long, and a quarter of an inch in diameter], it has at present a great deal of strength, due to its attraction of cohesion; but if Mr. Anderson will make part of this red-hot in the fire, we shall then find that it will become soft, just as sealing-wax will when heated, and we shall also find that the more it is heated the softer it becomes. Ah! but what does soft mean? Why, that the attraction between the particles is so weakened that it is no longer sufficient to resist the power we bring to bear upon it. [Mr. Anderson handed to the Lecturer the iron rod, with one end red-hot, which he shewed could be easily twisted about with a pair of pliers.] You see, I now find no difficulty in bending this end about as I like; whereas I cannot bend the cold part at all. And you know how the smith takes a piece of iron and heats it, in order to render it soft for his purpose: he acts upon our principle of lessening the adhesion of the particles, although he is not exactly acquainted with the terms by which we express it.
And now we have another point to examine; and this water is again a very good substance to take as an illustration (as philosophers we call it all water, even though it be in the form of ice or steam). Why is this water hard? [pointing to a block of ice] because the attraction of the particles to each other is sufficient to make them retain their places in opposition to force applied to it. But what happens when we make the ice warm? Why, in that case we diminish to such a large extent the power of attraction that the solid substance is destroyed altogether. Let me illustrate this: I will take a red-hot ball of iron [Mr. Anderson, by means of a pair of tongs, handed to the Lecturer a red-hot ball of iron, about two inches in diameter], because it will serve as a convenient source of heat [placing the red-hot iron in the centre of the block of ice]. You see I am now melting the ice where the iron touches it. You see the iron sinking into it, and while part of the solid water is becoming liquid, the heat of the ball is rapidly going off. A certain part of the water is actually rising in steam – the attraction of some of the particles is so much diminished that they cannot even hold together in the liquid form, but escape as vapour. At the same time, you see I cannot melt all this ice by the heat contained in this ball. In the course of a very short time I shall find it will have become quite cold.
Here is the water which we have produced by destroying some of the attraction which existed between the particles of the ice, – for below a certain temperature the particles of water increase in their mutual attraction, and become ice; and above a certain temperature the attraction decreases, and the water becomes steam. And exactly the same thing happens with platinum, and nearly every substance in nature; if the temperature is increased to a certain point, it becomes liquid, and a further increase converts it into a gas. Is it not a glorious thing for us to look at the sea, the rivers, and so forth, and to know that this same body in the northern regions is all solid ice and icebergs, while here, in a warmer climate, it has its attraction of cohesion so much diminished as to be liquid water. Well, in diminishing this force of attraction between the particles of ice, we made use of another force, namely, that of heat; and I want you now to understand that this force of heat is always concerned when water passes from the solid to the liquid state. If I melt ice in other ways, I cannot do without heat (for we have the means of making ice liquid without heat; that is to say, without using heat as a direct cause). Suppose, for illustration, I make a vessel out of this piece of tinfoil [bending the foil up into the shape of a dish]. I am making it metallic, because I want the heat which I am about to deal with to pass readily through it; and I am going to pour a little water on this board, and then place the tin vessel on it. Now if I put some of this ice into the metal dish, and then proceed to make it liquid by any of the various means we have at our command, it still must take the necessary quantity of heat from something, and in this case it will take the heat from the tray, and from the water underneath, and from the other things round about. Well, a little salt added to the ice has the power of causing it to melt, and we shall very shortly see the mixture become quite fluid, and you will then find that the water beneath will be frozen – frozen, because it has been forced to give up that heat which is necessary to keep it in the liquid state, to the ice on becoming liquid. I remember once, when I was a boy, hearing of a trick in a country alehouse; the point was how to melt ice in a quart-pot by the fire, and freeze it to the stool. Well, the way they did it was this: they put some pounded ice in a pewter pot and added some salt to it, and the consequence was, that when the salt was mixed with it, the ice in the pot melted (they did not tell me anything about the salt, and they set the pot by the fire, just to make the result more mysterious), and in a short time the pot and the stool were frozen together, as we shall very shortly find it to be the case here. And all because salt has the power of lessening the attraction between the particles of ice. Here you see the tin dish is frozen to the board – I can even lift this little stool up by it.
Fig. 21.
This experiment cannot, I think, fail to impress upon your minds the fact, that whenever a solid body loses some of that force of attraction by means of which it remains solid, heat is absorbed; and if, on the other hand, we convert a liquid into a solid, e. g., water into ice, a corresponding amount of heat is given out. I have an experiment shewing this to be the case. Here (fig. 21) is a bulb, A, filled with air, the tube from which dips into some coloured liquid in the vessel B. And I dare say you know that if I put my hand on the bulb A, and warm it, the coloured liquid which is now standing in the tube at C will travel forward. Now we have discovered a means, by great care and research into the properties of various bodies, of preparing a solution of a salt15 which, if shaken or disturbed, will at once become a solid; and as I explained to you just now (for what is true of water is true of every other liquid), by reason of its becoming solid, heat is evolved, and I can make this evident to you by pouring it over this bulb; – there! it is becoming solid, and look at the coloured liquid, how it is being driven down the tube, and how it is bubbling out through the water at the end; and so we learn this beautiful law of our philosophy, that whenever we diminish the attraction of cohesion, we absorb heat – and whenever we increase that attraction, heat is evolved. This, then, is a great step in advance, for you have learned a great deal in addition to the mere circumstance that particles attract each other. But you must not now suppose that because they are liquid they have lost their attraction of cohesion; for here is the fluid mercury, and if I pour it from one vessel into another, I find that it will form a stream from the bottle down to the glass – a continuous rod of fluid mercury, the particles of which have attraction sufficient to make them hold together all the way through the air down to the glass itself; and if I pour water quietly from a jug, I can cause it to run in a continuous stream in the same manner. Again, let me put a little water on this piece of plate-glass, and then take another plate of glass and put it on the water; there! the upper plate is quite free to move, gliding about on the lower one from side to side; and yet, if I take hold of the upper plate and lift it up straight, the cohesion is so great that the lower one is held up by it. See how it runs about as I move the upper one! and this is all owing to the strong attraction of the particles of the water. Let me shew you another experiment. If I take a little soap and water – not that the soap makes the particles of the water more adhesive one for the other but it certainly has the power of continuing in a better manner the attraction of the particles (and let me advise you, when about to experiment with soap-bubbles, to take care to have everything clean and soapy). I will now blow a bubble; and that I may be able to talk and blow a bubble too, I will take a plate with a little of the soapsuds in it, and will just soap the edges of the pipe, and blow a bubble on to the plate. Now, there is our bubble. Why does it hold together in this manner? Why, because the water of which it is composed has an attraction of particle for particle, – so great, indeed, that it gives to this bubble the very power of an india-rubber ball; for you see, if I introduce one end of this glass tube into the bubble, that it has the power of contracting so powerfully as to force enough air through the tube to blow out a light (fig. 22) – the light is blown out. And look! see how the bubble is disappearing, see how it is getting smaller and smaller.
Fig. 22. and Fig. 23.
There are twenty other experiments I might shew you to illustrate this power of cohesion of the particles of liquids. For instance, what would you propose to me if, having lost the stopper out of this alcohol bottle, I should want to close it speedily with something near at hand. Well, a bit of paper would not do, but a piece of linen cloth would, or some of this cotton wool which I have here. I will put a tuft of it into the neck of the alcohol bottle, and you see, when I turn it upside down, that it is perfectly well stoppered, so far as the alcohol is concerned; the air can pass through, but the alcohol cannot. And if I were to take an oil vessel, this plan would do equally well, for in former times they used to send us oil from Italy in flasks stoppered only with cotton wool (at the present time the cotton is put in after the oil has arrived here, but formerly it used to be sent so stoppered). Now, if it were not for the particles of liquid cohering together, this alcohol would run out; and if I had time, I could have shewn you a vessel with the top, bottom, and sides altogether formed like a sieve, and yet it would hold water, owing to this cohesion.
You have now seen that the solid water can become fluid by the addition of heat, owing to this lessening the attractive force between its particles, and yet you see that there is a good deal of attractive force remaining behind. I want now to take you another step beyond. We saw that if we continued applying heat to the water (as indeed happened with our piece of ice here), that we did at last break up that attraction which holds the liquid together; and I am about to take some ether (any other liquid would do, but ether makes a better experiment for my purpose), in order to illustrate what will happen when this cohesion is broken up. Now, this liquid ether, if exposed to a very low temperature, will become a solid; but if we apply heat to it, it becomes vapour, and I want to shew you the enormous bulk of the substance in this new form – when we make ice into water, we lessen its bulk, but when we convert water into steam, we increase it to an enormous extent. You see it is very clear that as I apply heat to the liquid I diminish its attraction of cohesion – it is now boiling, and I will set fire to the vapour, so that you may be enabled to judge of the space occupied by the ether in this form by the size of its flame, and you now see what an enormously bulky flame I get from that small volume of ether below. The heat from the spirit-lamp is now being consumed, not in making the ether any warmer, but in converting it into vapour; and if I desired to catch this vapour and condense it (as I could without much difficulty), I should have to do the same as if I wished to convert steam into water and water into ice: in either case it would be necessary to increase the attraction of the particles, by cold or otherwise. So largely is the bulk occupied by the particles increased by giving them this diminished attraction, that if I were to take a portion of water a cubic inch in bulk (A, fig. 23) I should produce a volume of steam of that size, B [1700 cubic inches; nearly a cubic foot], so greatly is the attraction of cohesion diminished by heat; and yet it still remains water. You can easily imagine the consequences which are due to this change in volume by heat – the mighty powers of steam and the tremendous explosions which are sometimes produced by this force of water. I want you now to see another experiment, which will perhaps give you a better illustration of the bulk occupied by a body when in the state of vapour. Here is a substance which we call iodine, and I am about to submit this solid body to the same kind of condition as regards heat that I did the water and the ether [putting a few grains of iodine into a hot glass globe, which immediately became filled with the violet vapour], and you see the same kind of change produced. Moreover, it gives us the opportunity of observing how beautiful is the violet-coloured vapour from this black substance, or rather the mixture of the vapour with air (for I would not wish you to understand that this globe is entirely filled with the vapour of iodine).
If I had taken mercury and converted it into vapour (as I could easily do), I should have a perfectly colourless vapour; for you must understand this about vapours, that bodies in what we call the vaporous, or the gaseous state, are always perfectly transparent, never cloudy or smoky: they are, however, often coloured, and we can frequently have coloured vapours or gases produced by colourless particles themselves mixing together, as in this case [the Lecturer here inverted a glass cylinder full of binoxide of nitrogen16 over a cylinder of oxygen, when the dark-red vapour of hypo-nitrous acid was produced]. Here also you see a very excellent illustration of the effect of a power of nature which we have not as yet come to, but which stands next on our list – Chemical Affinity. And thus you see we can have a violet vapour or an orange vapour, and different other kinds of vapour; but they are always perfectly transparent, or else they would cease to be vapours.
I am now going to lead you a step beyond this consideration of the attraction of the particles for each other. You see we have come to understand that, if we take water as an illustration, whether it be ice, or water, or steam, it is always to be considered by us as water. Well, now prepare your minds to go a little deeper into the subject. We have means of searching into the constitution of water beyond any that are afforded us by the action of heat, and among these one of the most important is that force which we call voltaic electricity, which we used at our last meeting for the purpose of obtaining light, and which we carried about the room by means of these wires. This force is produced by the battery behind me, to which, however, I will not now refer more particularly: before we have done we shall know more about this battery, but it must grow up in our knowledge as we proceed. Now, here (fig. 24) is a portion of water in this little vessel C, and besides the water there are two plates of the metal platinum, which are connected with the wires (A and B) coming outside, and I want to examine that water, and the state and the condition in which its particles are arranged. If I were to apply heat to it, you know what we should get; it would assume the state of vapour, but it would nevertheless remain water, and would return to the liquid state as soon as the heat was removed. Now, by means of these wires (which are connected with the battery behind me, and come under the floor and up through the table), we shall have a certain amount of this new power at our disposal. Here you see it is [causing the ends of the wires to touch] – that is the electric light we used yesterday, and by means of these wires we can cause water to submit itself to this power; for the moment I put them into metallic connection (at A and B), you see the water boiling in that little vessel (C), and you hear the bubbling of the gas that is going through the tube (D). See how I am converting the water into vapour; and if I take a little vessel (E), and fill it with water, and put it in the trough over the end of the tube (D), there goes the vapour ascending into the vessel. And yet that is not steam; for you know that if steam is brought near cold water, it would at once condense, and return back again to water. This then cannot be steam, for it is bubbling through the cold water in this trough; but it is a vaporous substance, and we must therefore examine it carefully, to see in what way the water has been changed. And now, in order to give you a proof that it is not steam, I am going to shew you that it is combustible; for if I take this small vessel to a light, the vapour inside explodes in a manner that steam could never do.
Fig. 24.
I will now fill this large bell-jar (F) with water; and I propose letting the gas ascend into it, and I will then shew you that we can reproduce the water back again from the vapour or air that is there. Here is a strong glass vessel (G), and into it we will let the gas (from F) pass. We will there fire it by the electric spark, and then after the explosion you will find that we have got the water back again: it will not be much, however, for you will recollect that I shewed you how small a portion of water produced a very large volume of vapour. Mr. Anderson will now pump all the air out of this vessel (G); and when I have screwed it on to the top of our jar of gas (F), you will see upon opening the stop-cocks (H´ H H) the water will jump up, shewing that some of the gas has passed into the glass vessel. I will now shut these stop-cocks, and we shall be able to send the electric spark through the gas by means of the wires (I, K) in the upper part of the vessel, and you will see it burn with a most intense flash. [Mr. Anderson here brought a Leyden jar, which he discharged through the confined gas by means of the wires I, K.] You saw the flash; and now that you may see that there is no longer any gas remaining, if I place it over the jar and open the stop-cocks again, up will go the gas, and we can have a second combustion; and so I might go on again and again, and I should continue to accumulate more and more of the water to which the gas has returned. Now, is not this curious? – in this vessel (C) we can go on making from water a large bulk of permanent gas, as we call it, and then we can reconvert it into water in this way. [Mr. Anderson brought in another Leyden jar, which, however, from some cause would not ignite the gas. It was therefore recharged, when the explosion took place in the desired manner.] How beautifully we get our results when we are right in our proceedings! – it is not that Nature is wrong when we make a mistake. Now, I will lay this vessel (G) down by my right hand, and you can examine it by and by: there is not very much water flowing down, but there is quite sufficient for you to see.
Another wonderful thing about this mode of changing the condition of the water is this – that we are able to get the separate parts of which it is composed, at a distance the one from the other, and to examine them, and see what they are like, and how many of them there are; and for this purpose I have here some more water in a slightly different apparatus to the former one (fig. 25), and if I place this in connection with the wires of the battery (at A B), I shall get a similar decomposition of the water at the two platinum plates. Now, I will put this little tube (O) over there, and that will collect the gas together that comes from this side (A), and this tube (H) will collect the gas that comes from the other side (B); and I think we shall soon be able to see a difference. In this apparatus the wires are a good way apart from each other, and it now seems that each of them is capable of drawing off particles from the water and sending them off, and you see that one set of particles (H) is coming off twice as fast as those collected in the other tube (O). Something is coming out of the water there (at H) which burns [setting fire to the gas]; but what comes out of the water here (at O), although it will not burn, will support combustion very vigorously. [The Lecturer here placed a match with a glowing tip in the gas, when it immediately rekindled.]
Fig. 25.
Here, then, we have two things, neither of them being water alone, but which we get out of the water. Water is therefore composed of two substances different to itself, which appear at separate places when it is made to submit to the force which I have in these wires; and if I take an inverted tube of water and collect this gas (H), you will see that it is by no means the same as the one we collected in the former apparatus (fig. 24). That exploded with a loud noise when it was lighted, but this will burn quite noiselessly – it is called hydrogen; and the other we call oxygen– that gas which so beautifully brightens up all combustion, but does not burn of itself. So now we see that water consists of two kinds of particles attracting each other in a very different manner to the attraction of gravitation or cohesion; and this new attraction we call chemical affinity, or the force of chemical action between different bodies. We are now no longer concerned with the attraction of iron for iron, water for water, wood for wood, or like bodies for each other, as we were when dealing with the force of cohesion: we are dealing with another kind of attraction, – the attraction between particles of a different nature one to the other. Chemical affinity depends entirely upon the energy with which particles of different kinds attract each other. Oxygen and hydrogen are particles of different kinds, and it is their attraction to each other which makes them chemically combine and produce water.
I must now shew you a little more at large what chemical affinity is. I can prepare these gases from other substances, as well as from water; and we will now prepare some oxygen. Here is another substance which contains oxygen – chlorate of potash. I will put some of it into this glass retort, and Mr. Anderson will apply heat to it. We have here different jars filled with water; and when, by the application of heat, the chlorate of potash is decomposed, we will displace the water, and fill the jars with gas.
Now, when water is opened out in this way by means of the battery – which adds nothing to it materially, which takes nothing from it materially (I mean no matter; I am not speaking of force), which adds no matter to the water – it is changed in this way: the gas which you saw burning a little while ago, called hydrogen, is evolved in large quantity, and the other gas, oxygen, is evolved in only half the quantity; so that these two areas represent water, and these are always the proportions between the two gases.
But oxygen is sixteen times the weight of the other – eight times as heavy as the particles of hydrogen in the water; and you therefore know that water is composed of nine parts by weight – one of hydrogen and eight of oxygen; thus: —
Now, Mr. Anderson has prepared some oxygen, and we will proceed to examine what is the character of this gas. First of all, you remember, I told you that it does not burn, but that it affects the burning of other bodies. I will just set fire to the point of this little bit of wood, and then plunge it into the jar of oxygen, and you will see what this gas does in increasing the brilliancy of the combustion. It does not burn – it does not take fire as the hydrogen would – but how vividly the combustion of the match goes on. Again, if I were to take this wax taper and light it, and turn it upside down in the air, it would in all probability put itself out, owing to the wax running down into the wick. [The Lecturer here turned the lighted taper upside down, when in a few seconds it went out.] Now, that will not happen in oxygen gas; you will see how differently it acts (fig. 26). [The taper was again lighted, turned upside down, and then introduced into a jar of oxygen.] Look at that! see how the very wax itself burns, and falls down in a dazzling stream of fire, so powerfully does the oxygen support combustion. Again, here is another experiment which will serve to illustrate the force, if I may so call it, of oxygen. I have here a circular flame of spirit of wine, and with it I am about to shew you the way in which iron burns, because it will serve very well as a comparison between the effect produced by air and oxygen. If I take this ring flame, I can shake by means of a sieve the fine particles of iron filings through it, and you will see the way in which they burn. [The Lecturer here shook through the flame some iron filings, which took fire and fell through with beautiful scintillations.] But if I now hold the flame over a jar of oxygen [the experiment was repeated over a jar of oxygen, when the combustion of the filings, as they fell into the oxygen, became almost insupportably brilliant], you see how wonderfully different the effect is in the jar; because there we have oxygen instead of common air.
Fig. 26.
