Kitabı oku: «On the various forces of nature and their relations to each other», sayfa 4

LECTURE IV.
CHEMICAL AFFINITY – HEAT

We shall have to pay a little more attention to the forces existing in water before we can have a clear idea on the subject. Besides the attraction which there is between its particles to make it hold together as a liquid or a solid, there is also another force, different from the former – one which, yesterday, by means of the voltaic battery, we overcame, drawing from the water two different substances – which, when heated by means of the electric spark, attracted each other, and rushed into combination to reproduce water. Now, I propose to-day to continue this subject, and trace the various phenomena of chemical affinity; and for this purpose, as we yesterday considered the character of oxygen, of which I have here two jars (oxygen being those particles derived from the water which enable other bodies to burn), we will now consider the other constituent of water; and, without embarrassing you too much with the way in which these things are made, I will proceed now to shew you our common way of making hydrogen. (I called it hydrogen yesterday – it is so called because it helps to generate water.)¹⁷ I put into this retort some zinc, water, and oil of vitriol, and immediately an action takes place, which produces an abundant evolution of gas, now coming over into this jar, and bubbling up in appearance exactly like the oxygen we obtained yesterday.

Fig. 27.

The processes, you see, are very different, though the result is the same, in so far as it gives us certain gaseous particles. Here, then, is the hydrogen. I shewed you yesterday certain qualities of this gas; now let me exhibit you some other properties. Unlike oxygen, which is a supporter of combustion, and will not burn, hydrogen itself is combustible. There is a jar full of it; and if I carry it along in this manner, and put a light to it, I think you will see it take fire, not with a bright light – you will at all events hear it, if you do not see it. Now, that is a body entirely different from oxygen: it is extremely light; for although yesterday you saw twice as much of this hydrogen produced on the one side as on the other, by the voltaic battery, it was only one-eighth the weight of the oxygen. I carry this jar upside-down. Why? Because I know that it is a very light body, and that it will continue in this jar upside-down quite as effectually as the water will in that jar which is not upside-down; and just as I can pour water from one vessel into another in the right position to receive it, so can I pour this gas from one jar into another when they are upside-down. See what I am about to do. There is no hydrogen in this jar at present, but I will gently turn this jar of hydrogen up under this other jar (fig. 28), and then we will examine the two. We shall see, on applying a light, that the hydrogen has left the jar in which it was at first, and has poured upwards into the other, and there we shall find it.

Fig. 28.

You now understand that we can have particles of very different kinds, and that they can have different bulks and weights; and there are two or three very interesting experiments which serve to illustrate this. For instance, if I blow soap bubbles with the breath from my mouth, you will see them fall, because I fill them with common air, and the water which forms the bubble carries it down. But now, if I inhale hydrogen gas into my lungs (it does no harm to the lungs, although it does no good to them), see what happens. [The Lecturer inhaled some hydrogen, and after one or two ineffectual attempts, succeeded in blowing a splendid bubble, which rose majestically and slowly to the ceiling of the theatre, where it burst.] That shews you very well how light a substance this is; for, notwithstanding all the heavy bad air from my lungs, and the weight of the bubble, you saw how it was carried up. I want you now to consider this phenomenon of weight as indicating how exceedingly different particles are one from the other; and I will take as illustrations these very common things – air, water, the heaviest body, platinum, and this gas: and observe how they differ in this respect; for if I take a piece of platinum of that size (fig. 29), it is equal to the weight of portions of water, air, and hydrogen of the bulks I have represented in these spheres. And this illustration gives you a very good idea of the extraordinary difference with regard to the gravity of the articles having this enormous difference in bulk. [The following tabular statement having reference to this illustration appeared on the diagram board.]

Fig. 29

Whenever oxygen and hydrogen unite together they produce water; and you have seen the extraordinary difference between the bulk and appearance of the water so produced, and the particles of which it consists chemically. Now, we have never yet been able to reduce either oxygen or hydrogen to the liquid state; and yet their first impulse, when chemically combined, is to take up first this liquid condition, and then the solid condition. We never combine these different particles together without producing water; and it is curious to think how often you must have made the experiment of combining oxygen and hydrogen to form water without knowing it. Take a candle, for instance, and a clean silver spoon (or a piece of clean tin will do), and if you hold it over the flame, you immediately cover it with dew – not a smoke – which presently evaporates. This perhaps will serve to shew it better. Mr. Anderson will put a candle under that jar, and you will see how soon the water is produced (fig. 30). Look at that dimness on the sides of the glass, which will soon produce drops, and trickle down into the plate. Well, that dimness and these drops are water, formed by the union of the oxygen of the air with the hydrogen existing in the wax of which that candle is formed.

Fig. 30.

And now, having brought you in the first place to the consideration of chemical attraction, I must enlarge your ideas so as to include all substances which have this attraction for each other – for it changes the character of bodies, and alters them in this way and that way in the most extraordinary manner, and produces other phenomena wonderful to think about. Here is some chlorate of potash, and there some sulphuret of antimony.¹⁸ We will mix these two different sets of particles together; and I want to shew you in a general sort of way some of the phenomena which take place when we make different particles act together. Now, I can make these bodies act upon each other in several ways. In this case I am going to apply heat to the mixture; but if I were to give a blow with a hammer, the same result would follow. [A lighted match was brought to the mixture, which immediately exploded with a sudden flash, evolving a dense white smoke.] There you see the result of the action of chemical affinity overcoming the attraction of cohesion of the particles. Again, here is a little sugar¹⁹, quite a different substance from the black sulphuret of antimony, and you shall see what takes place when we put the two together. [The mixture was touched with sulphuric acid, when it took fire and burnt gradually, and with a brighter flame than in the former instance.] Observe this chemical affinity travelling about the mass, and setting it on fire, and throwing it into such wonderful agitation!

I must now come to a few circumstances which require careful consideration. We have already examined one of the effects of this chemical affinity; but to make the matter more clear we must point out some others. And here are two salts dissolved in water²⁰. They are both colourless solutions, and in these glasses you cannot see any difference between them. But if I mix them, I shall have chemical attraction take place. I will pour the two together into this glass, and you will at once see, I have no doubt, a certain amount of change. Look, they are already becoming milky, but they are sluggish in their action – not quick as the others were – for we have endless varieties of rapidity in chemical action. Now, if I mix them together, and stir them, so as to bring them properly together, you will soon see what a different result is produced. As I mix them, they get thicker and thicker, and you see the liquid is hardening and stiffening, and before long I shall have it quite hard; and before the end of the lecture it will be a solid stone – a wet stone, no doubt, but more or less solid – in consequence of the chemical affinity. Is not this changing two liquids into a solid body a wonderful manifestation of chemical affinity?

There is another remarkable circumstance in chemical affinity, which is, that it is capable of either waiting or acting at once. And this is very singular, because we know of nothing of the kind in the forces either of gravitation or cohesion. For instance, here are some oxygen particles, and here is a lump of carbon particles. I am going to put the carbon particles into the oxygen; they can act, but they do not – they are just like this unlighted candle. It stands here quietly on the table, waiting until we want to light it. But it is not so in this other case. Here is a substance, gaseous like the oxygen, and if I put these particles of metal into it, the two combine at once. The copper and the chlorine unite by their power of chemical affinity, and produce a body entirely unlike either of the substances used. And in this other case, it is not that there is any deficiency of affinity between the carbon and oxygen; for the moment I choose to put them in a condition to exert their affinity, you will see the difference. [The piece of charcoal was ignited, and introduced into the jar of oxygen, when the combustion proceeded with vivid scintillations.]

Now, this chemical action is set going exactly as it would be if I had lighted the candle, or as it is when the servant puts coals on and lights the fire: the substances wait until we do something which is able to start the action. Can anything be more beautiful than this combustion of charcoal in oxygen? You must understand that each of these little sparks is a portion of the charcoal, or the bark of the charcoal, thrown off white-hot into the oxygen, and burning in it most brilliantly, as you see. And now let me tell you another thing, or you will go away with a very imperfect notion of the powers and effects of this affinity. There you see some charcoal burning in oxygen. Well, a piece of lead will burn in oxygen just as well as the charcoal does, or indeed better; for absolutely that piece of lead will act at once upon the oxygen as the copper did in the other vessel with regard to the chlorine. And here also a piece of iron: if I light it and put it into the oxygen, it will burn away just as the carbon did. And I will take some lead, and shew you that it will burn in the common atmospheric oxygen at the ordinary temperature. These are the lumps of lead which, you remember, we had the other day – the two pieces which clung together. Now these pieces, if I take them to-day and press them together, will not stick; and the reason is, that they have attracted from the atmosphere a part of the oxygen there present, and have become coated as with a varnish by the oxide of lead, which is formed on the surface by a real process of combustion or combination. There you see the iron burning very well in oxygen; and I will tell you the reason why those scissors and that lead do not take fire whilst they are lying on the table. Here the lead is in a lump, and the coating of oxide remains on its surface; whilst there you see the melted oxide is clearing itself off from the iron, and allowing more and more to go on burning. In this case, however [holding up a small glass tube containing lead pyrophorus.²¹, the lead has been very carefully produced in fine powder, and put into a glass tube, and hermetically sealed, so as to preserve it; and I expect you will see it take fire at once. This has been made about a month ago, and has thus had time enough to sink down to its normal temperature. What you see, therefore, is the result of chemical affinity alone. [The tube was broken at the end, and the lead poured out on to a piece of paper, whereupon it immediately took fire.] Look, look at the lead burning; why, it has set fire to the paper! Now, that is nothing more than the common affinity always existing between very clean lead and the atmospheric oxygen; and the reason why this iron does not burn until it is made red-hot is, because it has got a coating of oxide about it, which stops the action of the oxygen – putting a varnish, as it were, upon its surface, as we varnish a picture, absolutely forming a substance which prevents the natural chemical affinity between the bodies from acting.

I must now take you a little further in this kind of illustration – or consideration, I would rather call it – of chemical affinity. This attraction between different particles exists also most curiously in cases where they are previously combined with other substances. Here is a little chlorate of potash, containing the oxygen which we found yesterday could be procured from it. It contains the oxygen there combined and held down by its chemical affinity with other things; but still it can combine with sugar, as you saw. This affinity can thus act across substances; and I want you to see how curiously what we call combustion acts with respect to this force of chemical affinity. If I take a piece of phosphorus and set fire to it, and then place a jar of air over the phosphorus, you see the combustion which we are having there on account of chemical affinity (combustion being in all cases the result of chemical affinity). The phosphorus is escaping in that vapour, which will condense into a snow-like mass at the close of the lecture. But suppose I limit the atmosphere, what then? why, even the phosphorus will go out. Here is a piece of camphor, which will burn very well in the atmosphere, and even on water it will float about and burn, by reason of some of its particles gaining access to the air. But if I limit the quantity of air by placing a jar over it, as I am now doing, you will soon find the camphor will go out. Well, why does it go out? Not for want of air, for there is plenty of air remaining in the jar. Perhaps you will be shrewd enough to say, for want of oxygen.

This, therefore, leads us to the inquiry as to whether oxygen can do more than a certain amount of work. The oxygen there (fig. 30) cannot go on burning an unlimited quantity of candle, for that has gone out, as you see; and its amount of chemical attraction or affinity is just as strikingly limited: it can no more be fallen short of or exceeded than can the attraction of gravitation. You might as soon attempt to destroy gravitation, or weight, or all things that exist, as to destroy the exact amount of force exerted by this oxygen. And when I pointed out to you that 8 by weight of oxygen to 1 by weight of hydrogen went to form water, I meant this, that neither of them would combine in different proportions with the other; for you cannot get 10 of hydrogen to combine with 6 of oxygen, or 10 of oxygen to combine with 6 of hydrogen – it must be 8 of oxygen and 1 of hydrogen. Now, suppose I limit the action in this way: this piece of cotton wool burns, as you see, very well in the atmosphere; and I have known of cases of cotton-mills being fired as if with gunpowder, through the very finely-divided particles of cotton being diffused through the atmosphere in the mill, when it has sometimes happened that a flame has caught these raised particles, and it has run from one end of the mill to the other, and blown it up. That, then, is on account of the affinity which the cotton has for the oxygen; but suppose I set fire to this piece of cotton, which is rolled up tightly, it does not go on burning, because I have limited the supply of oxygen, and the inside is prevented from having access to the oxygen, just as it was in the case of the lead by the oxide. But here is some cotton which has been imbued with oxygen in a certain manner. I need not trouble you now with the way it is prepared; it is called gun-cotton.²² See how that burns [setting fire to a piece]; it is very different from the other, because the oxygen that must be present in its proper amount is put there beforehand. And I have here some pieces of paper which are prepared like the gun-cotton²³, and imbued with bodies containing oxygen. Here is some which has been soaked in nitrate of strontia – you will see the beautiful red colour of its flame; and here is another which I think contains baryta, which gives that fine green light; and I have here some more which has been soaked in nitrate of copper – it does not burn quite so brightly, but still very beautifully. In all these cases the combustion goes on independent of the oxygen of the atmosphere. And here we have some gunpowder put into a case, in order to shew that it is capable of burning under water. You know that we put it into a gun, shutting off the atmosphere, with shot, and yet the oxygen which it contains supplies the particles with that without which chemical action could not proceed. Now, I have a vessel of water here, and am going to make the experiment of putting this fuse under the water, and you will see whether that water can extinguish it. Here it is burning out of the water, and there it is burning under the water; and so it will continue until exhausted, and all by reason of the requisite amount of oxygen being contained within the substance. It is by this kind of attraction of the different particles one to the other that we are enabled to trace the laws of chemical affinity, and the wonderful variety of the exertions of these laws.

Now, I want you to observe that one great exertion of this power, which is known as chemical affinity, is to produce HEAT and light. You know, as a matter of fact, no doubt, that when bodies burn they give out heat; but it is a curious thing that this heat does not continue – the heat goes away as soon as the action stops, and you see thereby that it depends upon the action during the time it is going on. It is not so with gravitation: this force is continuous, and is just as effective in making that lead press on the table as it was when it first fell there. Nothing occurs there which disappears when the action of falling is over; the pressure is upon the table, and will remain there until the lead is removed; whereas, in the action of chemical affinity to give light and heat, they go away immediately the action is over. This lamp seems to evolve heat and light continuously; but it is owing to a constant stream of air coming into it on all sides, and this work of producing light and heat by chemical affinity will subside as soon as the stream of air is interrupted. What, then, is this curious condition of heat? Why is the evolution of another power of matter, of a power new to us, and which we must consider as if it were now for the very first time brought under our notice? What is heat? We recognise heat by its power of liquefying solid bodies and vaporising liquid bodies, by its power of setting in action, and very often overcoming, chemical affinity. Then, how do we obtain heat? We obtain it in various ways – most abundantly by means of the chemical affinity we have just before been speaking about; but we can also obtain it in many other ways. Friction will produce heat. The Indians rub pieces of wood together until they make them hot enough to take fire; and such things have been known as two branches of a tree rubbing together so hard as to set the tree on fire. I do not suppose I shall set these two pieces of wood on fire by friction; but I can readily produce heat enough to ignite some phosphorus. [The Lecturer here rubbed two pieces of cedar-wood strongly against each other for a minute, and then placed on them a piece of phosphorus, which immediately took fire.] And if you take a smooth metal button stuck on a cork, and rub it on a piece of soft deal wood, you will make it so hot as to scorch wood and paper, and burn a match.

Fig. 31.

I am now going to shew you that we can obtain heat, not by chemical affinity alone, but by the pressure of air. Suppose I take a pellet of cotton and moisten it with a little ether, and put it into a glass tube (fig. 31), and then take a piston and press it down suddenly, I expect I shall be able to burn a little of that ether in the vessel. It wants a suddenness of pressure, or we shall not do what we require. [The piston was forcibly pressed down, when a flame, due to the combustion of the ether, was visible in the lower part of the syringe.] All we want is to get a little ether in vapour, and give fresh air each time, and so we may go on again and again getting heat enough by the compression of air to fire the ether-vapour.

This, then, I think, will be sufficient, accompanied with all you have previously seen, to shew you how we procure heat. And now for the effects of this power. We need not consider many of them on the present occasion, because when you have seen its power of changing ice into water and water into steam, you have seen the two principal results of the application of heat. I want you now to see how it expands all bodies – all bodies but one, and that under limited circumstances. Mr. Anderson will hold a lamp under that retort, and you will see the moment he does so that the air will issue abundantly from the neck, which is under water, because the heat which he applies to the air causes it to expand. And here is a brass rod (fig. 32) which goes through that hole, and fits also accurately into this gauge; but if I make it warm with this spirit-lamp, it will only go in the gauge or through the hole with difficulty; and if I were to put it into boiling-water, it would not go through at all. Again, as soon as the heat escapes from bodies they collapse. See how the air is contracting in the vessel, now that Mr. Anderson has taken away his lamp: the stem of it is filling with water. Notice, too, now, that although I cannot get the tube through this hole or into the gauge, the moment I cool it by dipping it into water, it goes through with perfect facility; so that we have a perfect proof of this power of heat to contract and expand bodies.

Fig. 32.