The Mysteries of the Tea-Kettle

[Percival Leigh and Charles Dickens's "The Mysteries of the Tea-Kettle" appeared in Household Words on 16 November 1850. The essay, which the authors based upon Michael Faraday's own notes for his lecture at the Royal Institute, London, which the scientist shared with them, shows the novelist-editor working with one of his staff-writers to popularize the work of a pioneering scientist. Philip V. Allingham, Contributing Editor, Victorian Web, transcribed the essay and translated it into HTML. Follow for a discussion of the novelist's complex roles as author and editor of Household Words. GPL]

AT one of Mr. Bagges's small scientific tea-parties, Mr. Harry Wilkinson delivered to the worthy gentleman a lecture, based principally on reminiscences of the Royal Institution, and of a series of lectures delivered there, by PROFESSOR FARADAY addressed to children and young people. For it is not the least of the merits of that famous chemist and great man, PROFESSOR FARADAY, that he delights to make the mightiest subject clear to the simplest capacity; and that he shows his mastery of Nature in nothing more than I in being thoroughly imbued with the spirit of her goodness and simplicity.

This particular Lecture was on Natural Philosophy in its bearings on a kettle. The entertainment of a "Night with Mr. Bagges" was usually extemporaneous. It was so on this occasion. The footman brought in the tea-kettle. "Does it boil?" demanded Mr. Bagges.

"It have biled, Sir," answered the domestic.

"Have biled, Sir!" repeated Mr. Bagges. "Have biled! And what if it has 'biled,' or boiled, as I desire you will say in future? What is that to the purpose? Water may be [176/177] frozen, you simpleton, notwithstanding it has boiled. Was it boiling, Sir, eh? when you took it off the fire? That is the question, Sir."

"Yes, Sir, that was what I mean to say, Thomas."

"Mean to say, Sir! Then why didn't you say it, Sir? Eh? There — no, don't put it on, Sir; hold it still. Harry, reach me the thermometer," said Mr. Bagges, putting on his spectacles. "Let me see. The boiling point of water is two hundred and — what?"

"Two hundred and twelve, Fahrenheit," answered Master Wilkinson, "if commonly pure, and boiled in a metallic vessel, and under a pressure of the atmosphere amounting to fifteen pounds on every square inch of surface, or when the barometer stands at thirty inches."

"Gracious, what a memory that boy has!" exclaimed his uncle. "Well; now this water in the kettle — eh? — why, this is not above one hundred and fifty degrees. There, Sir, now set it on the fire, and don't bring me up cold water to make tea with again; or else," added Mr. Bagges, making a vague attempt at a joke, "or else — eh? — you will get yourself into hot water."

Mr. Thomas was seized with a convulsion in the chest, which he checked by suddenly applying his open hand to his mouth, the effort distending his cheeks and causing his eyes to protrude in a very ridiculous manner, whilst Mr. Bagges, disguised his enjoyment of the effects of his wit in a cough.

"Now let me see," said the old gentleman, musingly contemplating the vessel simmering on the fire; "how is it, eh, Harry, you said the other day that a kettle boils?"

"La!" interrupted Mrs. Wilkinson, who was of the party, "why, of course, by the heat of the coals, and by blowing the fire, if it is not hot enough."

"Aha!" cried her brother, "that's not the way we account for things, Harry, my boy, eh? Now, convince your mother; explain the boiling of a kettle to her: come."

"A kettle boils," said Harry," by means the action of currents."

"What are you talking about? Boiling a plum-pudding in a tea-kettle!" exclaimed the mystified mamma.

"Currents of heated particles — of particles of hot water," Harry explained. "Suppose you put your fire on your kettle — on the lid of it — instead of your kettle on your fire,— what then?"

"You would be a goose," said his mother.

"Exactly so —or a gosling," — rejoined her son; "the kettle would not boil. Water is a bad conductor of heat. Heat passes through the substance of water with very great difficulty. Therefore, it would have a hard matter to get from the top of a kettle of water to the bottom. Then how does it so easily get from the bottom to the top?"

Ah!" sighed Mr. Bagges. "In my young days we should have said, because the heat rises but that won't do now. What is all that about the — eh — what — law of ex — what — pansion — eh?"

"The law of expansion of fluids and gases by heat. This makes the currents that I spoke of just now, mamma; and I should have spelt the word to explain to you that I didn't mean plums. You know what a draught is?"

"I am sorry to say I do." If Mr. Bagges declared with much seriousness, instinctively carrying his hand to the region of the human body from the Latin for which is derived the term, Lumbago.

"Well," pursued Harry, "a draught is a current of air. Such currents are now passing up the chimney, and simply owing to that trifling circumstance, we are able to sit here now without being stifled and poisoned."

"Goodness!" ejaculated Mrs. Wilkinson.

"To be sure. The fire, in burning, turns into gases, which are rank poison — carbonic acid, for one; sulphurous acid, for another. Hold your nose over a shovelful of hot cinders if you doubt the fact. The gases produced by the fire expand; they increase in bulk without getting heavier, so much so that they become lighter in proportion than the air, and then they rise, and this rising of hot air is what is meant by heat going upwards. The currents of hot air that go up the chimney in this way have currents of cold air rushing after them to supply their place. When you heat water, currents are formed just as when you heat gas or air. The heated portion of the water rises, and some colder water comes down in its place; and these movements of the water keep going on till the whole bulk of it is equally hot throughout."

"Well, now," interrupted Mr. Bagges, "I dare say this is all very true, but how do you prove it?"

"Prove that water is heated by the rising and falling of hot currents? Get a long, slender glass jar. Put a little water, coloured with indigo, or anything you like, into the bottom of it. Pour clear water upon the coloured, gently, so as not to mix the two, and yet nearly to fill the jar. Float a little spirit of wine on the top of the water, and set fire to it. Let it blaze away as long as you like; the coloured water will remain steady at the bottom of the jar. But hold the flame of a spirit-lamp under the jar, and the coloured water will rise and mix with the clear, in very little time longer than it would take you to say Harry Wilkinson."

"Ah! So the water gets coloured throughout for the same reason that it gets heated throughout," Mr. Bagges observed," and when it gets thoroughly hot — what then?"

"Then it boils. And what is boiling?"

"Bubbling," suggested the young philosopher's mamma.

"Yes; but ginger-beer bubbles," said Harry, "but you wouldn't exactly call that boiling. [177/178] Boiling is the escaping of steam. That causes the bubbling; so the bubbling of water over the fire is only the sign that the water boils. But what occasions the escape of the steam?"

"The heat, of course — the — what is the right word? — the caloric," answered Mr. Bagges.

"True; but what heat? Why, the excess of heat over two hundred and twelve degrees — taking that as the average boiling point of water. You can heat water up to that point, and it remains water; but every degree of heat you cause to pass into it above that, turns a quantity of the water into steam; and flies off in the steam, unless the steam is hindered from escaping by extraordinary pressure. Blow the fire under that kettle as much as you will, and you will make the water boil faster, but you won't make it a bit hotter than two hundred and twelve degrees."

"Well, to be sure!" Mrs. Wilkinson exclaimed.

"If water," continued Harry, "could keep on getting hotter and hotter above the boiling point, why, we might have our potatoes charred in the pot, or our mutton boiled to a cinder. When water is confined in a strong vessel — and strong it must be to prevent a tremendous blow-up — confined, I say, so that no steam can escape, it may be heated almost red-hot ; and there is a vessel made for heating water under pressure, called Papin's Digestor, which will digest almost anything."

"What an enviable apparatus," exclaimed Mr. Bagges.

"Well," resumed Harry; "so the boiling point of water depends on the degree of force which the air or what not is pressing on its surface with. The higher the spot on which you boil your water, the lower the point it boils at. Therefore, water boiling at the top of a mountain is not so hot as water boiling at the mountain's base. The boiling point of water on the summit of Mont Blanc, is as low as one hundred and eighty-four degrees. So, if water must be at two hundred and twelve degrees, to make good tea, don't choose too high a hill to build a temperance hall on. The heavier, also, the air is, from the quantity of moisture in it, the hotter water becomes before it boils. If the atmosphere were carbonic acid gas, water would get much hotter without boiling than it can under

"Present arrangements," interposed Mr. Bagges.

"Consisting of a mixture of nitrogen and oxygen," continued Harry. "Water requires only a very low heat to make it boil in an exhausted receiver, out of which the air has been pumped, so as to leave none to press upon its surface. Owing to boiling depending upon pressure, you can actually make water boil by means of cold."

"What next?" sighed Mrs. Wilkinson.

"You can, indeed. Put a little boiling water in a salad-oil flask; so that the flask may be a quarter full, say. Cork the flask tightly. The boiling stops; and the upper three-fourths of the flask are full of vapour. Squirt a jet of ice-cold water upon the flask, above where the water is, and the water below will instantly begin to boil. The reason why, is this. The vapour in the flask presses on the surface of the hot water. The cold condenses the vapour — turns it back to water. That takes off the pressure for the time ; and then the hot water directly flies into vapour, and boils, and so on, till it cools down too low to boil any longer. What reduces the boiling point of water on a hill or a mountain is, that the pressure of the atmosphere decreases as you ascend. A rise of five hundred and thirty feet in height above the level of the sea, makes a difference of one degree; so, give me a kettle of water and a thermometer, and I'll tell you exactly how near the moon we are."

"I shouldn't think one could make good hot mixed punch up in a balloon, now," observed Mr. Bagges, reflectively.

"Then," Harry proceeded, "it requires more heat to make water boil in a glass vessel than it does in a metal one. A metal vessel's inner surface is made up of very small points and dents. Scratching the inside of the glass so as to give it a roughness something like what the metal has, makes the boiling point lower; and a few iron filings thrown into water boiling in glass at two hundred and fourteen degrees, will bring it down to two hundred and twelve. The filings, and the roughness of the glass, are so many more points for the heat to pass into the water from, and form steam, and the water does not cling to them so hard as it Clings to a smooth surface. Throw a lot of hay into a pan of hot water, and it makes a quantity of steam rise directly; and I have heard a doctor say that some poor people are in the habit of giving themselves cheap steam-baths by this means."

"A very good thing for rheumatic pairs, I should think; certainly a much more rational remedy than patent medicines or Government poison," Mr. Bagges remarked.

"There are some salts," continued Harry, "which, if dissolved in water, will prevent it from boiling till it is heated to two hundred and sixty-four degrees, as if they held the water back from flying into steam. So, then, the boiling of water maybe hindered, more or less, by pressure from without, and attraction from within. The boiling point of water depends on another important fact which the kettle always mentions before it boils, although we don't all of us understand the kettle's language. The singing of the kettle tells us" —

"That the water is going to boil," interrupted mamma.

"Yes, and that water contains air. The singing of the kettle is the noise made by the escape of the air, which is driven off by the heat. The air sticks and hangs in the water, till the heat expands it and makes it rise. Put a glass of water under the receiver of an air-pump, and exhaust the receiver. As you pump, the water begins to bubble, as if it were boiling; but the bubbles are the air contained in the water, being pumped out. The air-bubbles act like wedges between the little invisible drops that make up the whole water. If it were not for them, the water would be a mass which would hold together so hard that it would not go into steam, or boil, till it was heated to two hundred and seventy degrees, as may be proved by boiling some water quite deprived of air. And not only that, but when it did boil, it would boil all at once, and blow up with a tremendous explosion; which would be a still greater inconvenience in boiling a kettle."

"A pretty kettle of fish, indeed!" Mr. Bagges observed.

"So," said Harry, "strictly pure water would not be quite so great a blessing to us as you might think. Of course, you know, uncle, I don't mean to say that there is any advantage in the impurity of such water as the Thames, except when used for the purpose of fertilising the earth. I am speaking of water so pure as to contain no air. Water of such severe purity would be very unmanageable stuff. No fishes could live in it, for one thing. I have already given you one good reason why it would be unsuitable to our kettle; and another is, that it would not be good to drink. Then water, as we find it in the world, has a very useful and accommodating disposition to find its own level. Pump all the air out of water, however, and it loses this obliging character in a great measure. Suppose I take a bent glass tube, and fill one arm. of it with airless water. Then I turn the tube mouth upwards, and if the water were common water, it would instantly run from one arm into the other, and stand at the same level in both. But if the water has been exhausted of its air, it remains, most of it, in the one arm, and won't run till I give the table a smart rap, and shake it. So, but for the air contained in water, we could not make the water run up and down hill as we do. If water were deprived of air, London would be almost deprived of water."

"And water," observed Mr. Bagges, "would be robbed of a very valuable property."

"Good again," uncle. "Now, if we could see through the kettle, we should be able to observe the water boiling in it, which is a curious sight when looked into. To examine water boiling, we must boil the water in a glass vessel — a long tube is the best — heated with a spirit lamp. Then first you see the water in motion, and the air bubbles being driven off by the heat. As the water gets hotter, other bubbles appear, rising from the bottom of the tube. They go up for a little way and then they shrink, and by the time they get to the top of the water, you can hardly distinguish them. These are bubbles of steam, and they get smaller as they rise, because at first the water is colder above than below in proportion to the distance from the flame, and the cold gradually condenses the bubbles. But when the water gets thoroughly hot, the bubbles grow larger and rise quicker, and go of the same size right up to the top of the water, and there escape — if you choose to let them. And steam was allowed to escape so for many many ages, wasn't it uncle, before it was set to work to spin cotton for the world, and take us to America within a fortnight, and whirl us over the ground as the crow flies, and almost at a crow's pace."

"For all which," remarked Mr. Bagges, "we have principally to thank what's his name."

"Watt was his name, I believe, uncle. Well; heat turns water into steam, and I dare say I need not tell you that a quantity of water becoming steam, fills an immense deal more space than it did as mere water. Cold turns the steam back into water, and the water fills the some space as it did before. Water, in swelling into steam and shrinking back into water again, moves, of course, twice, and mighty motions these are, and mighty uses are made of them, I should rather think."

"I believe you, my boy," said Mr. Bagges.

"And now," asked Harry, "have you any idea of what a deal of heat there is in steam?"

"It is hot enough to scald you," answered his mamma, "I know that."

"Yes; and hot enough, too, to cook potatoes. But there is much more heat in it than that comes to. Take a kettle of cold water. See at what degree the thermometer stands in the water. Put the kettle on the fire and observe how long it takes to boil. It will boil at two hundred and twelve degrees; and therefore, during the time it has taken to boil, there has gone into it the difference of heat between two hundred and twelve degrees and the degree it stood at when first put on the fire. Keep up the same strength of fire, so that the heat may continue to go into the water at the same rate. Let the water boil quite away, and note how long it is in doing so. You can then calculate how much heat has gone into the water while the water has been boiling away. You will find that quantity of heat quantity enough to have made the water red-hot, if all the water, and all the heat, had remained in the kettle. But the water in your kettle will have continued at two hundred and twelve degrees to the last drop, and all the steam that it has turned into will not have been hotter — according to the thermometer — than two hundred and twelve degrees; whereas a red heat is one thousand degrees. The difference between two hundred [179/180] and twelve degrees and one thousand degrees is seven hundred and eighty-eight degrees; and what has become of all this heat? Why, it is entirely contained in the steam, though it does not make the steam hotter. It lies hid in the steam, and therefore it is called latent heat. When the steam is condensed, all that latent heat comes out of it, and can be felt, and the quantity of it can be measured by a thermometer. The warmth that issues from steam-pipes used to warm a house, is the latent heat of the steam that escapes as the steam turns back to water."

"Latent heat! latent heat!" repeated Mr. Bagges, scratching his head. "Eh? Now, that latent heat always puzzles me. Latent, lying hid. But how can you hide heat? When the zany in the pantomime hides the red-hot poker in his pocket, he cauterises his person. How — eh? — how can heat be latent?"

"Why, the word heat has two meanings, uncle. In the first place, it means hotness. Hotness cannot be latent, as the clown finds when he pockets the poker. In the second place, heat means a something the nature of which we don't know, which is the cause of hotness, and also the cause of another effect. Whilst it is causing that other effect, it does not cause hotness. That other effect which heat causes in the instance of steam, is keeping water in the form of steam. The heat that there is in steam, over and above two hundred and twelve degrees, is employed in this way. It is wholly occupied in preserving the water in an expanded state, and can't cause the mercury in the thermometer to expand and rise as well. For the same reason, it could give you no feeling of hotness above what boiling water would — if you had the nerve to test it. Whilst it is making steam continue to be steam, it is latent. When the steam becomes water again, it has no longer that work to do, and is set free. Free heat is what is commonly understood by heat. This is the heat which cooks our victuals, the heat we feel, the heat that singes Mr. Merriman. Latent heat is heat that doesn't warm, singe, or cook, because it is otherwise engaged. If you press gas suddenly into a fluid, the latent heat of the gas is set free. You seem to squeeze it out. Indeed, the same thing happens, if you violently force any substance into a closer form all at once. Everything appears to have more or less latent heat in it, between its little particles, keeping them at certain distances from each other. Compress the particles within a smaller compass, and a part of the latent heat escapes, as if it were no longer wanted. When a substance in a compressed state expands on a sudden, it draws in heat, on the other hand. When a lady bathes her forehead with eau-de-Cologne to cure a headache, the heat of the head enters the eau-de-Cologne, and becomes latent in it whilst it evaporates. If you make steam under high pressure, you can heat it much above two hundred and twelve degrees. Suppose you let off steam, so compressed and heated by a wide hole, from the boiler, and put your hand into it as it rushes out" —

"What? Why, you'd scald your hand off!" cried Mr. Bagges.

"No, you wouldn't. The steam rushes out tremendously hot, but it expands instantly so very much, that the heat in it directly becomes latent in a great measure; which cools it down sufficiently to allow you to hold your hand in it without its hurting you. But then you would have to mind where you held your hand; because where the steam began to condense again, it would be boiling hot."

"I had rather take your word for the experiment than try it, take your word for the experiment than try it, young gentleman," Mr. Bagges observed.

"Another very curious thing," proceeded Harry, "in regard to boiling, has been discovered lately. A kettle might be too hot to boil water in. Take a little bar of silver, heated very highly; dip it into water. At first, you have no boiling, and you don't have any at all till the silver has cooled some degrees. Put a drop of water into a platinum dish, heated in the same way, and it will run about without boiling till the heat diminishes; and then it bursts into steam. M. Boutigny, the French chemist, made this discovery. Vapour forms between the drop of water and the red-hot metal, and, being a bad conductor of heat, keeps the heat of the metal for some time from flowing into the water. Owing to this, water, and mercury even, may be frozen in a red-hot vessel if the experiment is managed cleverly. A little more than a couple of centuries ago, this would have been thought witchcraft."

"And the philosopher," added Mr. Bagges, would have been fried instead of his water-drop. Let me see — eh? — what do they call this singular state of water?"

"The spheroidal state," answered Harry. "However, that is a state that water does not get into in a kettle, because kettles are not allowed to become red hot, except when they are put carelessly on the fire with no water in them, or suffered to remain there after the water has boiled quite away!"

"Which is ruination to kettles," Mrs. Wilkinson observed.

"Of course it is, mamma, because at a red heat iron begins to unite with oxygen, or to rust. Another thing that injures kettles is the fur that collects in them. All water in common use contains more or less of earthy and other salts. In boiling, these things separate from the water, and gradually form a fur or crust inside the kettle or boiler."

"And a nice job it is to get rid of it," said his mamma. "Well; chemistry has lessened that difficulty," replied Harry. "The fur is mostly carbonate of lime. In that case, all you have [180/181] to do is boil some sal-ammoniac — otherwise muriate, or more properly hydrochlorate of ammonia — in the furred vessel. The hydrochloric acid unites with the lime, and the carbonic acid goes to the ammonia. Both the compounds formed in this way dissolve and wash away; and so you may clean the foulest boiler or kettle. This is a rather important discovery; for the effect of fur in a kettle is to oppose the passage of heat, and therefore to occasion the more fuel to be required to boil water in it, which is a serious waste and expense when you have a large steam-boiler to deal with. Dr. Faraday mentions the case of a Government steamer that went to Trieste, and during the voyage had so much fur formed in her boiler as to oblige all her coal to be consumed, and then the engineers were forced to burn spars, rigging, bulkheads, and even chopped cables, and to use up every shaving of spare timber in the ship. Soot underneath the kettle, as well as fur inside it, is a hindrance to boiling, as it is a bad conductor; and that is the reason why one can bear to hold a kettle of hot water, which is very sooty on its under surface, on the flat of the hand. So a black kettle doesn't give out its heat readily to what touches it, and so far it is good to keep water hot; but it gets rid of heat in another way; as I dare say you know, uncle."

"Eh?" said Mr. Bagges, "why, what? — no —I did know something about it the other day — but I've such a memory! — and — eh? — no — I've quite forgotten it."

"By radiation, you know. All warm bodies are constantly giving off rays of heat, as shining ones are giving off rays of light, although the heat — rays are invisible."

"How do we know that?" asked Mr. Bagges.

"Get a couple of concave mirrors — a sort of copper basins, polished inside. Stand them face to face, some yards apart. Put a hot iron ball — not red hot — in the focus of one mirror. Put a bit of phosphorus in the focus of the other. The phosphorus will take fire; though without the mirrors you might place it much nearer the hot iron, and yet it would not burn. So we know that there are rays of heat, because we can reflect them as we can rays of light. Some things radiate better than others. Those that have bright metal surfaces radiate worst, though such are what are used for reflectors. If their surfaces are blackened or roughened, they radiate better. A bright kettle gives off fewer rays of heat than a black one, and so far, is better to keep water hot in. But then, on the other hand, it yields more heat to the air, or the hob or hearth that it stands upon — if colder than itself. The bright kettle gives off heat in one way and the black in another. I don't know at what comparative rate exactly."

"Six of one, and half-a-dozen of the other," Mr. Bagges suggested.

"Now look at the wonderful relations of the kettle, uncle!"

"Relations? — Eh? — what the pot and the saucepan?" said Mr. Bagges.

"Oh, oh, uncle! No; its relations to the pressure of the atmosphere and every cause that affects it — to the conveyance, and conduction and radiation of heat — to latent heat or caloric, to the properties of water, to chemical decomposition —and to steam and its astonishing marvels, present and to come!"

"Well," said Mr. Bagges, "it is wonderful; and the kettle certainly is very respectably connected. Eh? And I hope to profit by the subject of our conversation; and so, I say, pour me out a cup of tea." [ends on page 181.]

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References

Leigh, Percival, and Charkes Dickens. "The Mysteries of the Tea-Kettle."Household Words (16 November 1850): 176-181.

Last modified 24 July 2023