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In like manner when two fluids of any kind； of different densities； come into contact； or are mixed with each other； that which is the lightest will be forced upwards by that which is the heaviest。
And as heat rarefies all bodies； fluids as well as solids； air as well as water； or mercury；it follows that two portions of the same fluid； at different temperatures； being brought into contact with each other； that portion which is the hottest being more rarefied or specifically LIGHTER than that which is colder； must be forced upwards by this last。And this is what always happens in fact。
When hot water and cold water are mixed； the hottest part of the mixture will be found to be at the surface above；and when cold air is admitted into a warmed room； it will always be found to take its place at the bottom of the room； the warmer air being in part expelled； and in part forced upwards to the top of the room。
Both air and water being transparent and colourless fluids； their internal motions are not easily discovered by the sight； and when these motions are very slow； they make no impression whatever on any of our senses； consequently they cannot be detected by us without the aid of some mechanical contrivance： But where we have reason to think that those motions exist； means should be sought； and may often be found； for rendering them perceptible。
If a bottle containing hot water tinged with log…wood； or any other colouring drug； be immersed； with its mouth open； and upwards； into a deep glass jar filled with cold water； the ascent of the hot water from the bottle through the mass of cold water will be perfectly visible through the glass。 Now nothing can be more evident than that both of these fluids are forced； or PUSHED； and not DRAWN upwards。Smoke is frequently said to be drawn up the Chimney；and that a Chimney draws well； or ill；but these are careless expressions； and lead to very erroneous ideas respecting the cause of the ascent of Smoke； and consequently tend to prevent the progress of improvements in the management of fires。The experiment just mentioned with the coloured water is very striking and beautiful； and it is well calculated to give a just idea of the cause of the ascent of Smoke。 The cold water in the jar； which； in consequence of its superior weight or density； forces the heated and rarefied water in the bottle to give place to it； and to move upwards out of its way； may represent the cold air of the atmosphere； while the rising column of coloured water will represent the column of Smoke which ascends from a fire。
If Smoke required a Chimney to DRAW it upwards； how happens it that Smoke rises from a fire which is made in the open air； where there is no Chimney？
If a tube； open at both ends； and of such a length that its upper end be below the surface of the cold water in the jar； be held vertically over the mouth of the bottle which contains the hot coloured water； the hot water will rise up through it； just a smoke rises in a Chimney。
If the tube be previously heated before it is plunged into the cold water； the ascent of the hot coloured water will be facilitated and accelerated； in like manner as Smoke is known to rise with greater facility in a Chimney which is hot； than in one in which no fire has been made for a long time。But in neither of these cases can it； with any propriety； be said； that the hot water is DRAWN up the tube。The hotter the water in the bottle is； and the colder that in the jar； the greater will be the velocity with which the hot water will be forced up through the tube； and the same holds of the ascent of hot Smoke in a Chimney。When the fire is intense； and the weather very cold； the ascent of the Smoke is very rapid； and under such circumstances Chimneys seldom smoke。
As the cold water of the jar immediately surrounding the bottle which contains the hot water； will be heated by the bottle while the other parts of the water in the jar will remain cold； this water so heated； becoming specifically lighter than that which surrounds it； will be forced upwards； and if it finds its way into the tube will rise up through it with the coloured hot water。The warmed air of a room heated by an open Chimney Fire…place has always a tendency to rise； （if I may use that inaccurate expression；） and finding its way into the Chimney frequently goes off with the Smoke。
What has been said； will； I flatter myself； be sufficient to explain and illustrate； in a clear and satisfactory manner； the cause of the ascent of Smoke； and just ideas upon that subject are absolutely necessary in order to judge； with certainty； of the merit of any scheme proposed for the improvement of Fire…places； or to take effectual measures； in all cases； for curing smoking Chimnies。For though the perpetual changes and alterations which are produced by accident； whim； and caprice； do sometimes lead to useful discoveries； yet the progress of improvement under such guidance must be exceedingly slow； fluctuating； and uncertain。
As to the causes of the smoking of Chimnies； they are very numerous； and various； but as a general idea of them may be acquired from what has already been said upon that subject in various parts of this Essay； and as they may； in all cases； （a very few only excepted；） be completely remedied by making the alterations in Fire…places here pointed out； I do not think it necessary to enumerate them all in this place； or to enter into those long details and investigations which would be required to show the precise manner in which each of them operates； either alone； or in conjunction with others。
There is however one cause of smoking Chimnies which I think it is necessary to mention more particularly。In modern built houses； where the doors and windows are generally made to close with such accuracy that no crevice is left for the passage of the air from without； the Chimnies in rooms adjoining to each other； or connected by close passages； are frequently found to affect each other； and this is easy to be accounted for。When there is a fire burning in one of the Chimnies； as the air necessary to supply the current up the Chimney where the fire burns cannot be had in sufficient quantities from without； through the very small crevices of the doors and windows； the air in the room becomes rarefied； not by heat； but by subtraction of that portion of air which is employed in keeping up the fire； or supporting the combustion of the fuel； and in consequence of this rarefaction； its elasticity is diminished； and being at last overcome by the pressure of the external air of the atmosphere； this external air rushes into the room by the only passage left for it； namely； by the open Chimney of the neighbouring room：And the flow of air into the Fire…place； and up the Chimney where the fire is burning being constant； this expence of air is supplied by a continued current down the other Chimney。
If an attempt be made to light fires in both Chimnies at the same time； it will be found to be very difficult to get the fires to burn； and the rooms will both be filled with Smoke。
One of the fires；that which is made in the Chimney where the construction of the Fire…place is best adapted to facilitate the ascent of the Smoke；or if both Fire…places are on the same construction；that which has the wind most favourable； or in which the fire happens to be soonest kindled；will overcome the other； and cause its Smoke to be beat back into the room by the cold air which descends through the Chimney。The most obvious remedy in this case is to provide for the supply of fresh air necessary for keeping up the fires by opening a passage for the external air into the room by a shorter road than down one of the Chimnies； and when this is done； both Chimnies will be found to be effectually cured。
But Chimnies so circumstanced may very frequently be prevented from smoking even without opening any new passage for the external air； merely by diminishing the draught； （as it is called；） up the Chimnies； which can best be done by altering both Fire…places upon the principles recommended and fully explained in the foregoing Chapters of this Essay。
Should the doors and windows of a room be closed with so much nicety as to leave no crevices by which a supply of air can enter sufficient for maintaining the fire； AFTER THE CURRENT OF AIR UP THE CHIMNEY HAS BEEN DIMINISHED AS MUCH AS POSSIBLE BY DIMINISHING THE THROAT OF THE FIRE…PLACE； in that case there would be no other way of preventing the Chimney from smoking but by opening a passage for the admission of fresh air from without；but this； I believe； will very seldom be found to be the case。
A case more frequently to be met with is where currents of air set down Chimnies in consequence of a diminution and rarefaction of the air in a room； occasioned by the doors of the room opening into passages or courts where the air is rarefied by the action of some particular winds。 In such cases the evil may be remedied； either by causing the doors in question to close more accurately；or； （which will be still more effectual；） by giving a supply of air to the passage or court which wants it； by some other way。
Where the top of a Chimney is commanded by high buildings； by clifts； or by high ground