This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
We will now look into C. M.'s paper.
In reference to my statement that the particles of water are unable to transmit heat to one another, C. M. asks, if this be so, "How does it come to pass that water poured upon ice imparts its heat to it, the ice being then a solid?" In reference to this question, I may remark that hot ice would be a curiosity. And the question implies, that by pouring hot water upon ice the latter becomes hot. I think, however, that it is impossible to heat ice either by pouring hot water on it or by any other means. When in contact with a hotter medium ice does not expand, and C. M. says "all bodies expand by heat." Ice, however, is an exception to this rule, as, under the influence of heat, the expanded particles of water of which ice is formed contract until reaching a certain point, when they resume their liquid condition and are no longer ice, the contracting and melting being confined to the external particles, those in the interior not being affected by the thawing of those on the exterior of the mass.
To test the correctness of this, drop a ball of ice into a vessel containing hot water, let it remain for three or four minutes, then take out what remains unthawed of the ball and drop it into a vessel containing cold water, and the remaining portion of the ball of ice just taken from the hot water will not raise the temperature of the cold, but will have an opposite effect, - thus proving that solid particles of water are unable to transmit heat to each other by conduction. And C. M. says solids transmit heat "by conduction only." Now as the particles of water in a solid state, in the form of ice, are unable to heat one another by conduction, the most reasonable inference is that they are unable to do so in their liquid state. The next matter claiming attention is where C. M. says, "the fact is, water transmits heat to water in the same degree as it does to any other body." Well, "facts are chiels that winna ding." Assertions, however, are not always facts, and we will consider the above as only an assertion, and take it to mean that water parts with beat to all bodies in the same degree.
It is true water parts with heat to all other substances with which it comes in contact that are of a lower temperature than itself, but the degree of rapidity with which it does so is measured by the conducting power of the material acted on; thus iron, being a good conductor of heat, is the material most used in the construction of the heating apparatus. I wonder how wooden pipes would act in warming our plant-houses? If it is a fact that water transmits heat to all bodies in the same degree, then wooden pipes should heat our plant-houses equally as well as those of iron. Again, C. M. assumes that, supposing the particles of water transmitted heat to each other in the same way as those of solid bodies, "it would make no difference whatever" to the circulation of the water in the pipes, as " the expansive properties of the water would remain the same." Now I venture to think the expansive properties would be considerably lessened. It is owing to the inability of the particles of water to heat one another that their expansion is so much greater than what takes place in solids when exposed to the same degree of heat. It is also a mistake to suppose that expansion is the cause of circulation.
Expansion is from the centre, and acts with equal force in all directions - it therefore cannot cause the water to move in one direction only; and that is what takes place in a properly-adjusted heating apparatus. Heat and expansion are the first promoters of circulation, but cold and contraction are as much concerned in the continuance of the process. Neither, however, is the cause, they are only the agents by which the cause is brought about - that is the difference in the specific gravity of the volumes of water at different points of the apparatus, which is the cause of the water circulating in the pipes.
C. M. next says, "If it were possible to apply sufficient heat to liquefy bodies, (I presume he means solid bodies), circulation would take place in the same manner." Well, there is no use in discussing impossibilities. I may remark, however, that C. M.'s impossibles are possibles, and vice versa. We could use for a boiler a blast-furnace. In it the "bodies " would liquefy - the difficulty would be about circulation. Yes, circulation and not liquefication would be impossible, unless we turned the whole apparatus into a blast-furnace, and cultivated those fabulous fire-eating animals called salamanders, instead of fruits and flowers. Then, to prove that I am wrong about the return-current in the flows when they are fixed upon an ascending scale from the boiler, C. M. gives what he no doubt considers the right theory of the circulation or movements of the water in the boiler and pipes. According to his theory circulation occurs in the following way: "The water in the boiler, on being heated, expands, consequently it then becomes lighter than the water in the pipes, hence its tendency to rise.
And as the water from the flow, which proceeds from the top of the boiler, cannot descend without mixing and equalising the temperature of both, therefore the water from the return, which is situated at the bottom, rushes in and gets heated likewise, and continues to expand and ascend the flow-pipe, equalising and forciug the cold before it." As this theory of circulation differs from all others that I have either heard or read about, I propose to distinguish it by calling it the equalised mixed forcing theory. It supposes that the hotter and relatively lighter water ascends from the boiler to the flow-pipes, which is true, that the colder water in the flow will not descend to the boiler, and that the heated water on entering the flow commences a twofold action - that is, equalising the temperature, and at the same time forcing the cold before it, which I venture to think is not true. The hotter and relative lighter water cannot force the colder and heavier water before it on an uphill course. Still C. M. thinks the above theory of circulation is correct, and in the closing sentence of his paper says that "That part of the structure which is situated farthest from the boiler will be the hottest, the pipes being at the highest elevation; " which goes far to prove that a continuous rise in the pipes does not hinder circulation, but the reverse.
The only inference that can be drawn from this sentence is, that the water on its journey from the boiler to the highest and farthest point of the apparatus suffers no diminution, but, on the contrary, increases in temperature. This may be so where the apparatus is fitted up on equalised mixed forcing principles. The mixing required to equalise the temperature of the hotter and colder water, and the force required on the part of the equalised water to drive the cold water before it uphill, may have the effect of generating heat, so that the water on reaching the point highest and farthest from the boiler may be hotter than when it left the point on which the fire acts. I think, however, that C. M. is mistaken in thinking so; but if he can prove that it is so, then, ye inventors of fuel-economising boilers, your occupation is gone, and coal bills will no longer vex the gardener! For if the water gets hotter after entering the flows, then all that will be required will be a little fuel to start it out of the boiler in the first instance, the mixing and equalising will do the remainder.
And if the water parts with no heat until it reaches the highest point of the apparatus, as indicated in C. M.'s paper, would it not be best to have this point as near the boiler as possible? By having it so, the mixing, equalising, and forcing would be reduced to a minimum, and consequently a more rapid circulation of the water in the pipes would be the effect.
J. Hammond. Brayton Hall.