212 THE GARDENER. [May 



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 conduc- 

 tor of heat, is the material most used in the construction of the heating appar- 

 atus. 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 ex- 

 pansive 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 expan- 

 sion 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 con- 

 traction 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 pro- 

 ceeds from the top of the boiler, cannot descend without mixing and equalis- 

 ing 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 foicing the cold before it." 

 As this theory of circulation differs from all others tuat 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 



