In reference to "C. M.'s" last paper on the above subject, I have to remark in the first place that if it had not been for the two last sentences in it, I would not have taken any notice of the others. The sentences referred to imply that his object in continuing the discussion is to elicit truth. Now, when writing the paper that has led to the present discussion on heating by hot water, my object was to put before the readers of ' The Gardener,' in as plain and short a manner as I possibly could, what I knew to be the truth on the subject with which I was dealing. "C. M.,"however, thought right to dispute my conclusions, and not in a style that would lead one to suppose his object was to bring out the truth of the matter, but in a way calculated to induce those who have not given much thought to this particular subject to suppose that my statements were wrong, and that he knew them to be so.

When reading "C. M.'s" paper in the April issue of 'The Gardener,' in opposition to mine in that of February, it struck me that he had not given the subject with which he was dealing much thought, and his last paper strengthens that belief. If "C. M." had got any great length in his studies on heating by hot water, he would know, and therefore believe, that opposing currents of hotter and colder water take place, without equalising, in the flow-pipes when they are laid on a continuous rise from the top of the boiler to their furthest points of extension in the compartments to be heated. I note that in his last paper he says, "mixing and equalising to a certain extent at the commencement of circulation;" and this qualification of his ideas on this particular point, as put forth in his first paper, is an evidence that he is making progress in the right direction; and I have no doubt in due time he will get on the "right tack," and throw the equalised, mixed, forcing theory of circulation overboard altogether.

I also observe that he maintains that hot ice "is not at air' a curiosity; but in his endeavour to prove this, he is obliged to depart from the generally accepted meaning of the word "hot," and when he is forced to do so in order to support his argument, it is pretty clear the latter is of an unsound nature. That " ice is not always of the same temperature" is a well-known fact; but, notwithstanding this truth, ice has no sensible heat, and the latter is the right definition of hot, and therefore hot ice would be a curiosity, whatever " C. M." may think or say to the contrary. That " ice is only a transformation of a body " is also true; but " CM." is slightly mistaken when he says, " if that body is converted into its normal condition, water, it expands by the application of heat, like all other bodies." All other bodies, whatever may be their actual temperature, expand if that temperature is raised, and contract if it is lowered. On the other hand, water must reach a temperature of 39 1/2° before expansion commences, so that, until reaching that temperature, water, like its transformation, ice, is an exception to "C. M.'s" rule that "all bodies expand by heat." That " a certain amount of heat becomes latent when ice is converted into water" is also true; but this fact does not assist "C. M." in proving that one particle of water is capable of transmitting heat to an adjoining particle by conduction; neither does it prove that "ice becomes hot when it is being converted into water;" and, moreover, the latent heat of water is of no use for heating our plant-houses, or anything else.

The following experiment will show the amount of sensible heat that becomes latent when ice is converted into water. Take a pound of ice and pound it fine; put it into a wooden vessel of suitable dimensions; pour on to it a pound of water at a temperature of 172°. The result will be two pounds of water - not at a mean temperature between the ice and the hot water, but at a temperature of 32o, thus showing that the ice did not become hot in the process of being converted into water. The ice absorbs the sensible heat, and it disappears somewhere and is lost.

"C. M." tries to make it appear that I admit that water transmits its heat to water, and all other bodies, in the same degree. What I said on this point was as follows: 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. If " C. M." construes this into an admission on my part that his facts are "chiels that winna ding," he is at liberty to do so - at least, so far as I am concerned. I would here, however, point out to "C. M." that misquoting his opponents is not the most likely way of convincing them that they are wrong. "C. M." "fails to see the difference between" the first promoters and the cause of circulation, as the latter takes place in the hot-water apparatus. Perhaps I will fail in making the difference plain to him. I will try, however. Heat and expansion cause the water to become relatively lighter than it was before heat was applied and expansion took place. Cold and contraction cause the water to become relatively heavier than it was before cooling and contracting took place.

Now it requires both agents - heat and cold - to maintain circulation; neither can do so independent of the other. It requires both to bring about the immediate cause - not the primary reason - of the water circulating through the apparatus. In the case of the hot-water apparatus, we make heat and expansion the first promoters of circulation, because we want to heat our plant-houses; but on a hot summer day, if we wanted to cool them, we would make cold, or rather the absence of heat and contraction, the first promoters of circulation, and in place of using fuel under the lowest point of the apparatus, we would use ice of as extreme cold as we could get. By the by, may I ask "C. M." how many degrees below zero does he fix " extreme cold, as the absence of heat?" On the top of the highest point circulation would take place just the same as if combustion was going on below the boiler, which shows that heat and expansion have no more to do with circulation than cold and contraction. I need scarcely point out that "C. M." is mistaken in supposing that expansion "moves in one direction only - that is, towards the highest point of elevation." If the force of expansion was towards the highest point only, it would not be necessary to have the bottom of a steam-boiler as strong as its top.