Allow me to reply as briefly as I can to Mr Hammond's article in the ' Gardener' for May, where he criticises my letter in the preceding number.

In answer to my question, Why water transmitted its heat to ice? he says, "I may remark that hot ice would be a curiosity." If I understand Mr Hammond's definition of hot to mean, to raise the temperature of, I may reply that ice becomes hot when it is being converted into water, which is not at all a "curiosity." I consider extreme cold as the absence of heat; and if the temperature of a body is raised by the application of heat, that body becomes hot according to his definition, although the general acceptation of the word means sensible heat. He thinks it is impossible to heat ice by any means. But I have to inform him that ice is not always at the same temperature. "Ice," he says," is an exception to the rule that all bodies expand by heat." Bub I must remind him that ice is only a transformation of a body, and if that body is converted to its normal condition, water, it expands by the application of heat, like all other bodies. His test of the conducting power of water is to drop a ball of ice into hot water for a few minutes, then take it out, and put what remains unthawed amongst cold water, and because the ice does not raise the temperature of the water (which must, according to the law of nature, already be at a higher temperature than the water), but has "an opposite effect," he "thus proves that the particles of water are incapable of transmitting heat to each other by conduction." But from their "having the opposite effect" we may infer they are capable of conducting cold - or in other words, that it has lowered the temperature of the water, which proves most eftectually that the heat has been absorbed by the ice.

I will also here inform Mr Hammond that there is a certain amount of heat become latent when ice is converted into water, which he can prove for himself by the following simple experiment. Procure a uniform source of heat, and fill a vessel with ice below 32° Fahr., having a thermometer placed in it. Apply the heat and the temperature will rise to 32°. At that point it will remain until all the ice is converted into water - that is, seven-ninths of the time that the same source of heat is required to raise the temperature of the water from 32° to the boiling point, 212°. He truly says, "Facts are chiels that winna ding;" but he thinks-my statement concerning the transmitting power of water is only "an assertion," and takes it to mean "that water parts with its heat to all bodies in the same degree," which is "true," he says. Now I fail to see the difference. If water parts with its heat to water, "or to all bodies in the same degree," I cannot see that it is of any material difference whether we call it parting or transmitting. His admission furnishes conclusive proof that my facts are "chiels that winna ding" by any false logic.

The material with which water comes in contact has got nothing to do with the transmitting power of water; that depends upon the conducting power of the material itself. He "wonders how wooden pipes would act in heating plant-houses. If it is a fact that water transmits heat to all bodies in the same degree, wooden pipes should heat our plant-houses equally as well as iron." He forgets that wood is a bad conductor; and supposing he had iron (which, he admits, is a good conductor) covered with wood, the wood would still be a bad conductor, but that would not alter the conducting power of the iron. I wish it to be understood that I do not consider water to be a good conductor. That is one of the reasons why it is of so much service in heating plant-houses, by parting with its heat slowly. Then he says, " It is also a mistake to suppose that expansion is the cause of circulation." Then further on he says, "Heat and expansion are the first promoters of circulation." I fail to see the difference between the promoter and the cause here. He says, expansion is from the centre and acts equally in all directions - it therefore cannot cause the water to move in one direction only. " But though heat and expansion are the promoters," he says, "neither is the cause, they are only agents.

That is the difference of the specific gravity of the water at different points of the apparatus." But what is the cause of that difference? Is it not the expansion which, instead of moving equally in all directions, moves in one direction only - that is, towards the highest point of elevation. The difference of the specific gravity of the two volumes of water, then, is the consequent result of expansion, which must be the primary cause.

In reference to what Mr Hammond is pleased to "distinguish" as my "mixed, equalised, forcing theory, "I cannot agree with him that there is a distinct line drawn between hot and cold water in the same pipe, either vertically or horizontally. Therefore I must adhere to my statement, that mixing and equalising must go on to a certain extent, at the commencement of circulation, until the water reaches the highest point of elevation, from whence it will return by its own specific gravity. Neither can I believe that there could be a return-current of cold water in the flow without being affected and inter-cepted by the warm current proceeding direct from the boiler while the legitimate course was open for it by the return. He does not think it is "true" that hotter and lighter water can force colder and heavier water on an uphill course, certainly not without mixing with it to a certain extent. I shall endeavour to prove that such does take place. It must either be forced or drawn uphill. Now, to prove that it is not drawn: there are generally airpipes at the highest point of elevation, and before the heavy column in the return would draw up the column in the flow, the air would rush in and fill up its place, which he can prove by a siphon.

The only inference he can draw from my statement, that the highest point of the structure will be the hottest, is, "that it suffers no diminution of heat until it reaches that point." It is needless to refer further to such an absurdity. Nevertheless, it is a well-known fact that a house situated above the level of the others is the hottest from its having a quicker circulation, which proves the value of elevation. An imperfect circulation would take place by Mr Hammond's principle of construction on a small scale; but on an extensive system it would result in disappointment, where there are pipes branching from the main flow to houses at different points of the apparatus. On his principle there is neither a vertical nor gradient rise from the main flow to the houses to induce the water to circulate, consecpiently the circulation would principally take place in the main flow and return, which are not unfrequently under outside paths. It is unnecessary to advert to the fact that the rapidity with which water circulates is in proportion to the elevation and the difference of the weight of the two columns of water, which has already been ably treated of by Mr Makenzie last month.

I do not wish to enter into a mere conflict of words with Mr Hammond, which, although it may affect to search after truth, in reality only raises such a dust that we are apt to lose sight of the subject. But with a single eye to truth, I have not the least objection. C. M.