If the fuel used be wood, or partly wood, it is during its passage through the upper half of the fire-room that its volatile parts are lost, and it becomes converted into charcoal. M. Ebelman ascertained that wood, at the depth of ten feet, in a fire-room twenty-six feet high, preserved its appearance after an exposure for 1 3-4 of an hour, and that the mineral mixed with it preserved its moisture at this depth; but three and a half feet lower, an exposure of 3 1-4 hours reduced the wood to perfect charcoal, and the ore to magnetic oxide. The temperature of the upper half of the fire-room, when wood is used, is lower than in the case of charcoal, from the great amount of heat made latent by the vapor arising from the wood. In the case of bituminous coal, Bunsen and Playfair find that it has to descend still lower before it is perfectly coked. After the wood is completely charred, or the coal become coked, the subsequent changes are the same that happen in the charcoal furnaces. _To be continued._ =ANIMALCULAE IN WATER.= [Illustration:] The fact is generally known that nearly all liquids contain a variety of minute living animals, though in some they are too small for observation, even with a microscope. In others, especially in water that has been long stagnant, these animals appear not only in hideous forms, but with malignant and voracious propensities. The print at the head of this article purports to be a microscopic representation of a single drop of such water, with the various animals therein, and some of the inventors and venders of the various improved filters for the Croton water, would have no objection to the prevalence of the opinion that this water contains all the variety of monsters represented in this cut. But the fact is far otherwise; and it is doubtful whether these animals could frequently be detected in the Croton water, with the best solar microscope. Nevertheless, the fact is readily and clearly established that the Croton water contains a quantity of deleterious matter, which is arrested by the filters; and, on this account, we cheerfully and heartily recommend the adoption of filters by all who use this water, from either the public or private hydrants. To this end we would call the special attention of our city readers to the improved filters noticed under the head of "New Inventions." =Length of Days.=