But why is it not necessary to continue applying caloric throughout the process of combustion, in order to keep up the electric energy of the wood, which is required to enable it to combine with the oxygen? MRS. B. The caloric which is gradually produced by the two electricities during combustion, keeps up the temperature of the burning body; so that when once combustion has begun, no further application of caloric is required. CAROLINE. Since I have learnt this wonderful theory of combustion, I cannot take my eyes from the fire; and I can scarcely conceive that the heat and light, which I always supposed to proceed entirely from the coals, are really produced as much by the atmosphere. EMILY. When you blow the fire, you increase the combustion, I suppose, by supplying the coals with a greater quantity of oxygen gas? MRS. B. Certainly; but of course no blowing will produce combustion, unless the temperature of the coals be first raised. A single spark, however, is sometimes sufficient to produce that effect; for, as I said before, when once combustion has commenced, the caloric disengaged is sufficient to elevate the temperature of the rest of the body, provided that there be a free access of oxygen. It however sometimes happens that if a fire be ill made, it will be extinguished before all the fuel is consumed, from the very circumstance of the combustion being so slow that the caloric disengaged is insufficient to keep up the temperature of the fuel. You must recollect that there are three things required in order to produce combustion; a combustible body, oxygen, and a temperature at which the one will combine with the other. EMILY. You said that combustion was one method of decomposing the atmosphere, and obtaining the nitrogen gas in its simple state; but how do you secure this gas, and prevent it from mixing with the rest of the atmosphere? MRS. B. It is necessary for this purpose to burn the body within a close vessel, which is easily done. --We shall introduce a small lighted taper (PLATE VII. Fig. 1.) under this glass receiver, which stands in a bason over water, to prevent all communication with the external air. [Illustration: Plate VII. Vol. I. p. 181. Fig. 1. Combustion of a taper under a receiver. Fig. 2. A Retort on a stand. Fig. 3. Preparation of oxygen gas. A Furnace. B Earthen Retort in the furnace. C Water bath. D Receiver. E.E Tube conveying the gas from the Retort through the water into the Receiver. F.F.F Shelf perforated on which the Receiver stands. Fig. 4. Combustion of iron wire in oxygen gas.] CAROLINE. How dim the light burns already! --It is now extinguished. MRS. B. Can you tell us why it is extinguished? CAROLINE.