The question is, indeed, not very easy to answer; but I conceive that the general carbonisation of calcareous matter may have been the effect of a general combustion, occasioned by some revolution of our globe, and producing an immense supply of carbonic acid, with which the calcareous matter became impregnated; or that this may have been effected by a gradual absorption of carbonic acid from the atmosphere. --But this would lead us to discussions which we cannot indulge in, without deviating too much from our subject. EMILY. How does it happen that we do not perceive the pernicious effects of the carbonic acid which is floating in the atmosphere? MRS. B. Because of the state of very great dilution in which it exists there. But can you tell me, Emily, what are the sources which keep the atmosphere constantly supplied with this acid? EMILY. I suppose the combustion of wood, coals, and other substances, that contain carbon. MRS. B. And also the breath of animals. CAROLINE. The breath of animals! I thought you said that this gas was not at all respirable, but on the contrary, extremely poisonous. MRS. B. So it is; but although animals cannot breathe in carbonic acid gas, yet, in the process of respiration, they have the power of forming this gas in their lungs; so that the air which we _expire_, or reject from the lungs, always contains a certain proportion of carbonic acid, which is much greater than that which is commonly found in the atmosphere. CAROLINE. But what is it that renders carbonic acid such a deadly poison? MRS. B. The manner in which this gas destroys life, seems to be merely by preventing the access of respirable air; for carbonic acid gas, unless very much diluted with common air, does not penetrate into the lungs, as the windpipe actually contracts and refuses it admittance. --But we must dismiss this subject at present, as we shall have an opportunity of treating of respiration much more fully, when we come to the chemical functions of animals. EMILY. Is carbonic acid as destructive to the life of vegetables as it is to that of animals? MRS. B. If a vegetable be completely immersed in it, I believe it generally proves fatal to it; but mixed in certain proportions with atmospherical air, it is, on the contrary, very favourable to vegetation. You remember, I suppose, our mentioning the mineral waters, both natural and artificial, which contain carbonic acid gas? CAROLINE. You mean the Seltzer water? MRS. B. That is one of those which are the most used; there are, however, a variety of others into which carbonic acid enters as an ingredient: all these waters are usually distinguished by the name of _acidulous_ or _gaseous mineral waters_.