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Water and fire are the foundation of health preservation. In daily life, if one uses water and fire without examining their benefits and harms, it is sufficient to injure the human body, yet this is precisely what people do not understand. Therefore, the quality of water varies in grades, and the nature of fire has distinctions between good and bad; this is indeed no deception. Let us set aside other matters for now and discuss the water and fire of Beijing. Generally, superior water obtains the qi of Yang, flows clearly with a distant source, has an aromatic odor, and tastes sweet; inferior water obtains the nature of Yin, has a nearby source but turbid flow, has a foul odor, and tastes bitter. The water in the capital consists of two types: sweet water and bitter water. Even the sweet water is not very good, while the bitter water is the most inferior. The reason for the bitter taste of water is that it contains much alkaline substance. If one takes the white frost from walls and ignites it, it burns; the substance contained in the water is exactly this, which is also Puxiao (Mirabilite). Its nature can dissolve the "Five Metals and Eight Minerals," hence named "Xiao" (Nitrate/Salt). Thus, it is good at pushing away stagnation and breaking down hardness. For those with weak Spleen, taking it often causes diarrhea, which is the verification. If one does not have such excess conditions but uses it morning and evening for health preservation, I fear the human organs would face something even more formidable than the Five Metals and Eight Minerals; its latent consumption and hidden depletion would strip away Yuan Qi (Original Qi) without people noticing, which is truly terrifying. Someone might say: Not necessarily. If it were as you say, I have not seen a lack of people in this place, nor have I seen everyone sick here. Why do you worry too much? I reply: Alas, this is precisely what is meant by "unnoticed." We can record the verification through longevity and premature death. In places where water and soil are clean and sweet, people must live long, and those with gray hair and regrown teeth are common everywhere; in regions where water and soil are bitter and inferior, Tian Nian (Heavenly Years/Lifespan) is secretly shortened, and octogenarians or centenarians are rarely seen. Although it is said that lands of longevity may not have all long-lived people, and lands of short life may not have all short-lived people, if strong individuals nourish themselves properly, will they not live longer? If weak individuals have poor diet, will they not die sooner? We cannot generalize about distant places, but comparing the capital with my hometown, the difference in longevity is indeed vast. Given this cause, can we say it is not due to water and soil?

Furthermore, the quality of fire naturally differs, so the ancient Kings used them differently across the four seasons. However, the capital uses coal, which cannot be changed. Although many places use coal, only the coal in the capital has an especially fierce nature, often causing suffocation deaths every year. The reason people cannot avoid it is simply because they do not use it correctly. The climate in the capital is cold, rooms are tightly pasted with paper, people sleep on heated kang beds, and coal is burned mostly indoors. It is precisely the smallest and tightest rooms that are most harmful. Why is this? Because the nature of water flows downward; if it does not drain from below, it fills upward from below. The nature of fire flares upward; if it does not vent from above, it fills downward from above. Therefore, coal poisoning usually occurs after midnight, when the gas gradually fills the room, descends to the nose, closes off respiration, and leads to unconscious death, which is truly lamentable. To avoid this poison, one must inspect the tightest parts of the room, which are extremely dangerous. By leaving a hole in the ceiling grid or tearing several strips of the window paper, the gas will naturally dissipate and not fill downward, thus removing the danger. However, overall, window gaps are inferior to roof vents because they allow faster ventilation. If someone is poisoned, they will likely have closed airways, struggling sounds, and inability to wake up spontaneously. One should quickly call to them, give cool water to drink, and immediate rescue is possible. Alternatively, quickly lay them on the ground so their nose inhales the earth's qi, which can also save them. However, waiting until emergency to treat may lead to delay and ineffectiveness. Is it not better to prevent it in advance? These are the harms of water and fire in the capital; discussing the capital allows inference for other places. Officials traveling to the capital or residents in foreign lands must know these two explanations as the foundation for cherishing themselves.