水火者，养生之本。日用之物，用水火而不察其利病，则适足以伤人，而实人所不知也
。故水品分差等，火性言优劣，固非欺我者也。姑无论其他，试以燕京之水火言之。凡水之
佳者，得阳之气，流清而源远，气香而味甘；水之劣者，得阴之性，源近而流浊，气秽而味
苦。而京师之水则有两种，曰甜水，曰苦水是也。即其甜者亦未甚佳，而其苦者乃为最劣。
盖水之味苦者，以其多碱。试取墙间白霜，火之皆燃，水中所有，即此物也。即朴硝也。其
性则五金八石皆能消化，因而命名曰硝。故善于推荡积滞，攻破 坚。凡脾弱之人服之
多泄，是所验也。使无其实，而朝夕用之以养生。吾恐人之脏腑，有更非五金八石之可比，
其为潜消暗耗，剥人元气于罔觉之中，大有可畏者。或曰∶未必然，果若所云，则吾未见斯
地之乏人，亦未见斯地之皆病，何子之过虑也？予曰∶噫，此正所谓罔觉也。请以寿夭而纪
其验，则水土清甘之处，人必多寿，而黄发儿齿者，比比皆然；水土苦劣之乡，暗折天年，
而耄耋期颐者，目不多见。虽曰寿乡未必全寿，夭乡未必皆夭。若以强者而滋养得宜，岂不
更寿？弱者而饮食不佳，岂不更夭？远者不能概知，第以京师较之吾乡，则其寿夭之殊，不无
大有径庭矣。职此之由，谓非水土之使然与？
又若火之良否，原自不同，故先王取用，四时有异。惟是京师用煤，必不可易。虽用煤
之处颇多，而惟京师之煤，气性尤烈，故每熏人至死，岁岁有之，而人不能避者无他，亦以
用之不得其法耳。夫京师地寒，房室用纸密糊，人睡火炕，煤多热于室内。惟其房之最小而
最密者，最善害人。其故何也？盖以水性流下，下而不泄，则自下满而上；火性炎上，上而
不泄，则自上满而下。故凡煤毒中人者，多在夜半之后，其气渐满，下及人鼻，则闭绝呼吸
，昧然长逝，良可慨悯。凡欲避其毒者，惟看房室最密之所，极为可虑。但于顶格开留一窍
，或于窗纸揭开数楞，则其气自透去，不能下满，乃可无虑矣。然总之窗隙不如顶核，为其
透气之速也。设有中其毒者，必气闭声挣，不能自醒，速当呼之，饮以凉水，立可解救。或
速令仆地，使其鼻吸地气，亦可解救。然待其急而救疗，恐有迟误而无济于事。孰若预有以
防之为愈也。此京师水火之害，举京师而言，则他处可以类推矣。凡宦游京国及客处异地者
，不可不知此二说，以为自珍之本。
水火是养生的根本。日常生活中使用水火，如果不考察其利弊，就足以伤害人体，而这正是人们所不了解的。因此，水的品质有优劣之分，火的性质也有好坏之别，这确实不是欺骗我们的话。暂且不论其他，试着以北京城的水火来说明。凡是优质的水，得自阳气，水流清澈且源头长远，气味芳香而味道甘甜；劣质的水，得自阴气，源头近便且水流浑浊，气味腥臭而味道苦涩。而京城的水主要有两种，一种是甜水，一种是苦水。即使是甜水也不算很好，而苦水则是最为低劣的。大概水味发苦的原因，是因为含有较多的碱性物质。试着取墙壁上的白霜，用火点燃都会燃烧，水中含有的就是这个东西，也就是朴硝（芒硝）。它的性质能够消化五金八石等矿物，因此命名为“硝”。所以它善于推荡体内的积滞，攻破坚硬的结块。凡是脾胃虚弱的人服用后大多会腹泻，这就是验证。如果并没有这种实证，却早晚用它来养生，我恐怕人的脏腑，会有比五金八石更难以承受的情况，它在不知不觉中潜消暗耗，剥蚀人的元气，这是非常令人畏惧的。有人说：未必如此，如果真的像您所说，那么我没见过这个地方人口匮乏，也没见过这里的人都生病，为什么您这么过度忧虑呢？我说：唉，这正是所谓的“不知不觉”。可以用寿命长短来记录其验证，那么在水土清洁甘美的地方，人一定长寿，头发变白、牙齿再生的老人，到处都是；在水土苦涩低劣的地方，暗中折损天命之年，到了八九十岁甚至百岁高龄的人，眼睛很少见到。虽然说是长寿之乡未必所有人都长寿，短命之乡未必所有人都短命。但如果身体强壮的人滋养得当，难道不会更长寿吗？身体虚弱的人饮食不佳，难道不会更短命吗？远处的情形不能一概知晓，仅拿京城与我家乡相比，那么他们寿命长短的差异，确实有很大的差距了。由于这个原因，能说不是水土造成的吗？
另外关于火的好坏，原本就有不同，所以先王在选用时，四季都有差异。只是京城使用煤炭，是不可改变的。虽然用煤的地方很多，但只有京城的煤，气性特别猛烈，所以常常熏死人，每年都有发生，而人们无法躲避的原因没有其他，也是因为使用的方法不得当罢了。京城地寒，房屋用纸严密糊住缝隙，人睡火炕，煤炭多在室内燃烧发热。正因为房间最小且最密闭的地方，最善于害人。这是什么缘故呢？因为水性向下流动，如果在下方不排泄，就会从下向上充满；火性炎上，如果在上方不排泄，就会从上向下充满。所以凡是煤炭毒气中人，多发生在半夜之后，毒气逐渐充满，下到人的鼻子，就会闭绝呼吸，昏迷长逝，实在令人感慨怜悯。凡是想要躲避这种毒气的人，只有查看房间最密闭的地方，极为值得忧虑。只要在房顶格间留出一个孔窍，或者将窗纸揭开几条缝隙，那么毒气自然透散出去，不会向下充满，就可以没有忧虑了。然而总之，窗户缝隙不如屋顶通气孔，因为屋顶透气更快。如果有中毒的人，一定是气息闭塞、声音挣扎，不能自己醒来，应当迅速呼唤他，喂饮凉水，立刻可以解救。或者迅速让他趴在地上，使他的鼻子吸入地气，也可以解救。然而等到紧急情况才去救治，恐怕会有延误而无济于事。哪里比得上预先防备更好呢。这就是京城水火的危害，举京城而言，其他地方可以类推了。凡是做官游历京城以及客居异地的人，不可不知道这两个说法，作为珍重自身的根本。
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.