Calomelas (mercurous chloride, Hg2Cl2), also named 汞粉 or 水银粉
Nature and flavour. Acrid, cold; classed as toxic. Lung and Large Intestine systems.
A white crystalline sublimate of mercurous chloride — mercury that has been made into a salt and condensed. Classical texts describe it as attacking toxin, killing parasites and astringing sores, and it is recorded externally for scabies, ringworm, weeping and itching skin complaints, and the sores of the traditional account of syphilis; internally it is recorded for oedema with constipation and for expelling worms. The internal use belongs to the historical record and to a period that did not know what mercury does. Calomel is also worth recognising as a genuinely global drug: it was a mainstay of nineteenth-century Western medicine as a purgative and as a treatment for syphilis, taken in quantities that would now be unthinkable, and mercurialism from it — tremor, ulcerated mouth, loss of teeth — was described at the time. Calomel also appeared in teething powders, and the resulting illness in children, acrodynia or pink disease, is one of the clearer instances of a paediatric poisoning identified from its pattern rather than from a label.
性味归经。辛，寒；有毒。归肺、大肠经。
氯化亚汞的白色结晶升华物 —— 就是把汞变成盐再冷凝下来。古籍记载攻毒杀虫、收湿敛疮，外用治疥疮、癣、渗液瘙痒的皮肤病，以及传统所论梅毒的疮疡；内服记载用于水肿兼便秘与驱虫。内服用途属于历史记载，也属于那个还不知道汞能做什么的年代。轻粉也确实值得作为一味世界性药物来认识：它在十九世纪西方医学里是通便剂与梅毒治疗的主力，服用量之大今天难以想象；当时已经描写过由它引起的汞中毒 —— 震颤、口腔溃烂、牙齿脱落。轻粉也曾出现在婴幼儿出牙粉中，由此在儿童身上造成的疾病（肢端疼痛症，又称红皮病）是儿科中毒史上从症状模式而非从标签识别出病因的较清晰例子之一。
一种汞化合物，凡汞制剂适用的注意事项此处一条不减。它经皮吸收，粉尘或蒸气则经肺吸收，在体内蓄积，最终落在神经系统与肾上。儿童比成人更敏感 —— 肢端疼痛症（手足疼痛发红、烦躁、发育停滞）正是历史上儿童接触轻粉的特征表现，这也是此药及同类药物在有儿童的家庭中毫无位置的原因。不得内服，也不作自用。不宜大面积、用于破损皮肤或反复使用，不要在密闭敷料下长时间接触。绝不可近火或加热。孕期与哺乳期及肾病患者避免使用。它在光照下会变暗，该变化反映的是向金属汞与溶解度更高的汞盐分解 —— 陈旧变色者已非新品的同一物品，这也是不要使用来源不明药材的又一个理由。