Al'kaloids, a class of substances that occur in plants, some of which are very poisonous, and many are very valuable medicines. All of them are bases, that is, they unite with acids to form salts, and in many cases the salts are used medicinally. All alkaloids contain carbon, nitrogen and hydrogen and usually oxygen also. Some of the most important alkaloids are theine of coffee and tea, nicotine of tobacco, morphine from opium, quinine from Peruvian bark, as well as strychnine, atropine, cocaine etc.
