The discovery of specific remedies has always, and most justly, been considered one of the most important benefits to be conferred on the practice of medicine. Much dispute has been carried on respecting their nature, but all are agreed about their existence. They have been defined by Dr. Young to be medicines which cure diseases, “without any perceptible connexion between the immediate effect and the benefit obtained.” While their operation is thus obscure, the mode of their employment, and their peculiar virtues, must be subjects of much doubt and uncertainty; while the accidents to which they are liable, in common with other medicines, must occasion great embarrassment and perplexity. But from the moment their modus operandi can be connected with any known general law of the constitution, a great part of these doubts disappear, a light is afforded for directing their good effects, and a clew is obtained for tracing their injurious properties, and applying the necessary antidote. The medical history of iodine will fully exemplify the above observations. This medicine was first introduced into practice by Dr. Coindet of Geneva. Whilst making researches for other purposes, he found that the fucus vesiculosus had been recommended by Russel in the cure of goitre. From this plant, and other species of the same family, the soda, with which iodine is generally found combined, is extracted. As the sponge, whose virtues have long been established by certain experience at Geneva,[1] is also a maritime plant, Dr. Coindet suspected that iodine might be the active principle of them both; and by this analogy he was first led to employ it in the cure of bronchocele. The success which attended its use in the first instance was very remarkable; and it seems to have been exhibited cautiously and warily, for some considerable time had elapsed before the alarm was given of its noxious effects.