Metallo-therapy has been defined as a mode of treating various affections, chiefly those of a nervous character, by the external application of metals. It was recommended by Galen and other medical writers, but they attributed its curative powers to the magical inscriptions which the metals bore.

Mesmer experimented with magnets extensively, but soon abandoned their use, as he found that he could obtain equally good results without them.

The so-called "metallic tractors" originated with Dr. Elisha Perkins (1740-1799), a practising physician of Norwich, Connecticut, and consisted of two rods, one of brass, and the other of steel. In cases of rheumatism and various neuroses, the affected portions of the body were lightly stroked by means of the tractors, and many remarkable cures were reported. The new therapeutic method was endorsed by many reputable practitioners, both in the United States and Europe, and its fame spread like wild-fire.

It was soon discovered, however, that wooden tractors were fully as efficacious as the metallic ones, and that the many vaunted cures were psychic. Thus Perkins's tractors afford a striking example of the curative force of suggestion.

Thereby (wrote John Haygarth, M.D., Fellow of the Royal Medical Society of Edinburgh, in a brief treatise on the Imagination, published in the year 1800) is to be learned an important lesson in Medicine, namely, the wonderful and powerful influence of the passions of the mind, upon the state and disorders of the body. This fact, he continued, was too often overlooked in Practice, where sole dependence was placed upon material remedies, without utilizing mental influence. To the latter, this sagacious physician, writing more than a century ago, was shrewd enough to ascribe the marvellous cures attributed to the remedies of quacks, whose magnificent and unqualified promises inspire weak minds with confidence.

In one of his Lowell Institute lectures, at Boston, November 14, 1906, Dr. Pierre Janet described the development of metallo-therapy in France between the years 1860 and 1880. Metallic discs were applied to the patient's body. These discs were of different kinds, sometimes being composed of two or more metals. In some cases a magnet was used. Different subjects, it was found, did not manifest sensitiveness to the same metals, some being cured by iron, others by copper, while the greatest number were susceptible to gold. Many interesting facts relating to these cures were noted, such as periods of transition and oscillation in the maladies, and most curious of all, a kind of transference. For example, should a paralysis or a contraction seat itself on the right side, the application of the discs would effect a cure, but the malady would often return to the opposite side. And there were other curious phenomena. A modification of sensation was invariably observed.

Under the influence of the metal disc, the shin and muscles, which before were numb, regained their normal states, and the return of sensation preceded the cure, and was an indispensable condition. One can obtain exactly the same results with discs composed of inert substances. An old-fashioned letter-wafer, for instance, applied to the hand, has produced similar effects. According to Dr. Janet, these phenomena are wholly due to psychic agencies, partly akin to suggestion and partly different. They depend upon the mechanism of attention. This faculty, when directed upon any organ, will bring into prominence sensations not ordinarily felt.

Consciousness is limited, in that it does not always take cognizance of all the existing sensations. This explains the phenomenon of transference, in that the suppression of those sensations which were prominent brings to the surface others which were not before recognized by the consciousness.

As a result of the introduction of metallo-therapy in the hospitals of Paris, an enormous number of hysterical patients applied for treatment, influenced partly, no doubt, by the love of notoriety.