In 1831 Faraday in London made one of the most important discoveries in physical science ever made, the discovery that if a current of electricity is changed in strength, or if a conductor carrying a current be moved, an instantaneous magnetic effect is felt in the vicinity; and that this magnetic effect will cause an instantaneous current in any closed conducting circuit that may be near. Faraday also discovered that a similar instantaneous current will be set up in a closed circuit if a magnet be moved in its vicinity. This discovery is usually spoken of as the discovery of electro-magnetic induction; and the instantaneous currents are said to be "induced." About the same time Professor Henry in Princeton discovered that an electric circuit will act not only on other circuits in its vicinity, but on itself; that the fact of being increased or decreased will set up instantaneous currents that tend to oppose the increase or decrease. Thus, while Faraday is credited with the discovery of electro-magnetic induction, Henry is credited with the discovery of self-induction. It has been claimed by some that Henry discovered electro-magnetic induction before Faraday did. This question is of great interest but it is outside the scope of this modest volume. While both discoveries were of prime importance, and were also analogous, that of electro-magnetic induction has played the more conspicuous part. With it began the endeavor to develop electric currents by the relative motion of coils of wire and magnets, that resulted in the invention of the dynamo, and the later invention of electric lights and motors. In the same year the discovery (or was it the invention?) of chloroform was made by Guthrie in America, Soubeiran in France and Liebig in Germany. A curious fact connected with the early history of chloroform is that, although its anæsthetic properties were known in general, and although the idea of using gases and vapors and medicines to deaden pain was many centuries old yet nevertheless, chloroform was not put to practical use until about 1846 when Dr. Morton, a dentist, of Boston, adopted it as an anæsthetic. Of all the single inventions ever made, chloroform has unquestionably done more than any other, invented till that time, to give relief from agony.