About 1859, Kirchhoff and Bunsen invented the spectroscope, an optical instrument for forming and analyzing the spectra of the rays emitted by bodies and substances. In 1860 Gaston Planté invented his famous "secondary battery," formed by passing an electric current through a cell composed of two sheets of lead immersed in dilute sulphuric acid, the two sheets separated by non-conducting strips of felt. The acid being decomposed, hydrogen formed on one plate, while oxygen attacked the other plate and formed peroxide of lead. There being now two dissimilar metals in an acid solution, a Voltaic battery had been created, that gave a current which passed through the liquid in a direction the reverse of the current ("charging current") that had caused the change. Planté's secondary battery was an important and practical contribution to the Machine; but the credit for the basic invention does not belong to Planté, but to Sir William Grove, who had invented the "Grove's gas battery." In this battery, two plates of platinum were immersed in dilute acid, and submitted to a charging current that decomposed the liquid and formed an actual though practically ineffective "secondary battery"; the two elements being oxygen and hydrogen. In the next year Philip Reis invented the singing telephone, by which he could transmit _musical tones_ over considerable distances. Whether or not Philip Reis invented the speaking telephone has been a much controverted question, for the reason that speech was occasionally transmitted over Reis's telephone,--though not by intention. The invention that Reis conceived, developed and produced was a singing telephone only; the apparatus by which he sometimes transmitted speech was his singing telephone, slightly disadjusted. That Reis should have failed to invent the telephone is amazing, in the same sense that it is amazing that Galileo did not invent the thermometer and the barometer; and the fact is extremely instructive in enabling us to see distinctly what constitutes invention. To make an invention, a man must himself create a thing that is new, and produce it in a concrete form, such that "persons skilled in the art can make and use it." Reis did not do this: and yet Philip Reis's telephone could be made to speak in a few seconds, by simply turning a little thumb-screw! Reis did not know this, and consequently could not give the information to "persons skilled in the art." Reis did not invent the speaking telephone, for the fundamental reason that his original conception, although correct for his singing telephone, was wholly incorrect for a speaking telephone; because the speaking telephone requires a continuous current, while Reis's conception included an intermittent current.