280. Many animals possess the five senses, but only man possesses constructive, creative power, and is able to build on the information gained through the senses. It is the constructive, creative power which raises man above the level of the beast and enables him to devise and fashion wonderful inventions. Among the most important of his inventions are those which relate to electricity; inventions such as trolley car, elevator, automobile, electric light, the telephone, the telegraph. Bell, by his superior constructive ability, made possible the practical use of the telephone, and Marconi that of wireless telegraphy. To these inventions might be added many others which have increased the efficiency and production of the business world and have decreased the labor and strain of domestic life. [Illustration: FIG. 197.--A simple electric cell.] 281. Electricity as first Obtained by Man. Until modern times the only electricity known to us was that of the lightning flash, which man could neither hinder nor make. But in the year 1800, electricity in the form of a weak current was obtained by Volta of Italy in a very simple way; and even now our various electric batteries and cells are but a modification of that used by Volta and called a voltaic cell. A strip of copper and a strip of zinc are placed in a glass containing dilute sulphuric acid, a solution composed of oxygen, hydrogen, sulphur, and water. As soon as the plates are immersed in the acid solution, minute bubbles of gas rise from the zinc strip and it begins to waste away slowly. The solution gradually dissolves the zinc and at the same time gives up some of the hydrogen which it contains; but it has little or no effect on the copper, since there is no visible change in the copper strip. If, now, the strips are connected by means of metal wires, the zinc wastes away rapidly, numerous bubbles of hydrogen pass over to the copper strip and collect on it, and a current of electricity flows through the connecting wires. Evidently, the source of the current is the chemical action between the zinc and the liquid. Mere inspection of the connecting wire will not enable us to detect that a current is flowing, but there are various ways in which the current makes itself evident. If the ends of the wires attached to the strips are brought in contact with each other and then separated, a faint spark passes, and if the ends are placed on the tongue, a twinge is felt.