This brilliant invention caused Franklin to conceive another brilliant invention, the utilization of the discovery he had just made in combination with his previous discovery of the power of points to collect electricity. He embodied his conception in what we now call "lightning rods," by erecting on the highest points of houses thin metal rods or conductors, the lower ends of which were buried in the earth, while their upper ends were sharpened to points, and made to project upward, above the houses. Franklin's theory was that the points would collect the electricity from the clouds and allow it to pass harmlessly through the conductors into the ground. The invention worked perfectly, and has been utilized everywhere ever since. Naturally, Franklin's epochal discoveries stirred the scientific world in Europe, and gave a great impetus to the study of electricity and the other physical sciences. One of the earliest important discoveries that followed (made by Mr. Cavendish) was that the electrical spark could decompose water and atmospheric air, and make water by exploding mixtures of oxygen and hydrogen. An epochal discovery was made by Mr. Cavendish about 1787, when he exploded a mixture of oxygen and nitrogen and obtained nitric acid. In 1790 Galvani discovered that, if two dissimilar metals were placed in contact at one end of each, and if the free ends are put into contact with the main nerve of a frog's hind leg and the thigh muscle respectively, spasmodic muscular movements would ensue. In investigating the cause of this phenomenon, Volta discovered that if the lower ends of two dissimilar metals were immersed in a liquid they would assume opposite electrical states; so that if their outer ends were joined by a conducting wire, electricity would pass along it. This led him at once to the invention of the Voltaic cell. The enormous value of the Voltaic cell in building up the science of electricity need hardly be pointed out. It is still used in electric telegraphy as a source of current. During the eighteenth century, the relations between chemistry and heat were very ill defined; but they were cleared up gradually by the researches of such men as Black in Scotland, Priestley and Cavendish in England, and Lavoisier in France. Black's work was mainly in making investigations of the phenomena of heat. In the course of them he discovered the important fact that different substances require different amounts of heat to be applied to a given mass to raise its temperature 1°. From this discovery arose the science of calorimetry, which deals with the specific heats of all substances, solid, liquid and gaseous, and which is necessary to the present science of heat and the arts that depend upon it. About 1774 Dr. Priestley discovered oxygen.