Many tests were made, and the result was a convincing proof that electricity is the same whatever its source, the different effects being due to difference in pressure and quantity. "But in no case," said Faraday, "not even in those of the electric eel and torpedo, is there a production of electric power without something being used up to supply it." Faraday's professional work would have made him wealthy. In one year he made £1000 ($5000), and the amount would have increased had he sold his services at their market value. But then there would have been no Faraday the discoverer. The world would have had to wait, no one knows how long, for the laying of the foundations of electrical industries. He chose to give up wealth for the sake of discovery. He gave up professional work with the exception of scientific adviser to Trinity House, the body which has charge of Great Britain's lighthouse service. Nor did he carry his discoveries to the point of practical application. As soon as he discovered one principle, he set out in pursuit of others, leaving the practical application to the future. Faraday loved the beauty of nature. The sunset he called the scenery of heaven. He saw the beauty of electricity, which he said lies not in its mystery, but in the fact that it is under law and within the control of the human intellect. A Wonderful Law of Nature Not long after Faraday made his first dynamo, Robert Mayer, a physician from Germany, was making a voyage to the East Indies which proved to be a voyage of discovery. He had sailed as the ship's physician, and after some months an epidemic broke out among the ship's company. In his treatment he drew blood from the veins of the arms. He was startled to see bright-red blood issue from the veins. He might almost have believed that he had opened an artery by mistake. It was soon explained to him by a physician who had lived long in the tropics that the blood in the veins of the natives, and of foreigners as well, in the tropics is of nearly the same color as arterial blood. In colder climates the venous blood is much darker than the arterial. He reasoned upon this curious fact for some time, and came to the conclusion that the human body does not make heat out of nothing, but consumes fuel. The fuel is consumed in the blood, and there the heat is produced. In the tropics less heat is needed, less fuel is consumed, and therefore there is less change in the color of the blood. When a man works he uses up fuel. If a blacksmith heats a piece of iron by hammering, the heat given to the iron and the heat produced in his body are together equal to the heat of the fuel consumed in his blood. The work a man does, as well as the heat of his body, comes from the burning of the fuel in his blood.