Page:Popular Science Monthly Volume 20.djvu/215

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203
A HALF-CENTURY OF SCIENCE.

of temperature, be concluded that the same method would prove applicable to gases. He thus found himself in possession of a new and independent method of procedure. It need, perhaps, he hardly added that, when submitted to this new test, his former conclusions on the interaction of heat and gaseous matter stood their ground.

The determination of the mechanical equivalent of heat is mainly due to the researches of Mayer and Joule. Mayer, in 1842, pointed out the mechanical equivalent of heat as a fundamental datum to be determined by experiment. Taking the heat produced by the condensation of air as the equivalent of the work done in compressing the air, he obtained a numerical value of the mechanical equivalent of heat. There was, however, in these experiments, one weak point. The matter operated on did not go through a cycle of changes. He assumed that the production of heat was the only effect of the work done in compressing the air. Joule had the merit of being the first to meet this possible source of error. He ascertained that a weight of one pound would have to fall 772 feet in order to raise the temperature of one pound of water by 1° Fahr. Hirn subsequently attacked the problem from the other side, and showed that if all the heat passing through a steam-engine was turned into work, for every degree Fahr. added to the temperature of a pound of water, enough work could be done to raise a weight of one pound to a height of 772 feet. The general result is that, though we can not create energy, we may help ourselves to any extent from the great storehouse of nature. Wind and water, the coal-bed and the forest, afford man an inexhaustible supply of available energy.

It used to be considered that there was an absolute break between the different states of matter. The continuity of the gaseous, liquid, and solid conditions was first demonstrated by Andrews in 1862. Oxygen and nitrogen have been liquefied independently and at the same time by Cailletet and Raoul Pictet. Cailletet also succeeded in liquefying air, and soon afterward hydrogen was liquefied by Pictet under a pressure of 650 atmospheres, and a cold of 170° Cent, below zero. It even became partly solidified, and he assures us that it fell on the floor with "the shrill noise of metallic hail." Thus, then, it was shown experimentally that there are no such things as absolutely permanent gases.

The kinetic theory of gases, now generally accepted, refers the elasticity of gases to a motion of translation of their molecules, and we are assured that, in the case of hydrogen at a temperature of 60° Fahr., they move at an average rate of 6,225 feet in a second; while, as regards their size, Loschmidt, who has since been confirmed by Stoney and Sir W. Thomson, calculates that each is at most 150000000 of an inch in diameter.

We can not, it would seem, at present hope for any increase of our knowledge of atoms by any improvement in the microscope. With