A HALF-CENTURY OF SCIENCE. 203 



of temperature, be concluded that the same method would prove appli- 

 cable 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 con- 

 densation 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 tempera- 

 ture of one pound of water by 1 Fahr. Hirn subsequently attacked 

 the problem from the other side, and showed that if all the heat pass- 

 ing 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 inex- 

 haustible 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 y^ uVbtstf 

 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 



