98 ESSAYS BIOGRAPHICAL AND CHEMICAL 



be due to the motions of the molecules of a gas; and 

 that when the gas is compressed, the impacts of its mole- 

 cules on the walls of the containing vessel are more 

 numerous, and that the work done in compressing a gas 

 appears as heat, owing to the more numerous impacts of 

 its molecules. Following on this, it was necessary to 

 Revise an absolute scale of temperature, and that we also 

 owe to Lord Kelvin. It is based on what is known as the 

 ' Second Law of Thermodynamics ' that heat cannot be 

 transferred from a cold to a hot body without expending 

 work. Following these ideas, Lord Kelvin was led to 

 consider the probable age of the earth, based on an 

 estimate of its original temperature, and the rate at which 

 heat would be lost by radiation. His opinion is that the 

 earth may have been habitable twenty million years ago, 

 but could not have been habitable as long ago as four 

 hundred million years. 



The province of electro-magnetism owes very much to 

 Lord Kelvin. It was he who developed the medium sug- 

 gested by Faraday into a means of representing electro- 

 magnetic forces by analogy with the distortion of an elastic 

 solid. After he had worked out in this manner the con- 

 nection between energy and electro-magnetism, he devised 

 our present system of electrical units volts, amperes, 

 farads, coulombs, etc., and invented machines to deter- 

 mine their numerical values. If it be permitted to assign 

 their relative importance to his contributions to practical 

 science, this must be pronounced the greatest. Without 

 it the science of electricity would be helpless as commerce 

 without a monetary system, and without weights and 

 measures. His work is the foundation of wireless tele- 

 graphy, and of many applications of the electric current. 

 It was he who taught the world how to transmit rapid 

 and trustworthy signals through cables; and he was a 

 pioneer of cable telegraphy. In the old days of cables 



