Page:Popular Science Monthly Volume 73.djvu/110

106 observation to be now conducted out of the earth yearly is so great that if this action had been going on with any apparent uniformity, the history of life on the earth could not exceed a few thousand million years." Another consideration leading to similar conclusions was based on the shape and rigidity of the earth. With Sir William Thomson, the age of the earth continued to be a question studied with great predilection. His aim was not so much to determine the exact age as to fix an upper age limit. As the years passed by, investigation supplied much of the knowledge which was at first wanting regarding the thermal properties of rocks, and Sir William Thomson was able greatly to reduce this upper limit.

"The Physical Condition of the Earth" was the topic of Sir William Thomson's presidential address in 1876, before Section A of the British Association. He took the gradual increase of temperature downwards to be on an average 1° C. for 30 meters of descent and gave reasons for his belief that for great depths the rate of increase does not diminish. He concludes that if at great depths the temperature does not exceed 4,000° C., then the geological age of the earth does not exceed 90 million years. This argument involves some very uncertain factors. Sir William Thomson has shown quite conclusively that the earth's interior is solid, but at what temperature the substance of the earth would begin to melt under the high internal pressures was a matter of pure conjecture.

About 1885 Carl Barus, of the United States Geological Survey, made a series of very important experimental researches on the physical properties of rocks at high temperatures, for the purpose of supplying trustworthy data for geological theory. Mr. Clarence King, in an article published in the American Journal of Science, used the data on specific heats, thermal conductivities and temperatures of fusion of rocks, which had been supplied by Barus, for a more accurate determination of the age of the earth. King concludes from these experimental data on diabase, "that we have no warrant for extending the earth's age beyond 24,000,000 years." A computation made by Lord Kelvin led to about the same figure. These results were embodied by him in his address of 1897 before the Victoria Institute.

What was the attitude of geologists toward these researches? In England, geologists did not pretend to be able to find any flaw in the argument of Lord Kelvin, but they were in a position described in the well-known couplet,

 A man convinced against his will Is of the same opinion still."