Page:Popular Science Monthly Volume 61.djvu/195

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thing like seven years. That these recondite phenomena have been disentangled and reduced to precise numerical statement is at once conclusive evidence of method in the madness of terrestrial motions and of exquisite refinement in astronomical science.

The much discussed question of the age of the earth may now be said to have risen from the level of figures of speech to the higher plane of numerical expression. We are not able, and we may never be able, to assign the age of the earth in years, or in thousands of years as our most respected teachers have done in the recent past; but we may say without fear of anything worse than literary contradiction that the age of the earth must be reckoned in millions of years. Probably some hundreds of millions of years have elapsed since the earth became habitable to organic forms. Nature has plenty of time for her operations; and old as the earth must be in comparison with the centuries of human affairs, it is still active with the energy manifested in the earliest geological times. The processes of erosion and sedimentation, and that of secular contraction from loss of internal heat, are still asserting themselves occasionally (frequently, if a million years be used as the time unit) by such appalling outbursts as that which has just overwhelmed St. Pierre. And these processes, though fraught with calamities here and there to our race, must go on for millions of years yet to come.



Within the last few years several particularly attractive biographies of distinguished chemists have been issued as volumes of the 'Century Science Series,' edited by Sir Henry E. Roscoe; they differ from other works in the same line in that they portray the men and their careers more graphically and concisely, and embody at the same time the results of the latest researches. One of these, written by Roscoe himself, bears the title 'John Dalton and the rise of Modern Chemistry' (New York, 1895).

Half a dozen memoirs of the Founder of the Atomic Theory had previously appeared, the most noteworthy being those by W. C. Henry (1854), by R. Angus Smith (1856), and by H. Lonsdale (1874). The first named forms one of the volumes printed for the Cavendish Society, the second includes a History of the Atomic Theory from the days of the Greeks to the time of Dalton, and is embellished with the most satisfactory of the printed portraits.

Roscoe's charming work is enlivened with facsimiles, extracts from letters, papers and books by Dalton, reminiscences by contemporaries, appreciations by later chemists and amusing anecdotes, the whole so simply and yet so vigorously written as to make a delightful narrative. There has always been much speculation as to the mental processes which led Dalton to conceive of the great theory indissolubly connected with his name, and this problem has only been quite recently solved by the discovery in the rooms of the Literary and Philosophical Society of Manchester (where the whole of the experimental work was carried on by Dalton) of his laboratory and lecture notebooks, in a number of volumes. It has been supposed that it was the experimental discovery of the law of combination in multiple proportions that led Dalton to the idea that chemical combination consists in the approximation of atoms of definite and characteristic weight, the atomic theory being thus adopted to explain the facts ascertained by chemical analysis. But an examination of these newly discovered manuscript notes shows that he arrived at these ideas from purely physical considerations along the line of the Newtonian doctrine of the atomic constitution of matter; he conceived of chemical combination as taking place