definite measure for geologic epochs, to ascertain how long ago the glacial age was, and how long it lasted. At present there are only surmises that the glacial epoch ended from 10,000 to 50,000 years ago. The twentieth century will probably be able to settle this.
Chemistry too as a science was nineteenth century product. There were guesses and ingenious surmises, but there were no known general laws, such as of definite proportions, of atomic weights, of energy in reactions and the like. It became possible to measure approximately the sizes of molecules and atoms, to know definitely their rates of vibration, and molecular structure is, for many compounds, made out about as well as if they could be dissected and their atoms handled like so many parts of an engine or dynamo.
As knowledge grew on the basis of experiment, generalization of course was attempted, and as physical phenomena were inextricably interwoven with the chemical, constant modifications were required. Not a few propositions found their way into books and general use which had to be abandoned. Thus, it was assumed that when molecules of salt, NaCl, were dissolved in water, each molecule retained its identity and moved as a whole in the liquid. We now know this is not true, but each atom becomes practically independent and moves like a gaseous particle in the air, producing pressure in the same way and for the same reason. The new knowledge has made it needful to revise again some of the notions that were held, and so profound is the change required that some years will be needful to bring chemistry as a science into satisfactory relations with physics. That is not all. We have all been taught and have probably had no misgivings in saying that matter is indestructible. Much philosophy is founded on that proposition. But we are now confronted with the well vouched for phenomenon from two independent workers that under certain conditions a certain mass of matter loses weight, not by mechanical removal of some of its molecules, but by the physical changes which take place in it. This is a piece of news that is almost enough to paralyze a scientifically minded man, for stability of atoms, unchanging quantity and quality, seems to be at the basis of logical thinking on almost all matters. In the 'Arabian Nights' one may expect that the unexpected will happen—genii may be summoned to do this or that, matter may be created or annihilated at will—and the conception gives one pleasure though one knows it to be impossible, and one thinks it impossible because one has never known such changes in matter, and because one has been taught that matter is indestructible. The amount of change is slight in the experiments related, yet well within the possibility of measuring, and one may be sure that from now on the most expert and careful and patient experimenters will attack this question and verify or disprove it. If it be disproved, we shall be philosophically where we have so long been. If it be proved,
