TWENTIETH CENTURY SCIENCE PROBLEMS. 241 



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 mole- 

 cules 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 chem- 

 istry 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, unchang- 

 ing 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 concep- 

 tion gives one pleasure though one knows it to be impossible, and one 

 thinks it impossible because one has never known such changes in mat- 

 ter, 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, 



