TWENTIETH CENTURY SCIENCE PROBLEMS. 249 



desert of the gardens. How much better to-day is the world for their 

 energy, their strenuousness and their power ? Are we any more lovable 

 or stronger or wiser? 



I have sometimes amused myself wondering what question I would 

 ask an inhabitant of Mars, if communication with that planet could 

 be established. If but one question could be answered, what should 

 that question be? Is there any one question which everybody would 

 be willing to have asked and forego every other one? Would it be a 

 question of astronomy or of biology or of philosophy? Each one 

 should settle for himself what that question ought to be, if its answer 

 was to be of interest to all mankind. If it were of a religious or 

 philosophical kind, think what happiness or misery would befall the 

 most of us when the answer came. It would be almost like a judg- 

 ment day and half the world or more would be thrown into a suicide 

 mood. Ignoring momentous questions, what others are we really most 

 concerned to have answered? Do we not all want to know of the na- 

 ture of life, of mind, and of all the activities of nature displayed in 

 phenomena? Does not everybody ask, 'What is electricity?' 'What is 

 life?' I do not remember ever to have heard the question 'What is 

 gravitation?' though it is certainly one of the most obscure of all the 

 great activities of nature. Not a particle of matter escapes its hold, 

 and the law of inverse squares we have all learned so glibly, we take 

 on the basis of uniform experience. How can such action be the out- 

 come of inherent properties of matter, and what must be the texture 

 and distribution in the ether so compelling? 



Surmises by the score have been made, but none are satisfied with 

 any attempt to find a reason or the antecedents of the phenomenon. 

 It conditions every phenomenon of every kind that comes to our knowl- 

 edge in a gravitative way, but hitherto it has quite eluded the most 

 ingenious of guessers, and most persons who have been concerned with 

 its problems have either abandoned attempts at its solution or have 

 unwarrantably concluded it is insoluble. There is no good reason why 

 it should be thought of as an ultimate problem, and its solution be- 

 longs to the twentieth century or some of its successors. In my 

 judgment its rationale will be found some time. 



When, a hundred years ago, men said that heat was caloric, it is 

 plain on a little thinking that such an answer brought us no nearer 

 the real solution. Giving the thing a new name was not an explana- 

 tion. We have been taught for a generation that heat is a mode of 

 motion, and when we now think of the phenomena we think of brisk 

 changes of position of the minute particles of a body, and that idea 

 reveals heat as a condition of matte?', not a thing in itself any more 

 than the spin of a top is to be thought of as a thing to be described 

 apart from the top. 



A hundred years ago light was thought to be a kind of corpuscle 



