Now all known substances are “transparent” to gravitation.  You can use screens of various sorts to cut off the light or heat, or electrical influence of the sun, or the warmth of the earth from anything; you can screen things by sheets of metal from Marconi’s rays, but nothing will cut off the gravitational attraction of the sun or the gravitational attraction of the earth.  Yet why there should be nothing is hard to say.  Cavor did not see why such a substance should not exist, and certainly I could not tell him.  I had never thought of such a possibility before.  He showed me by calculations on paper, which Lord Kelvin, no doubt, or Professor Lodge, or Professor Karl Pearson, or any of those great scientific people might have understood, but which simply reduced me to a hopeless muddle, that not only was such a substance possible, but that it must satisfy certain conditions.  It was an amazing piece of reasoning.  Much as it amazed and exercised me at the time, it would be impossible to reproduce it here.  “Yes,” I said to it all, “yes; go on!” Suffice it for this story that he believed he might be able to manufacture this possible substance opaque to gravitation out of a complicated alloy of metals and something new—­a new element, I fancy—­called, I believe, helium, which was sent to him from London in sealed stone jars.  Doubt has been thrown upon this detail, but I am almost certain it was helium he had sent him in sealed stone jars.  It was certainly something very gaseous and thin.  If only I had taken notes...

But then, how was I to foresee the necessity of taking notes?

Any one with the merest germ of an imagination will understand the extraordinary possibilities of such a substance, and will sympathise a little with the emotion I felt as this understanding emerged from the haze of abstruse phrases in which Cavor expressed himself.  Comic relief in a play indeed!  It was some time before I would believe that I had interpreted him aright, and I was very careful not to ask questions that would have enabled him to gauge the profundity of misunderstanding into which he dropped his daily exposition.  But no one reading the story of it here will sympathise fully, because from my barren narrative it will be impossible to gather the strength of my conviction that this astonishing substance was positively going to be made.

I do not recall that I gave my play an hour’s consecutive work at any time after my visit to his house.  My imagination had other things to do.  There seemed no limit to the possibilities of the stuff; whichever way I tried I came on miracles and revolutions.  For example, if one wanted to lift a weight, however enormous, one had only to get a sheet of this substance beneath it, and one might lift it with a straw.  My first natural impulse was to apply this principle to guns and ironclads, and all the material and methods of war, and from that to shipping, locomotion, building, every conceivable