“You found out what it was!” Drai stated, rather than asked.
“I found out something which will let me figure out what it is, very shortly,” replied Ken.
“But what did you do? Why did you go out twice?”
“You must have seen me putting a sample into the pressure bomb. I sealed it in, and brought it inside so it would all evaporate and so that the pressure gauge on the bomb would be at a temperature where I could trust it. I read the pressure at several temperatures, and weighed the bomb with the sample. I had already weighed it empty — or rather, with the near-vacuum this planet uses for air inside it. The second time I opened the door was to let off the sample, and to make a check at the same temperature with a sample of the planet’s air — after all, it must have contributed a little to the pressure the first time.”
“But what good would all that do?”
“Without going into a lot of detail, it enabled me to find out the molecular weight of the substance. I did not expect that to be very conclusive, but as it happened I think it will be; it’s so small that there aren’t many possible elements in it — certainly nothing above fluorine, and I think nothing above oxygen. I’ll concede that I may be off a unit or so in my determination, since the apparatus and observing conditions were not exactly ideal, but I don’t think it can be much worse than that.”
“But what is it?”
“The molecular weight? Between eighteen and nineteen, I got.”
“What has that weight, though?”
“Nothing at all common. I’ll have to look through the handbook, as I said. Only the very rarest elements are that light”