The Deadly Dust by Murray Leinster

It was dark. The car had come back for Murfree and he had sent it away again. He paced up and down. He chewed his fingers. To know of certain doom awaiting one's country and to have as one's ally a man who can do anything that can be imagined in the way of physics -- and much that cannot be imagined -- and not to be able to think of anything either possible or impossible to avert the doom -- it was maddening.

Bud Gregory grinned amiably.