In our room we worked far into the night. Craig carefully swabbed out the bottom and sides of each bottle by inserting a little piece of cotton on the end of a long wire. Then he squeezed the water out of the cotton swab on small glass slides coated with agar-agar, or Japanese seaweed, a medium in which germ-cultures multiply rapidly. He put the slides away in a little oven with an alcohol-lamp which he had brought along, leaving them to remain overnight at blood heat.

I had noticed all this time that he was very particular not to touch any of the bottles on the outside. As for me, I wouldn't have touched them for the world. In fact, I was getting so I hesitated to touch anything. I was almost afraid to breathe, though I knew there was no harm in that. However, it was not danger of infection in touching the bottles that made Craig so careful. He had noted, in the dim light of the station lamps, what seemed to be finger-marks on the bottles, and they had interested him, in fact, had decided him on a further investigation of the bottles.

"I am now going to bring out these very faint finger-prints on the bottles," remarked Craig, proceeding with his examination in the better light of our room. "Here is some powder known to chemists as 'grey powder'—mercury and chalk. I sprinkle it over the faint markings, so, and then I brush it off with a camel's-hair brush lightly. That brings out the imprint much more clearly, as you can see. For instance, if you place your dry thumb on a p