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method of growing one species of germ away from all other contaminating ones that are always threatening to sneak in!"

But how to cage one kind of microbe? All manner of weird machines were being invented to try to keep different sorts of germs apart. Several microbe hunters devised apparatus so complicated that when they had finished building it they probably had already forgotten what they set out to invent it for. To keep stray germs of the air from falling into their bottles some heroic searchers did their inoculations in an actual rain of poisonous germicides!

Until, one day, Koch—who frankly admitted it was by accident—looked at the flat surface of half of a boiled potato left on a table in his laboratory. "What's this, I wonder?" he muttered, as he stared at a curious collection of little colored droplets scattered on the surface of the potato. "Here's a gray colored drop, here's a red one, there's a yellow, a violet one—these little specks must be made up of germs from the air. I'll have a look at them."

He stuck his short-sighted eyes down close to the potato so that his scraggly little beard almost dragged in it; he got ready his thin plates of glass and polished off the lenses of his microscope.

With a slender wire of platinum he fished delicately into one of the gray droplets and put a bit of its slimy stuff in a little pure water between two bits of glass, under his microscope. Here he saw a swarm of bacilli, swimming gently about, and every one of these microbes looked exactly like his thousands of brothers in this drop. Then Koch peered at the bugs from a yellow droplet on the potato, and at those of a red one and a violet one. The germs from one were round, from another they had the appearance of swimming sticks, from a third microbes looked like living corkscrews—but all the microbes in one given drop were like their brothers, invariably!