To return to geyser-spring experiments, I built a simple quart flask surmounted by a four-foot glass tube. At the top the tube rose through a cork in the bottom of a two-foot pan. In the side of the cork of the flask was a second tube with a hose leading up to a reservoir bottle of water. The reservoir bottle could be raised or lowered. If the water in it was level with the pan, there was hydrostatic equilibrium: the pan a pool, the bottle a source, the flask and tube full. When we applied heat to the bottom of the flask, the water boiled, the pan overflowed, and some cold water from the bottle chilled the flask. The pan had become a boiling spring. Next we lowered the reservoir bottle. The reduced head of water permitted no overflow at the pan, and steam bubbles accumulated in the four-foot upright tube. The boiling point was controlled by four feet of water pressure. If the bubble lift reduced this to three feet, there was a lower boiling point, the pressure was reduced by overflow above, and the whole flaskful boiled. The geyser tube became a regular geyser at intervals of a minute and a half, with eruptions enduring twenty seconds. This was a miniature of Old Faithful in the Yellowstone. Old Faithful is bigger, its intervals average sixty-five minutes, and they range from thirty-one to eighty-one minutes. It jets up 150 feet for a period of four minutes. It throws out 3,000 barrels of water at each eruption. Our little machine threw up about a pint to a height of four feet. We hear much about soaping geysers as an artificial stimulus. The apparatus in our laboratory showed the effect of soap right away. When some soap was put in the pan, the intervals of a minute and a half shortened to one minute. Soapsuds accumulated in the tube and depressed the water to the neck of the flask. The multiple bubbles, film against film, made the water system viscous. The myriads of tiny steam bubbles formed so fast that they shortened the lifting time for the column. If the height of the reservoir bottle was so adjusted that the geyser didn’t quite know whether it was a geyser or a boiling spring, the soap made the decision, and the thing went off with a bang. This simple group of experiments makes springs very real. The Yellowstone explosive springs differ from other springs in having superheated steam from live lavas to heat them. The rock is cracked and the water is doing a job of solution and deposition. It deposits stout tough silica around some openings and builds them up against the head of groundwater, and they become geysers. It deposits lime dissolved off underlying limestone at Mammoth, and this makes sculptured terraces but not explosive springs because the temperatures are not so hot. In both lime and silica regions, blue-green algae, which love hot water, decoratively sculpture the pools.