YEAltS ago, as a boy in a Somerset village, I sometimes amused myself and i~pressed my fiiends by filling a jam-jar with water, turning it upside down in the village horse-pond, and poking the mud beneath it with a stick. Bubbles of manh-gas, released from the bed of the pond, rose through the water and soon fiIIed the jar. When I held a lighted match near the mouth of the inverted jar, the gas took fire and bumt quietly with an almost invisible flame. Occasionally, however, when a good deal of air had got into the jar, the mixture exploded with a very pleasing pop.