According to shape: 1, _sackhöhlen_ or chamber caverns, into which one enters immediately at the entrance; 2, _ganghöhlen_, or passage caves terminating in a chamber; 3, _röhrenhöhlen_, or passage caves where the passages continue further than the chamber. He is a strong advocate of the winter's cold theory. He says: "The ice of caves is formed by the cold of winter, and remains despite the heat of summer, as through local circumstances the quantity of heat brought to the ice is not great enough to melt it by the time when ice and snow in the open at the same altitude have already disappeared." "In order that ice may form in a cave in winter, two factors are necessary. There must be water present in some form or other, and in some way the outside cold air must be able to sink into the cave." "When the bottom of a cave is below the entrance, the outside cold winter air sinks into the cave from its weight, when the temperature of the cave air is higher than that of the outside air; and it will remain there during the warmer weather, as the warm outside air on account of its lighter weight cannot drive out the cold heavy cave air." "The most important factor for the formation of ice is the drip water. The more drip flows into a cave during the cold season, the more ice is formed; the more drip, on the contrary, flows into the cave during the warm season, the more ice is destroyed." "The warmth, which the roof of the cave gives out, is also a cause which helps to melt the ice, and a cause in fact which works the harder, the higher the temperature of the roof and the dirtier the ice floor." "If direct rays of the sun penetrate a cave, they scarcely warm up the air which they traverse, but they raise the temperature of the floor or of the walls, which they touch. They are therefore a very important factor, which may bring about the melting of the ice." "The snow slope at the mouth of a cave offers some protection against the rays of the sun, especially if it is no longer white, but covered with all sorts of dirt." "The larger the mass of ice, the longer is its duration." "A certain thickness to the roof is of importance in preserving the ice. If it is less than 8 meters, then it is well if it is covered with outside vegetation." I entirely agree with these _dicta_ of Professor Fugger.