The rain which pours down from this cloud is often mixed with ashes and is as black as ink. The ashes have a color which varies between light-gray, yellow-gray, brown, and almost black, and they consist of minute spherules of lava ejected by the force of the gases and rapidly congealed by contact with the air. Larger drops of lava harden to volcanic sand—the so-called “lapilli” (that is, little stones), or to “bombs,” which are often furrowed by the resistance offered by the air, and turn pear-shaped. These solid products, as a rule, cause the greatest damage due to volcanic eruptions. In the year 1906 the weight of these falling masses (Fig. 4) crushed in the roofs of houses. A layer of ashes 7 m. (23 ft.) in thickness buried Pompeii under a protective crust which had covered it up to days of modern excavations. The fine ashes and the muddy rain clung like a mould of plaster to the dead bodies. The mud hardened afterwards into a kind of cement, and as the decomposition products of the dead bodies were washed away, the moulds have provided us with faithful casts of the objects that had once been embedded in them. When the ashes fall into the sea, a layer of volcanic tuffa is formed in a similar manner, which enshrines the animals of the sea and algæ. Of this kind is the soil of the Campagna Felice, near Naples. Larger lumps of solid stones with innumerable bubbles of gases float as pumice-stone on the sea, and are gradually ground down into volcanic sand by the action of the waves. The floating pumice-stone has sometimes become dangerous or, at any rate, an obstacle to shipping, through its large masses; that was, at least, the case with the Krakatoa eruption of 1883. [Illustration: Fig. 3.—Eruption of Vesuvius in 1872. (After a photograph.)] Among the gases which are ejected in addition to water vapor, carbonic acid should be mentioned in the first instance; also vapors of sulphur and sulphuretted hydrogen, hydrochloric acid, and chloride of ammonium, as well as the chlorides of iron and copper, boric acid, and other substances. A large portion of these bodies is precipitated on the edges of the volcano, owing to the sudden cooling of the gases. The more volatile constituents, such as carbonic acid, sulphuretted hydrogen, and hydrochloric acid, may spread over large areas, and destroy all living beings by their heat and poison. It was these gases, for example, which caused the awful devastation at St. Pierre, where 30,000 human lives were destroyed on May 8, 1902, by the eruption of Mont Pelée. The ejection of hydrogen gas, which, on emerging from the lava, is burned to water by the oxygen of the air, has been observed in the crater of Kilauea.