The lava in a volcanic vent is always in a highly heated, usually incandescent, condition. Seen by night, its freshly exposed surface is glowing red, sometimes apparently white-hot. But by exposure to the atmosphere the surface is rapidly chilled, appearing dull red by night, and black by day. Many persons are surprised to find that a flowing stream of lava presents the appearance of a great mass of rough cinders, rolling along with a rattling sound, owing to the striking of the clinker-like fragments against each other. When viewed by night, the gleaming, red light between these rough, cindery masses betrays the presence of incandescent materials below the chilled surface of the lava-stream. No fact in connection with lavas is more striking than the varying degrees of liquidity presented by them in different cases. While some lava-streams seem to resemble rivers, the material flowing rapidly along, filling every channel in its course, and deluging the whole country around, others would be more fitly compared to glaciers, creeping along at so slow a rate that the fact of their movement can only be demonstrated by the most careful observation. Even when falling over a precipice such lavas, owing to their imperfect liquidity, form heavy, pendent masses like a 'guttering' candle, as is shown by fig. 18, which is taken from a drawing kindly furnished to me by Capt. S. P. Oliver, R.A. The causes of these differences in the rate of motion of lava-streams we must proceed to consider. [Illustration: Fig. 18.--Cascade of Lava tumbling over a cliff in the Island of Bourbon.] [Sidenote: TEMPERATURE OF LAVA-STREAMS.]