352 THE POPULAR SCIENCE MONTHLY. 



rial which is called scoria. If the lava is glassy it hecomes pumice, a 

 mass of minute glass bubbles drawn out in one direction by the move- 

 ment of the mass while it was still in a plastic state. Fragments of 

 scoria and pumice are often thrown by a violent escape of steam to 

 a height of hundreds or thousands of feet into the atmosphere. While 

 going up and coming down, they encounter each other and wear each 

 other away by their frequent rubbing, with a noise which is one of 



Fig. 3. Minute Liquid-Cavity in a Crystal, with a moving Bubble. (The path of the bubble 



is indicated hy the dark line.) 



the most noteworthy accompaniments of volcanic eruptions. ]\Ir. 

 Poulett Scrope, who watched the Vesuvian eruption of 1822 for nearly 

 a month, remarks that at first fragments of enormous size were thrown 

 out, but that they were gradually reduced by constant re-ejections, till 

 at last only the most impalpable dust issued from the vent a dust 

 which was so excessively finely divided that it went everywhere, even 

 into the most closely fastened boxes. Mr. Whymper estimates that 

 no less than two million tons of dust must have been ejected during 

 a single slight outburst of Cotopaxi which he witnessed ; and Profess- 

 or Bonney calculates from actual examination that it would take from 

 four to twenty-five thousand particles of this same dust to make up a 

 grain in weight ! 



The temperature and consistency of lava-streams vary greatly, and 

 the variations give rise to differences in the appearance of the cooled 

 mass. The surface of the stream cools rapidly in the air, so that it 

 appears dull-red at night and black by day like a great mass of rough 

 cinders while all is of a white heat beneath, and may be so seen at 

 night shining through the rough, cindery masses. Some streams are 

 very liquid, resembling rivers and filling every channel in their course ; 

 while others, cooler and stiffer, might be more fitly compared to gla- 

 ciers, creeping along so slowly that the fact of their movement can be 

 established only by the most careful observation. The stiff lavas leave 

 a crust wrinkled and folded like coils of rope, and are then frequently 

 called " ropy lavas." The very liquid, fast-floAving lavas leave a sur- 

 face covered with rough, cindery masses presenting jagged projections. 

 Admirable examples of the ropy lava are afforded by the Vesuvian 

 lava-stream of 18G8 (Fig. 4), and by the lava-cascade of the Island of 



