472 THE POPULAR SCIENCE MONTHLY. 



mass of matter involved, it may nevertheless serve to measure the 

 very low order of its quantity. Many snuff-boxfuls were thrown out 

 of Krakatoa. Moreover, the dust-particles may be very sparsely scat- 

 tered ; the miles of air through which they are spread compensating 

 for the wide space between them. The fineness of the solid dust is a 

 legitimate result of what is now known of the constitution of lavas. 

 Microscopic examination of igneous rocks has shown lithologists how 

 well a volcanic explosion can produce diffracting dust ; high magnify- 

 ing power, applied to rocks that are presumably old buried lavas which 

 failed to reach the surface, reveals the presence of the minutest cavi- 

 ties containing liquids or gases or both, so small and so closely packed 

 that myriads would be contained in a cubic inch : under the decreas- 

 ing pressures found as lavas rise through a vent to escape at the sur- 

 face outlet, the occluded gases and vapors would escape, and in so 

 doing would shatter the lavas to the finest imaginable dust. It is 

 probably by this intimate process, as well as by ordinary forms of 

 mechanical violence, that Krakatoa was, figuratively, blown to atoms. 

 The greater and coarser part of the dust darkened the sky for a day 

 or two and soon fell on the surrounding lands and seas ; a finer rem- 

 nant was carried high into the air by the outrushing gases, and then 

 spread far and wide over the earth to produce the marvelous sunsets ; 

 does the finest residue still hang aloft and give us Bishop's ring ? 

 How can it be suspended so long ? 



Kiessling's experiments have led him to believe that the coronal 

 diffraction does not take place immediately around the volcanic dust- 

 particles, but rather around the minute globules of water or ice con- 

 densed on these particles as nuclei. Recent researches have shown 

 that water-vapor may remain in the gaseous state below the tempera- 

 ture proper to its condensation, provided there is no solid or liquid mat- 

 ter present on which the condensation can begin ; the change from the 

 gaseous to the liquid state seems to desire the presence of some point 

 of beginning, such as is furnished by ordinary dust, or by the far finer, 

 ultra-microscopical particles always present in the air. It is in part 

 for this reason that great cities in damp countries must be hopelessly 

 foggy; however perfect the combustion in their numerous furnaces, 

 unburned ash in very fine division must fly up the chimneys as long as 

 wood and coal are used, and the finer the ash the better for the fog, 

 when the coolness for condensation arrives. Now, in connection with 

 this, there is a very peculiar point to be considered, concerning the 

 distribution of water-vapor in the atmosphere. Water-vapor is a light, 

 elastic, condensible gas, and its elastic lightness is always tending to 

 throw it to an altitude where the cold of its expansion would require 

 a part of it to condense into the liquid or solid state. It can, as yet, 

 hardly be said that some minute point of beginning is absolutely 

 necessary for all such condensation, but it may be safely asserted that 

 the presence of dust aids and increases the rapidity of the process ; 



