496 POPULAR SCIENCE MONTHLY. 



The formation of the vapor drops that make the cloud was long 

 a puzzle to science, but modern research has at last succeeded in solving 

 this mystery. It has been found by experiments that if pure dry air, 

 and pure vapor of water, be mixed in a clean vessel, and then cooled 

 down below the temperature of saturation, the drops of mist are not 

 generally able to form. Purity means that all the particles of dust 

 which float in the air have been perfectly filtered out, and that all 

 traces of electricity have been removed from the air and the vapor 

 before mixing them. It was further discovered that if fine dust powder 

 is injected into the pure mixture, without changing the temperature 

 or the pressure, the drops of water developed at once; also that if 

 minute charges of electricity, carried on particles of matter which may 

 be as small as one-thousandth part of the mass of an atom of hydrogen, 

 are introduced, the drops are able to condense. It is inferred that 

 nuclei of some kind, dust particles or electric particles, called ions 

 or electrons, are required for the formation of water drops suspended 

 in dry air, one nucleus for each drop. Hence, it is possible by counting 

 the number of minute drops that form in a cubic inch, to estimate the 

 number of motes of dust in the air, and even the number of ions 

 charged with electricity in a given volume. The number of the ions 

 contained in the air may be enormous, ranging from 20 per cubic 

 centimeter to many millions. We perceive further that these minute 

 drops coalesce to form rain, which falls from the clouds to the ground, 

 and that they carry down the dust previously blown up by the winds 

 and so purify the atmosphere from all sorts of small floating particles. 

 They also bring electric charges to the earth, and this has something 

 to do with producing the atmospheric electrical potential which always 

 exists. These ions are a natural portion of the atmosphere itself, being 

 continuously produced in it, even when no special cause seems to be 

 present, and they have much to do with explaining some of the strange 

 characteristics of atmospheric electricity which have so long baffled all 

 efforts to comprehend. Investigators are now paying the closest atten- 

 tion to these ions from every point of view. 



After clouds have been formed by the condensation of the aqueous 

 vapor, which has been lifted by evaporation from the surface of water 

 at the earth into the air, they take on very different shapes according 

 to circumstances. They may be broadly divided into two classes, the 

 cumulus type and the stratus type. Cumulus clouds are the fleecy, wool- 

 pack clouds usually seen on a warm summer afternoon ; the stratus are 

 the horizontal veils or sheets that cover the sky more or less completely 

 and which may develop into a general rain. The cumulus of the lower 

 strata, one mile high above the ground, may grow upwards so as to 

 form large domes, and in hot weather they become cumulo-nimbus 

 when the heads are very lofty, some having been observed to reach six 



