November 27, 1891.J 



SCIENCE. 



297 



the clouds above, before the flash. Otherwise they could 

 not have reached the earth at so nearly the same time. In 

 fact, Professors Ayrton and Perry show [Phil. Mag., 1878, 

 v., 197) that condensation is a cause for increase of electrical 

 potential, and this may produce the flash, and not the flash 

 the condensation. 



If, then, the lightning is the source of the sudden and in- 

 creased downrush of rain, in thunderstorms the same evi- 

 dence precludes the thunder also as a cause. If General 

 Dyrenforth's heavy cannonading and concussions could 

 evoke the rain-drops, then much more should we expect the 

 increase with the terrific reverberations of the thunder. But 

 all the proof is against his sound theory. The heavy deto- 

 nations, then, upon which General Dyrenforth bases his 

 theory are unnecessary, and the success of his experimenting 

 must be looked for in causes other than the noises of the ex- 

 plosion. 



One turns naturally, then, to the products of the explo- 

 sives. In General Dyrenforth's experiments, minute solid 

 particles of silica and carbon were liberated as results of the 

 explosions of the dynamite and rackarock. This fine dust, 

 entering into the upper air-layers, might have served as nu- 

 clei about which the moisture could gradually condense to 

 finally form rain-drops. When this has been accomplished 

 the rain fell, and not before. Even where the immense 

 oxy-hydrogen balloons were exploded, the dynamite bat- 

 teries were for hours steadily throwing fine powders of silica 

 and carbon into the air. 



Now it is well known that hail-stones, which are products 

 of the vapor condensation, often show a nucleus of a particle 

 of dust, and in volcanic regions frequently of a granule of 

 ashes. In these cases a dust-particle was the centre of the 

 condensation. This fact furnishes strong evidence to sup- 

 port the theory, that very small particles of dust may form 

 the nuclei of the rain drops and that the sudden presence of 

 fine powder in the upper strata of air will lead to condensa- 

 tion if sufficient moisture be present. 



It is well known that during the first few strokes of an air- 

 pump, a vaporous cloud appears in the receiver. Some ten 

 years ago Mr. John Aitken, in studying the London fogs, 

 proved that if the air in the receiver be first filtered through 

 cotton-wool so as to be dust-free, then no vapor cloud ap- 

 peared (Nature, Dec. 30, 1880, 195). He came to the con- 

 clusion, and stated it clearly, that no condensation will take 

 place unless some solid nucleus as dust be present. 



The writer has recently performed some laboratory experi- 

 ments similar to those of Aitken, to seek corroboration of 

 his results, and to determine any relative difference in the 

 properties of different dusts as regards their power for con- 

 densing moisture. I find with Aitken that condensation 

 under the receiver of the air-pump does not take place in 

 .dust-free air, and, further, that, with different powders in- 

 troduced, the amount of apparent condensation varied. The 

 experiments were then repeated without the air-pump as fol- 

 lows: Into a large glass sphere filtered air was introduced, 

 and then a steam jet discharged into it. No trace of con- 

 densation was present. Then air containing products of sul- 

 phur combustion was put in, and a heavy condensation be- 

 came visible. In a similar way, vapor clouds appeared with 

 ordinary atmospheric air and with air containing gun-powder 

 smoke. 



In this way it was found that such powders as carbon, 

 silica, sulphur, and common salt are particularly capable of 

 precipitating the moisture, while the burning of sulphur or 

 gunpowder gave heavy visible clouds of vapor. 



Laboratory experiments cannot represent conditions which 

 hold on a larger scale in Nature. Still they may be sug- 

 gestive. So from these experiments it may be legitimate to 

 reason that the finest dust introduced artificially into the 

 higher regions of the atmosphere will furnish centres for con- 

 densation, and by gradual agglomeration of moisture induce 

 a rain-fall. It must, however, be borne in mind, that there 

 must be sufficient water vapor in the atmosphere above to 

 gradually collect upon the dust. Therefore not under every 

 atmospheric condition could a rain-fall be hoped for. 



To prove and to make practical use of this dust theory, 

 elaborate and expensive experiments would not be necessary. 

 In place of the costly outfit required by General Dyrenforth 

 for producing his terrific noises, upon which his sound theory 

 depends; in place of the heavy mortars transported to the 

 plains; in place of the immense retorts with acids and chemi- 

 cals for producing oxygen and hydrogen gases neces.sary for 

 his balloons, there could be substituted the relatively inex- 

 pensive fire balloons. By sending up a few of these there 

 could be carried aloft a mile or so apart a quantity of impal- 

 pable powders. Then at the heighth of about a mile any 

 feasible means of scattering this powder into the air might 

 furnish the occasion for an artificial rain-fall. The burning 

 of sulphur or gun-powder by fuses timed for the proper 

 height of the balloons should also be tried. 



It might be urged against this theory, that many instances 

 may be cited where dust has been superabundant even in the 

 upper air layers, and no increase of rain-fall noticed ; that 

 volcanic eruptions emit quantities of finest ashes to the 

 atmosphere above, as did Krakatoa a few years ago, whose 

 dusty breath circled the earth for many a month, and yet no 

 unusual aqueous precipitations were observed. But it is to 

 be remembered that if the number of the dust particles is 

 excessive, the amount of moisture in the air, which is always 

 limited, will be divided among so many that the agglomera- 

 tion upon each will not be sufficient to cause it to fall as 

 rain-drops. 



If this dust theory be true, the amount of powder borne 

 aloft and exploded from the balloons need not be beyond the 

 limits of practicable experimenting. At least the experi- 

 ments necessary to test the theory would be incomparably 

 cheaper than General Dyrenforth's, and, if successful, arti- 

 ficial rains could be ordered at a cost which General Dyren- 

 forth's explosive bombardments cannot approximate. 



Ltjcien I. Blake. 



Department ol Physics and Electrical Engineering, 

 State University of Kansas. 



NOTES AND NEWS. 

 Dr. a. H. Beals has been appointed professor of pedagogics 

 and natural science, Georgia Normal and Industrial College, Mil- 

 ledgeville, Ga. 



— At the sixty-fourth meeting of German naturalists and 

 physicians at Ilalle, on the 23d of September, Dr. Below gave an 

 important address on "Health in the Tropics." He came to the 

 conclusion that the opening up of the tropics for Europeans was 

 practically a question of hygiene, and that, with proper sanitary 

 precautions, the most apparently unhealthy districts may be ren- 

 dered salubrious. 



— At the recent meeting of the Italian Congress of Internal 

 Medicine initial arrangements were made for the next Interna- 

 tional Medical Congress, which is to meet in Rome in 1893. In 

 what month of the year it will be held is au important question 

 not yet decided, according to The Lancet. At midsummer, or in 

 the early autumn, Rome is not likely to be found attractive to 

 those who dread subtropical heat in a malarious vicinity. If held 

 in the spring, or the late autumn, many teachers of medicine will 

 not be able to attend. The last fortnight of September is what the 



