November 3, 192 1] 



NATURE 



313 



frequency of sulphur. It is well known, too, that 

 coloured forms can be obtained of many 

 simple salts which normally are colourless, 

 such as sodium chloride, by supplying energy 

 to them. 



The phenomena of fluorescence and phosphor- 

 escence are also due to molecular phases. If a 

 molecule absorbs a phase quantum which, for in- 

 stance, is ten times the molecular quantum, this 

 energy can be radiated in two ways. It may either 

 be radiated as lo molecular quanta, when the 

 fluorescence will be in the infra-red, or it may be 

 radiated partly as one quantum characteristic of a 

 lower phase — say, that phase with frequency five 

 times the molecular frequency — and partly as mole- 

 cular quanta. In the second case the fluorescence 

 will be visible. 



Since the essential characteristic of the phases 

 of a molecule from the chemical point of view is 

 their force fields, the variation in which causes 

 their different reactivities, it might be argued that 



this theory is only a re-statement of the secondary 

 valency hypothesis. Such an argument would not, 

 however, be sound, for the secondary valency 

 hypothesis does not explain absorption. At best 

 it only succeeds in showing that different distribu- 

 tions of secondary valency can generally be 

 written where the same molecule has been found 

 to exhibit different absorption under different con- 

 ditions. The present theory establishes the exist- 

 ence of different phases of any inorganic or organic 

 molecule, each of which has its own energy con- 

 tent, its own reactivity, its own frequency and 

 power of absorbing light. The theory attempts 

 to correlate all the phenomena of absorption and 

 to place them on a quantitative basis, and in this 

 attempt it would seem to meet with some success. 

 Although in this article we are not concerned with 

 the chemical aspect of the differences in the force 

 fields of the phases, it may also be claimed that 

 this theory offers a quantitative explanation of the 

 phenomena of reaction and reactivity. 



Artificial Production of Rain. 



By Dr. Harold Jeffreys. 



IN an article in the Times of October 17 an 

 account is given of the achievements ot Mr. 

 Charles M. Hatfield in producing rain. The 

 method used is not described in any detail. A 

 tank filled with certain unspecified " chemicals " 

 was exposed at a height of 25 ft. above the 

 ground, and it is claimed that this had the effect 

 of producing 8 in. of rain in three months at 

 Medicine Hat, 22 miles away. The theory of 

 the method is that the apparatus draws clouds 

 from other parts to the Medicine Hat district and 

 causes them to precipitate their moisture there. 

 Xo direct observations of the motions of clouds 

 are mentioned in confirmation of this theory, 

 though they should not have been difficult to 

 obtain. 



The official raingauge at Medicine Hat during 

 May, June, and July, the period of the contract, 

 recorded 48 in., which was 13 in. below the 

 normal for the station for those months. Further 

 comment on the success of the experiments is un- 

 necessary. 



The financial side of Mr. Hatfield's contract 

 with the United States Agricultural Association 

 of Medicine Hat is interesting, for the associa- 

 tion was apparently prepared to pay Mr. Hatfield 

 as if 8 in. of rain had fallen. Still more interest- 

 ing is the fact that he was promised 4000 dollars 

 for^ in., and 6000 dollars for 6 in. Since the 

 normal rainfall is 6-i in., Mr. Hatfield would 

 have been much more likely than not to make a 

 substantial profit even if he had done nothing at 

 all. 



It may be mentioned that at Calgary, Alberta, 

 the rainfall was 3-0 in. below normal ; at Edmon- 

 ton it was 3-1 in. above; and at Qu'Appelle 



j (Sask.), 300 miles to the east, it was 3-85 in. 



! above normal. 



i NO. 2714, VOL. 108] 



It is also stated that at Los Angeles, in the first 

 four months of 1905, Mr. Hatfield guaranteed 

 18 in. of rain, and that his own raingauge showed 

 29-49 in. If this is correct the rainfall must have 

 been extremely local, for the official raingauge at 

 Los Angeles in those months showed only i4t^8 in. 

 Still, this was 44 in. above normal. At San 

 Diego, however, which is 200 miles away, the 

 excess was 4-6 in., and it appears likely that the 

 abnormality at both stations was due to 

 more widespread causes than Mr. Hatfield's 

 chemicals. 



Attempts have on many previous occasions been 

 made to produce rain by artificial means, but the 

 results have been uniformly unsuccessful. The 

 reason is not difficult to see. To make the water 

 vapour in the air condense it is necessary to cool 

 the air in some way to a temperature below the dew 

 point. This may be done in two ways. One may 

 cool the air directlv, for instance by the evapora- 

 tion of liquid carbon dioxide or liquid air. This 

 certainly would produce a little condensation ; the 

 fatal objection to it is that it would be thousands 

 of times cheaper to distil sea water. The other 

 method is to raise the air. The pressure de- 

 creases with height, and to reduce the pressure 

 on a particular mass of air is known to cool it. 

 The difficulty is to raise it enough. To produce 

 an inch of rain over an area of 100 square miles 

 requires the condensation of 6 million tons of 

 vapour, and to achieve this some hundreds of 

 millions of tons of air must be lifted up. The 

 distance it must be raised depends on how nearly 

 saturated it was originally, but it could not be 

 less than a kilometre in ordinary- fine weather 

 conditions. We have no source of energy at our 

 command great enough to achieve this. 



It is often suggested that rain may be produced 



