November 12, 19 14] 



NATURE 



29; 



minor extent locally by rainfall ; and the water is 

 ejected, when the water-bearing layers are reached by 

 bores, in consequence of gas pressure due to the intro- 

 duction of hot plutonic water and to rock pressure due 

 to the weight of the overlying shales on the water- 

 bearing sands and sandstones. The question is prac- 

 tically important, for the policy to be followed in the 

 administration of these well waters depends upon 

 which explanation is adopted. The advocates of the 

 first theor\' insisted that there was no evidence of any 

 reduction on the jield of the wells and that no diminu- 

 tion need be feared. Some of the advocates of the 

 alternative theory insisted that there would be a 

 serious fall in the discharge. 



The problems connected with the supply have been 

 referred to an Inter-State Commission, which has 

 collected much valuable evidence and an atlas of most 

 useful maps and plates. It considered, amongst other 

 questions, the corrosion of the tubes used for lining 

 the wells, which in some localities rapidly decay ; the 

 irregular distribution of this corrosion illustrates the 

 great local variations in the nature of the bore waters. 

 Prof. Fawsitt, who gave evidence before the Com- 

 mission, attributes the corrosion to the action of 

 carbon dioxide in the presence of free oxygen. 



One of the most striking maps presented to the 

 Commission is that showing the widespread diminution 

 from the Queensland wells ; many have ceased to flow 

 and others are greatly reduced in volume. At first 

 the advocates of the water-pressure theory attributed 

 such cases to the choking of the bore or to the escape 

 of water around the bore tubes ; but the Commission 

 rejects these explanations. The distribution of the 

 dwindling supplies in Queensland shows that the cause 

 is very widespread, and is subject to local variations 

 which it is difficult to explain except upon the gas- 

 and-rock-pressure hvpothesis. The Commission has 

 adopted the water-pressure theory, but it makes two 

 great concessions towards the later theor\\ Its re- 

 port remarks that " the gas in these waters must 

 undoubtedly to some extent assist in bringing the 

 water to the surface." The Commissioners add that 

 the rise of the water is primaril)' due to hydraulic 

 pressure ; but as they recognise that gas pressure 

 assists, the old calculations as to the water levels 

 which have been represented as the strongest argu- 

 ment in favour of the water-pressure theon,- can no 

 longer carry much weight. The Commission has also 

 adopted a definition of artesian water which abandons 

 the basis of the older hypothesis. 



"'Artesian and Sub-Artesian Water. — Water struck 

 in bores may be under ordinary atmospheric pressure, 

 or it mav be under a pressure exceeding that of the 

 atmosphere. In the former case the water may be 

 termed ordinary- ground water ; and in the latter case 

 the water may be termed either artesian or sub- 

 artesian. Artesian water is subject to a natural pres- 

 sure sufficient to force it above the surface of the 

 ground " (p. ix.). According to this definition all 

 water which rises to the surface of the ground, 

 wliether through any limestone source or boiling vol- 

 canic spring, is artesian- — an extensio ad absurdum of 

 the term. 



TONISATION} 



TONISATIOX is the process by which ions — par- 

 ^ tides charged with electricity — are produced in a 

 ilid, liquid or gas. This address is confined to the 

 msideration of ions in gases and deals with two 

 questions : (i) the nature of the ions, (2) the process 

 by which thev are produced. The evidence as to the 



'•■ Abstract of the presidential address delivered before the Physical 

 ^ ciety on October 23 by Sir J. J. Thomson, O.M., F.R.S. 



nature of ions is derived from experiments on their 

 mobility which have shown : — 



a. The mobility of a positive ion depends only on 

 the gas through which the ions are moving and not 

 on the nature of the gas out of which the ions are 

 formed. 



/3. The mobility of the ions in a gas at constant 

 density is independent of the temperature. 



•y. There is a considerable number of gases in which 

 the mobility is very approximately inversely propor- 

 tional to the square root of the density of the gas. 



8. In the case of negative ions there is an abnormal 

 increase of mobility when the pressure is reduced 

 below a certain value, and there is some evidence that 

 this is also the case for positive ions at very low 

 pressures. 



Two theories of the mobilities of ions were con- 

 sidered, one founded on the view that the action 

 between ions and molecules is analogous to impacts 

 between hard elastic spheres, the other on Maxwell's 

 theory of forces between ions and molecules varying 

 inversely as the fifth power of the distance between 

 them. 



It is shown that (a) follows from the second theory" 

 provided the ion is a cluster of which the mass is con- 

 siderably greater than that of a molecule of the gas 

 through which it is moving. On this supposition it 

 follows also from the first theory if we suppose in 

 addition that all ions in a given gas are of the same 

 size. 



ifi) follows at once from the second theory ; to 

 explain it on the first theory we must suppose that 

 the size of the ion varies with the temperature in a 

 definite way. The necessar}- relation can be deduced 

 from thermodynamical principles if we suppose that 

 the force between an ion and a molecule is analogous 

 to that between a charged point and a sphere which 

 is either a conductor of electricity or has a high 

 specific inductive capacity. 



(7) requires on theory one that the ions in these 

 gases should be of the same size, on theor\- two that 

 the molecules of these gases should exert the same 

 force on a charged point at a given distance. 



(8) follows on either theory if the ion dissociates at 

 low pressures so that free corpuscles are present in 

 the gas. 



With regard to the process of ionisation, the first 

 stage of this in the vast majority' of cases consists in 

 the detachment of an electron or corpuscle. Evidence 

 as to the method by which this takes place is afforded 

 by determinations of the velocity with which the 

 electrons are ejected from the body. 



In the case of ionisation by light or Rontgen rays, 

 this velocity depends primarily on the wave-length of 

 the radiation, not upon its intensity, nor, at any rate 

 to any great extent, on the nature of the molecule 

 from which the corpuscle is ejected. This velocitv is 

 far greater, even when every allowance is made for 

 resonance, than can be accounted for if we suppose 

 that the energy of the light is uniformly distributed, 

 and that the corpuscle acquires its veiocitv bv the 

 action on it of the electric force in the wave. An- 

 other explanation not open to these objections was put 

 forward. 



When ionisation is due to the action of moving 

 electrified particles, whether positive or negative, the 

 results are quite different. The velocity of the ejected 

 particles (^rays, as they are sometimes called) does 

 not seem to van*- much, if at all, with the velocity of 

 the particles which eject them, and is of the same 

 order whether these particles are f>ositive ravs or the 

 much swifter kathode rays. The method of ejection 

 by the impact of such particles was considered, and 

 suggestions as to a possible theory and some of its 

 consequences thrown out. 



XO. 2350, VOL. 94] 



