486 



NATURE 



[October 21, 1909 



I understand, Dr. Chree's basis for his most anxious 

 doubts ; and the object of my letter was to try to remove 

 at least this form of doubt from the minds of astronomers 

 and responsible magneticians. 



One more quite minor point I may take the opportunity 

 of mentioning, though it is connected rather with a letter 

 of Dr. Chree's in the Times than with his letter in 

 Nature. 



(7) Disturbance of terrestrial rainfall — say an increase 

 for a short period due to influx of cosmic nuclei — need not 

 be supposed to modify the usual local distrihution of rain, 

 but only to increase its amount in the customary localities. 



This I only venture to say very tentatively, and with 

 no dogmatism at all. It is clear that the total rainfall 

 all over the earth during a long period cannot exceed what 

 the sun can evaporate in approximately the same period, 

 and therefore depends more on the sun's total activity than 

 on anything else. It is also clear that rain is a local 

 circumstance, and that the conditions which determine 

 whereabouts rain shall fall are mainly local. But I ques- 

 tion whether either of these propositions really negatives 

 the idea that cosmic causes may occasionally affect the 

 rainfall during any given month, even in a specified 

 locality. Oliver Lodge. 



Why has the Moon no Atmosphere? 



Proctor (" The Moon, "p. 334) says :— " It has been held, 

 and not without some degree of evidence in favour of the 

 theory, that in our Moon we have a picture of our Earth, 

 as she will be at some far distant date . . . when her 

 oceans and atmosphere have disappeared through the 

 action of the same circumstances (whatever they may be) 

 which have caused the Moon to be airless and oceanless." 



The following considerations suggest what the circum- 

 stances referred to may have been, and present what seems 

 a possible cause for the absence of an atmosphere. 



(i) Apart from all theory, we know that the sun exerts 

 a repulsive force on matter around him. The phenomena 

 of comets' tails show this as clearly as the streamers from 

 a flagstaff show that a wind is blowing. Kepler first 

 suggested the existence of this force. Sir John Herchel, 

 in his essay on comets, said, more than forty years ago, 

 that " they have furnished us with a proof, amounting to 

 demonstration, of the existence of a repulsive force directed 

 . . . from the Sun." 



(2) Maxwell in 1S73 deduced from his electromagnetic 

 theory the pressure of light, which Arrhenius in 1900 

 applied to explain the formation of comets' tails. Each 

 p.-aticle projected from the comet, under the influence of 

 the sun's heat when nearing the sun, being submitted to 

 two opposing forces, viz. gravitation and the pressure of 

 light, he pointed out that since the pressure varied as 

 the surface, while the weight varied as the volume, i.e. 

 one compared with the other, varied as the square of a 

 number compared with its cube, then, when the particles 

 were small enough, the repulsive force of the pressure 

 might be many times as great as the force of attraction, 

 and drive away the particles with great velocity. 



As a common example of such action, I may remark 

 that we have the case of a wind blowing on a newly 

 macadamised road or on a single stone on the road. 

 \\'hile the stones are unbroken the wind cannot move 

 them, but when they are crumbled to powder it sweeps 

 them awav in clouds. 



(3) In this way the sun exercises a sort of sifting process 

 in space, sweeping away very small particles and drawing 

 the larger ones towards him. 



(4) Assuming that the moon had an atmosphere for 

 many ages, the particles would be acted on by the repulsive 

 force of the sun radiating from its centre, and bv gravitv 

 diiected to the centre of gravity of the moon. During the 

 time that the moon retained its atmosphere it is evident 

 that gravity must have been the preponderating force. 

 The atoms, as Dalton called them, were not small enough 

 to allow the pressure of light to prevail over the weight. 



(5) But the atmosphere has disappeared, and we have to 

 account for this fact. Can the particles have been in any 

 wav reduced in size? 



If we are sure that the chemical atoms, as Dalton called 

 NO. 2086, VOL. 81] 



them, despite the protests of such men as Davy, WoUaston 

 and Berzelius, cannot be decomposed or disintegrated, then 

 an hypothesis to the contrary must be rejected. 



(6) But the recent discoveries in radio-activity are opposed 

 to this. It has been shown that the radio-active elements 

 are disintegrating slowly and gradually from their own 

 internal energy. The process has been going on for in- 

 definite time, although only lately discovered accidentally 

 because of certain radiations. Have we reason to believe 

 that it is limited to these elements? Prof. Rutherford 

 has pointed out that the existence of rayless changes in 

 these elements " indicates .the possibility that undetected 

 changes of a similar character may be taking place in the 

 non-radio-active elements" ("Radio-activity," p. 455, 

 2nd edition). If we suppose that such changes took place 

 at the outer surface of the moon's atmosphere, resulting 

 in particles sufficiently small, then part after part of the 

 atmosphere may have'been stripped away until the present 

 condition has been reached. 



(7) The same process may be going on now with the 

 earth's atmosphere, notwithstanding the greater force of 

 gravitv. 



Briefly, if the atmosphere of the moon was ever driven 

 awav— the repulsive force of the sun (pressure of light) is 

 the only driving force we know of— the component particles 

 must have been originally too heavy to be driven off, and 

 were therefore in some way reducible ; the transformations 

 in the radio-active elements suggest a possible process. 



Thus the present condition of the moon is an argu- 

 ment for the disintegration of some of the non-radi»-active 

 elements, and the argument is the stronger in proportion to 

 the difficultv of finding a solution otherwise to this old 

 astronomical problem. .\lexander Johnson. 



Montreal, Can.ada, September 30. 



A " Canaan Stone.'' 



Captain B , of the Brixham (Devon) trawler fleet, 



recently showed me what he termed a " Canaan stone." 

 He told me that in the hands of his wife's mother it had 

 effected many miraculous cures of diseases of the eye, and 

 that by its use she had been especially successful in curing 

 cataract. The stone was a polished sphere of agate, 

 translucent, and of a faintly greenish-yellow tint, contain- 

 ing several red-brown patches due to the presence of iron. 

 It was about |-inch in diameter, and had been drilled 

 through the centre, as though it had at one time formed 

 part of a necklace. The treatment simply consisted in 

 '' striking " [i.e. gently rubbing) the eye with the stone. 

 No pravers or incantations were used, but it was essential 

 that different parts of the stone should be used in different 

 diseases, aijd the part used also varied with the colour of 

 the patient's eyes. The stone was rubbed actually on the 

 conjunctiva, not on the lids. The secret of the exact 

 method of treatment died with the old lady, who is re- 

 ported to have h.-id quite an extensive ophthalmic practice, 

 and I was appealed to in order that I might explain the 

 secret to the present owners of the stone. Bej-ond the fact 

 that the stone had been bought by its late owner from 

 a man in Cornwall for 40/., no history was available. 



The following extract from the Book of Tobit sug- 

 gested itself to me as a possible explanation of the origin 

 of the belief in the curative value of the stone ; — 



" When Tobias and Raphael came to the river Tigris, a 

 fish leaped out of the water and would have devoured him, 

 but the young man laid hold of it and drew it to land. 

 The Angel bade Tobias open the fish, and take the heart, 

 and the liver, and the gall, and put them up safely. . . . 

 And the Angel said . . . .'\s for the gall : it is good to 

 anoint a man that a whiteness in his eyes shall be healed. 

 . . . Tobias met his father at the door, and strake of the 

 gall on his father's eyes . . . and Tobit recovered his 

 sight." 



It does not require a great stretch of the imagination 

 to see a resemblance between this translucent, greenish- 

 vellow stone, with its red-brown patches, and the distended 

 gall-bladder of a fish, excised with small portions of liver 

 adherent to its surface. The expression "to strike," for 

 to anoint or rub, is still quite common in Devon. In the 

 country districts a usual treatment for sprains or abrasions 



