December 17, 1908] 



NA TURE 



207 



radium, which constitute the most important feature of 

 radio-active emission, are only able to penetrate a limited 

 and definite distance into solid materials. They then lose 

 their characteristic properties, if, indeed, they are not 

 altogether stopped. This distance has been measured 

 experimentally, and Prof. Joly has shown that the distance 

 is just the same as that to which the alteration round 

 the zircon crystals extends. Thus we have full quantita- 

 tive confirmation of the theory which attributes it to 

 radio-activity. 



I will now pass from the discussion of a very minute 

 phenomenon to the discussion of a large-scale one. It will 

 be familiar to many of you that, in the opinion of some, 

 at least there is reason for changing the views which 

 have been held for two generations concerning the earth's 

 internal heat. We know that there is, at any rate, some 

 radium in the earth, and that radium gives out heat. 

 Thus it cannot be disputed that some part of the earth's 

 internal heat must be due to this cause ; the only question 

 which remains is whether this part is large or small, 

 whether, in fact, the earth's internal heat is chiefly to be 

 accounted for as a small remnant of the much greater 

 internal heat which it once possessed, or whether there 

 is enough radio-active material in the earth to supply 

 most of the annual loss by conduction through the crust 

 and radiation into space. 



.'\s I mentioned before, I have made a large number 

 of determinations of the quantity of radium in the rocks 

 of which the superficial portions of the earth are con- 

 stituted. These are found to be so rich in radium that 

 the difliculty is not so much to account for the internal 

 heat of the earth, as determined by underground observa- 

 tions of temperature, but rather to understand why it is 

 not much hotter. I have suggested, as an explanation, 

 that this general distribution of radio-active material, 

 which pervades the outer parts of the earth, is in reality 

 superficial, extending only to some moderate number of 

 miles in depth, though no doubt much deeper than the 

 deepest mines. I am not wholly satisfied, however, of 

 the sufficiency of this explanation. Radium, and the series 

 of products of which it is one, are not the only radio- 

 active materials in the earth ; there is another series, of 

 which thorium is a member, and there is good reason to 

 suppose that thorium is present in rocks in such quantity 

 as to add appreciably to the evolution of heat. Taking 

 this into account, we should probably find, if we had 

 e.xait data for calculation, that the thickness of rock con- 

 taining radio-active material was so small that the material 

 of the interior would somewhere have exuded, in the course 

 of the violent dislocations and earth movements which 

 geology reveals to us. No material, however, appears 

 anywhere at the earth's surface which can plausibly be 

 regarded as representative of the unknown interior if the 

 suggested hypothesis is accepted. It cannot be denied 

 that the subject is at present obscure. Possibly an cx- 

 plan.ition may be found by supposing that the activity 

 of uranium may be arrested at high temperatures. We 

 have at present no adequate experimental evidence on the 

 subject. It is known that there is very little effect of this 

 kind on radium. If, how-ever, the activity of uranium 

 were arrested at a high temperature, the supply of radium 

 and all the other members of the series would fall off, 



I and thus the aggregate heat production of the whole series 



I might be greatly diminished. 



I I shall now pass to another branch of the subject. The 



' investigations of Sir William Ramsay and Mr. Soddy have 

 proved that there is continuous evolution of helium from 

 the radium emanation. We have good reasons, into 

 which, however, I do not propose to enter, for considering 

 that the same is true of radio-active changes in general, 

 at all events those in which there is an emission of radia- 

 tion. Ilelium is probably evolved at each stage of the 

 transformation of uranium, and at each stage of the trans- 

 formation of thorium ; and it results that the natural 

 minerals and ores in which these elements are found con- 

 tain a store of helium, which has accumulated in them 

 and remains locked up in their pores. 



.As already mentioned, I have succeeded in determining 

 the presence of radium in granite. Thus it becomes 

 natural to inquire whether the corresponding amount of 



, helium is to be found there too. \othing of the kind 



I NO. 2042, VOL. 79] 



had ever come under observation before, and it was, there- 

 fore, with some interest that I made the experiment. 

 Vou see before you a vacuum tube of helium prepared 

 from ordinary granite. The characteristic yellow glow 

 will satisfy anyone acquainted with the appearance of a 

 helium discharge of the presence of the gas. 



The facility with which helium was detected in granite 

 suggested further experimental problems. The un- 

 doubtedly radio-active elements are at present confined to 

 uranium and thorium, and their respective fainilies of 

 descendants. Evidence has been produced, by myself 

 among others, which suggests that lead and some other 

 elements possess a feeble radio-activity of their own ; but 

 this evidence is somewhat equivocal. It seemed highly 

 desirable to attack the question in a new way, and the idea 

 suggested itself of looking for helium in the naturally 

 occurring ores of all the elements, common and rare. 

 This had indeed been done, to some extent, from quite 

 a different point of view, by Sir William Ramsay and his 

 coadjutors, in their first investigations on helium ; but 

 their observations were directed to finding a practical 

 source of the gas, and were not carried out with any- 

 thing approaching the minuteness required for the present 

 purpose. 



The upshot has been to prove the presence of helium 

 in almost every mineral examined, and even in such un- 

 promising materials as rock crystal, or common quartz 

 sand. The quantity found in the various cases has varied 

 very widely. In fact, minerals may be found having any 

 helium content, from thorianite, which contains lo cubic 

 centimetres per gram, down to rock crystal, which con- 

 tains about a ten-millionth part of that quantity. 



I have here a small tube of helium obtained from clear, 

 colourless rock crystal, and you will have no difliculty in 

 seeing the characteristic yellow glow as before. 



.Are we to regard the helium in common minerals as 

 due to a feeble radio-activity of the common elements? 

 No doubt such an hypothesis is tempting, but it must be 

 rejected. Radium is present everywhere in traces, and 

 these traces are in general sufficient to account for the 

 minute quantities of helium. This is illustrated in the 

 table below, which gives in round numbers the actual 

 amount of helium extracted from various minerals by 

 heat, and the amount of helium reckoned relatively to 

 the radium. 



I'ellum ratio. 

 Helium pre"iT. Lt. latin of h'livim 

 ,„. I er kil to ra^iuin. 



.Aril trary ■;ca'e. 



There is reason to think, as already mentioned, that 

 the presence of thorium would constitute another source 

 of helium ; but it is believed that this complication does 

 not produce any appreciable effect in these cases. You 

 will see that minerals like quartz, though they contain 

 actually only an infinitesimal quantity of either substance, 

 still show about the same proportion of helium to radium 

 as the minerals which are rich in both. We may conclude 

 that helium is connected with radium in the poor minerals 

 as in the rich ones. 



I have, however, encountered an interesting exception 

 to this rule in the mineral beryl. Beryl is, in all essentials, 

 the same as emerald ; the latter name is kept for stones 

 which are of a clear, deep green colour ; but scientifically 

 the distinction is of no importance. Some beryls contain 

 enormouslv more helium than can be accounted for by 

 the small 'traces of radium in them. Nor do they contain 

 anv appreciable quantitv of other radio-active material. 

 What view, then, can we take of the presence of helium 

 in this mineral? It is, to me at least, diflficult to believe 

 that the gas can have been introduced from without. If 

 not, can it have been generated from radium formerly 

 existing in the beryl, but now exhausted? This, too, 

 seems unlikely, for it would imply that beryls are older 



