12 



gives off radiations and small particles (of helium), thereby chang- 

 ing into a special kind of lead. The time that uranium takes to do 

 this is known — it is many millions of years. By measuring the 

 amount of lead in the uranium, the time that has passed can be 

 estimated. When this takes place right inside a rock, the amount 

 of helium (a gas) also gives a confirmatory figure. There are several 

 other methods as well, one involving 'isotopes'.* 



It is interesting to note that while, with all advances in technique, 

 readjustments of estimated past time occur, they are on the whole 

 of a comparatively minor order, so that it appears likely that we 

 really do know a good deal about the relatively enormous stretches 

 of time that have passed in our making. 



Almost everyone today accepts that our sun is a star, that in the 

 universe there are countless billions of other similar stars, and 

 that our sun started, somewhere and somehow, as an enormous 

 mass of very hot gas. This, whirling and moving at an enormous 

 speed through space, gradually cooled. Portions flew off at inter- 

 vals, and these are now the planets, of which our earth is one. 

 These smaller masses cooled more quickly than the sun itself. 

 Originally, of course, our earth was so hot it was almost all gas. As 

 it cooled liquid first formed at the centre ; then the surface became 



* A method that is proving of great value in dating remains of once-Hving 

 organisms has been evolved in recent times. It is based on the fact that carbon 

 in the structure of living organisms, both plants and animals, has been found 

 (1946) to contain a constant small amount of a radioactive isotope of atomic 

 weight 14. Compared with uranium this has a short life, the period of half 

 change being only 5,600 years. Because of its presence the carbon in organic 

 remains such as the bones of a skeleton, or of a tree trunk preserved by some 

 means such as being buried in a swamp, will steadily show less and less radio- 

 activity as time goes on. The amount is so very small that its measurement 

 demands great skill and many precautions. It has been possible to test the 

 method by the use of remains of accurately known age, and in the hands of an 

 expert it yields remarkable results. A striking application of this method has 

 recently caused a considerable scientific sensation. Early this century the 

 biological world was aroused by the discovery in deposits at Piltdown in 

 England of the bones of a skull claimed by many experts to be of an early 

 type of man since named the 'Piltdown Man', dating back close on a million 

 years. While some doubts were expressed about its validity, most British experts 

 accepted this view, and the bones remained a treasured possession in the 

 British Museum. The carbon-isotope method has led to the discovery that this 

 skull is made up of bones of different ages, all comparatively recent, none 

 really old. The whole thing was a deliberate fraud, there never was any 

 'Piltdown Man'. A book has recently (1955) been published gi\ing the whole 

 storv. 



