424 PROTOPLASM 



Heavy Water. — The discovery of an isotope of hydrogen led 

 immediately to its application to biology. 



Isotopes were first described by J. J. Thomson, who called 

 attention to the fact that neon may exist in two forms with prop- 

 erties alike in all respects, except weight. His discovery was 

 followed by the work of Aston to whom most of our present 

 knowledge of isotopes is due. Lead that has come into existence 

 through the breaking down of uranium has an atomic weight 

 different from that of ordinary lead and also from that of lead 

 which is the result of the breaking down of thorium. These three 

 substances are all lead in that they have the same chemical prop- 

 erties but are of different atomic weights, by which is meant that 

 their atomic nuclei are of different composition but their outer 

 fields of electrons are the same. Such substances are isotopes, 

 and it is now known that the majority of the elements consist of 

 isotopes. The atomic weights of the isotopes of an element are 

 whole numbers; a mixture of them usually results in a non- 

 integral value; thus, chlorine, as we know it, has an atomic 

 weight of 35.5 because it consists of an isotope of weight 35 and 

 one of weight 37. Magnesium has an atomic weight of 24.32, 

 which is the net average of three magnesium isotopes having the 

 respective weights 24, 25, and 26. Among the 92 elements, 66 

 have been studied as to their possible isotopic character and 

 only 24 have been found to be "simple" elements. Mercury 

 is an extreme case, with six isotopes of atomic weights — 197, 

 198, 199, 200, 202, 204— the net average of the mixture being 

 200.6, the atomic weight of mercury. 



The latest additions to isotopes are those of hydrogen, the most 

 significant one of recent discovery having a mass of two. The 

 discovery of the second hydrogen atom, of twice the weight of 

 the former one, was made by Urey and was based on a new line 

 in the hydrogen spectrum. H^ is the symbol of the heavier of 

 the two hydrogen isotopes. It probably consists of a neutron 

 added to the nucleus of the lighter hydrogen. Ordinary water 

 contains very little heavy hydrogen — about 1 atom to every 

 6,000 atoms of light hydrogen. 



There is yet another isotope of hydrogen (and there may be 

 more) of atomic weight 3, but it is very rare (H^ is six thousand 

 times as abundant as H^, and H^ is thirty thousand times as 

 abundant as H''). 



