56 MEASURING THE PRODUCTIVITY OF THE SEA 



which are not electrically charged. These are without importance to the 

 number of rotating electrons and so have nothing to do with the chemical 

 classification of the atom. Physically, however, it is a different matter; 

 a neutron weighs about the same as a proton, while, compared to the 

 nuclear particles, the electron weighs very little. 



In the nucleus of C^', then, there are six protons and six neutrons, 

 totalling 12; in C^^ there are six protons plus eight neutrons, totalling 14. 

 An atom of C^^ thus weighs one-sixth more than an atom of C^". 



The various physical modifications of a chemically identical element 

 are known as isotopes, and there are several carbon isotopes; namely, C^", 

 C", C^^ C^^ and C. Of these only two are stable — C*- and C*^ The 

 others are unstable, C^" and C^^ in a very high degree and C*** much less. 

 In a nucleus of a C*'' atom, however, a neutron may suddenly be converted 

 into a proton during the emission of an electron. We then no longer have 

 six protons in the nucleus but seven, and the carbon atom has become a 

 nitrogen atom. 



In a given quantity of C^* it takes 5,000 years for half the atoms to 

 become nitrogen. This is a very long time, and yet a number of conver- 

 sions take place every moment. Every minute in one gram of C^* there are 

 2X10^^ conversions (two followed by 13 ciphers — an astronomical figure). 

 In as little as one-millionth of a gram there are 20,000,000 conversions 

 from carbon atom to nitrogen atom per minute. The reason why there 

 are so many, of course, is the great number of atoms present. In one 

 gram of C" there are no fewer than 43 X 10^^ (43 followed by 21 ciphers). 



While in chemical analyses a comparatively large amount of material 

 must be used (rarely less than one-millionth of a gram), with a physical 

 method employing C^^ much smaller quantities will suffice (down to one 

 100,000th part of a millionth of a gram). 



The small quantity of C^* is possible because in fact a measurement 

 is made every time a single C*^ atom is converted into nitrogen. This 

 conversion, as stated above, is associated with the emission of an electron 

 from the nucleus. The emission takes place with so much force that the 

 electron may penetrate the thin window of a Geiger-Miiller tube, where 

 it will ionize the air and effect a brief electric discharge in the tube. The 

 discharge is amplified in roughly the same manner as a radio receiver 

 amplifies impulses, and the effect is to set going a mechanical counter. 



Because C^* emits electrons we call it radioactive, though elements 

 can be radioactive in other ways. They are often dangerous to work with, 

 but in the concentrations used on the Galathea C^* is perfectly harmless, 

 provided a few simple precautions are taken. 



