Over the millions and billions of years that the Earth’s crust had been formed, the atoms were reacting and if they had different weights to different isotopes, they’d react at different speeds, there would be a fractionation. Just as you can make heavy water by a process, that is going on naturally. So if you get the water from seawater or you get it from the Baltic or you get it from snow or you distil it and keep distilling it, you will alter the atomic weight of hydrogen minutely, perhaps in parts per million. But sometimes those variations occur much more noticeably.

If you get a sample of boron dug up from compounds in Turkey it has a different atomic weight from... the boron that you’d get from the lakes... dry lakes in California. And so people were getting different values at the highest order of instrumentation... different values for the atomic weights so that one could only publish a range of values and sometimes that limited. Now, there is a caveat I have to put on there – there are 19, possibly 20 elements which are what are called mono-isotopic. There is only one type of atom of that element that occurs in nature. There are 19 of those and they have their atomic weight now known to at least nine significant figures because it can be measured by a mass spectrometry very, very accurately indeed. For all others, in principle, there is a variation. For some, the variation is large; for some it is not so large and is only noticeable.