 Another strand of the philosophical discussion probes at the contemporary IUPAC definition of elements. According to IUPAC, to be gold is to have atomic number 79, regardless of atomic weight. A logical and intended consequence of this definition is that all isotopes sharing an atomic number count as the same element. Needham (2008) has recently challenged this identification by pointing to chemically salient differences among the isotopes. These differences are best illustrated by the three isotopes of hydrogen: protium, deuterium and tritium. The most striking chemical difference among the isotopes of hydrogen is their different rate of chemical reactions. Because of the sensitivity of biochemical processes to rates of reaction, heavy water (deuterium oxide) is poisonous whereas ordinary water (principally protium oxide) is not. With the development of more sensitive measuring techniques, it has become clear that this is a general phenomenon. Isotopic variation affects the rate of chemical reactions, although these effects are less marked with increasing atomic number. In view of the way chemists understand these differences in behavior, Needham argues that they can reasonably be said to underlie differences in chemical substance. He further argues that the criteria of sameness and difference provided by thermodynamics also suggest that the isotopes should be considered different substances. However, notwithstanding his own view, the places in Mendeleev’s periodic table were determined by atomic number (or nuclear charge), so a concentration on atomic weight would be highly revisionary of chemical classification (Hendry 2006a). It can also be argued that the thermodynamic criteria underlying the view that isotopes are different substances distinguish among substances more finely than is appropriate for chemistry (Hendry 2010c).