The chemistry of substances and the philosophy of mass terms

entity (as in "gold is an element"). A predicative mass noun takes quantifiers and various modifiers and denotes a set of quantities of the substance. An argument about whether there are rigid designators will be about nominal mass terms; but, of course, the quantities of substance are referred to contingently. (Compare also Salmon, 1982, p. 86.) This distinction is well documented with respect to physical quantities (see for example Palacios, 1964) and is basically an expression of the belief that the "concept" is fixed, but what there is changes all the time. It can, however, be applied to both the concepts of "substance" and of "molecule". 21 Mercuric oxide can be obtained in red and in yellow form, the difference being caused by the size and surface structure of the powder particles. This case, the same phase displaying different colours, should be clearly distinguished from cases like diamond and graphite the same substance being displayed in two different phases. 22 It adds to the confusion that it is not always clear whether the term 'H20' refers to one molecule, to a set of similar molecules, to the chemical composition, or to some concept or other. 23 This point has been discussed at great length in the literature. See Zemach (1976), Mellor (1977), Platts (1983), Smith (1981), Wiggins (1980, pp. 211-212). Of course, by stipulating in a scientific theory (as summarized in the main text) that isotopes are different substances, this does not exclude the possibility that the discussion about isotopes refers to a real problem no matter how sensible the definition of substance may seem. I think at least two problems are hidden in this discussion. Firstly, there is the question whether, if it is accepted that substances do have essential properties, how do we know that scientists have actually discovered any of these properties (see for example Averill, 1982). Secondly, there is Locke's statement (as referred to by Platts, 1983; see also Fales, 1981) that our natural kind classifications are as interestingly informative about ourselves as about the natural orders of things. If the essential properties have to have an explanatory function, wouldn't their relevance be determined by the interests of the investigators? If so, there is of course no problem with the isotopes. Sometimes atomic number is the "essential explanatory property"; at other times the isotope number is. It would not only be so that there have been radical This content downloaded from 207.46.13.80 on Thu, 11 Aug 2016 06:25:15 UTC All use subject to http://about.jstor.org/terms THE CHEMISTRY OF SUBSTANCES 319 changes in what scientists consider to be the essence of things (Zemach, 1976) that might only indicate that they are making progress. It would also depend on which scientists you asked. The consequence of this would be that what are called essential properties are only so relative to our knowledge or way of looking at the world (Smith, 1981; Farrell, 1983). And if the extension a term has is relative to its use within a given linguistic community, then in 1750 the extension of 'water', as used by OscarE and OscarTE is the same (sense determines reference), no matter whether we say now that in fact in 1750 the molecular structure was different on earth and on twin-earth. 24 Putnam (1982, p. 63). Note that on the account given in the previous section, (6) is still false. This is so because pure water is defined by its physical properties, which are different for different isotopes. Hence water, if pure, will not contain D20, whereas according to Putnam, containing some D20 is part of what it is to be water. The fact that water as it occurs on earth consists of a mixture of H20, D20, and some other isotope combinations of hydrogen and oxygen, is due to the history of the earth (which might well have been different). It is not, so to say, that H20 and D20 both occur in the same quantum mechanical formula, in the way that we can envisage that H20 and H402 do occur in the same formula. Therefore the substance with composition H20 is definitely different from the substance with composition D20. Of course, this does not exclude the possibility to argue for the statement that the combination of H20 and D20 (and a few other things) in a certain (or, more probably, in any) proportion constitute a natural kind as well. If the philosophical concern is primarily to "save" a statement that is both a posteriori and necessary, a solution might be to consider: "Water contains hydrogen atoms" (Tye, 1983). But as there are many things that contain hydrogen atoms, this statement is extremely uninformative about what water is. 25 Putnam (1982, p. 64). See also Farrell (1983): "two samples of liquid are both samples of the same liquid if they share, or have relevantly very similar, molecular structures"; (italics added). It would also seem that it is difficult to take (6) as an identity statement in the sense of Kripke. 26 Weyl (1963). The expression "a few sharp lines" refers to the lines in a valence diagram, representing the bonds between the atoms in the molecule. 27 Putnam (personal communication, 1983). It is not clear to me whether there is general consensus on this point in the philosophical literature. 28 Following Maccoll (1964) and adding (d). Many publications give similar classifications. As noted before philosophers are usually very vague about what sort of molecular structure they have in mind. It may be speculated that in most cases level (a) is intended. The following two quotations are explicit examples of this. "Molecules will be said to match if they contain atoms of the same elements in the same topological combinations" (Quine, 1977). "Thus 'molecule of H20' denotes a natural kind, mem bership in which depends on whether a given molecule is made up of certain kinds of atoms related in certain ways" (Hirsch, 1982, p. 266). Note that in both quotations reference is made to "kinds" (viz., "molecule", and "molecule of H20"). These may be proper natural kinds on one view or another. But nothing follows from this, necessarily, with respects to the kinds "substance" or "water". 29 Wheland, as quoted in Kursanov et al. (1959). Although the mesomer concept is considered useful as a mathematical tool, it is still argued that: "The basic assertion of the resonance theory, that 'resonance' in some way can determine the properties of molecules, is devoid of meaning." This content downloaded from 207.46.13.80 on Thu, 11 Aug 2016 06:25:15 UTC All use subject to http://about.jstor.org/terms