There is a sense in which Needham is right. It does not explain why substances combine at all! The demand that a science explain everything in its domain leads to an infinite regress. It is not sensible to criticise modern electron theory because it does not explain the charge on the electron. Explanatory goals in science should be geared to what is realistic given the theoretical and experimental resources.
Newton was wise to refrain from explaining gravity because there was no promising path towards achieving such an explanation and because there were many opportunities to explain many phenomena by taking gravity for granted. Berthollet had already pinpointed the futility of attempting to access inter-atomic force laws empirically.
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Dalton was wise to avoid them in his chemistry and on shaky ground when he retained them in his physics of gases. Unlike Needham, I claim Dalton was wise to steer clear of hypotheses about forces of chemical affinity and did succeed in explaining the laws of proportion. The fact that there was no alternative explanation on offer at the time tells in favour of Dalton.
But let us recall that by the mid-nineteenth century the only explanation on offer of known optical phenomena, including the recognition that light reaches us from distant stars, was that light is a transverse wave in an elastic aether. But there is no aether. One natural demand for the adequacy of an explanation of some phenomenon by a theory, in addition to the demand that the theory entail the phenomenon, is that there is evidence for the theory independent of the phenomenon explained.
If we include the physical aspects of his atomism this is no longer true, but hardly helps the case for Dalton given the fate of those aspects of his atomism that we have described above. Heschel, H. Davy and J. Brock, , pp. The detailed chemistry in these works follows a common pattern. The sections on each chemical substance begin with a description of the key chemical properties and mode of preparation of the substance, details the results of the analysis of the proportions of elements in the substance where it is a compound and then concludes by suggesting an atomic constitution for each compound.
What Dalton takes to be the atomic constitution of a compound is a result arrived at after the chemistry has been done. This contrasts with what others were able to do with an appropriate deployment of chemical formulae, as we shall see. It was Berzelius who introduced into chemistry formulae of the kind now commonplace for representing the composition of compounds.
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By the time he did so, in , he was able to take advantage of the addition of a further experimental law that had been added to the three laws governing combining weights. An atomist will typically take the hydrogen atom as a standard so that the atomic weight of any other substance will be the weight of an atom of it compared to the weight of an atom of hydrogen. From this point of view, a formula of H 2 O for water indicates that a compound atom of water consists of two atoms of hydrogen combined with one of oxygen.
This yields the measured proportions by weight for an atomic weight of 16 for oxygen. But there is no compulsion to take the weight of a hydrogen atom as the standard. The formula H 2 O will then indicate two portions of hydrogen for every one of oxygen. Of course, if HO is taken as the formula for water then the atomic weight of oxygen will be 8 rather than Two years later , he re-iterated this point.
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I placed beside the corpuscular theory, a theory of volumes; because that theory is in some measure connected with facts that may be verified. To those who think that the theory of volumes may be fatal to the corpuscular theory, I would observe, that both are absolutely the same thing; but that the theory of volumes has this immediate advantage over the other that it may be more easily verified. But that draws into question the extent to which chemical atomism can be said to be supported by that evidence. Berzelius himself did not use this as a reason for denying atomism.
Inspired by the phenomenon of electrolysis he presumed that atoms were held together in compounds by electrostatic forces. In any case, he clearly separated this hypothetical part of his theory from the account of combining weights, claiming not to attach too much significance to the former, at least in I do not consider the conjectures which I hazarded on the electro-chemical polarity of the atoms as of much importance. I scarcely consider them in any other light than as an ideal speculation deriving some little probability from what we know of the chemical effects of electricity.
The main point is that Berzelian formulae can be used to express weight and volume relations involved in chemical composition without a commitment to atomism. Klein , p. To her list can be added doses Donovan , combining quantities Brande and stoichiometrical numbers Gmelin as noted by Goodman , p. Berzelian formulae were not much used in chemistry before the late s, not even by Berzelius himself Klein, , p.
This is understandable in light of the fact that, in inorganic chemistry where they were first introduced, they express little more than combining weights and volumes that can be just as well expressed in other ways. As Klein has argued in detail, this was to change when formulae were adapted for use in the much more complicated area of what is now referred to as organic chemistry. A large number of elements figure in the composition of inorganic compounds, with each compound consisting of fixed proportions of just a small number of those elements.
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By contrast, organic compounds are made up of complicated combinations of a small number of elements, mainly carbon, hydrogen and oxygen and to a lesser extent nitrogen. As a consequence, knowledge of the proportions of elements in a compound is by itself an inadequate indication of its properties. A further complication is that a reaction involving the production of some organic substance of interest is unavoidably accompanied by parallel reactions involving the production of by-products. This adaptation of formulae succeeded to such a degree that, by around , a fairly unique set of formulae adequately characterising the properties and composition of organic compounds had emerged.
In the following sections I take up the issue of the relevance of the story for atomism. The complexity of organic reactions due to the many by-products invariably accompanying the preparation of some product was confronted by using chemical equations to track the formation of each product. The numbers of occurrences of C, H and O on each side of the equation must balance, so that the weight of each element remains unchanged. Equations representing the formation of the various by-products can be represented by other balanced equations. In this way the messy process involving several parallel reactions and the formation of a mixture of products is comprehended by representing it as a superposition of identified reactions independent of each other and each represented by a balanced equation.
Klein , has shown how, in the late s, Jean Dumas and Polydore Boullay first used this technique to understand the formation of ether and its by-products from alcohol, thereby bringing order to a reaction that had caused confusion for decades.
Thereafter, the use of chemical equations became commonplace and indispensable. So-called radicals were understood as groupings of elements that remained intact through a chemical reaction and played a role similar to that of elements in inorganic chemistry. So, for instance, series of compounds could be understood as resulting from various additions to the methyl radical, CH 3 , so that we have methyl alcohol, CH 3 OH, methyl chloride, CH 3 Cl and so on using modern atomic weights.
A fruitful idea was that of homologous series, an example of which is that involving the successive addition of CH 2 to the methyl radical to form ethyl, propyl, butyl and higher order compounds. Using this device, the properties, and even the existence and method of preparation, of higher order substances could be predicted on the basis of knowledge of the lower order ones. A further use of formulae that was to have wide ramifications involved the concept of substitution.
This device was soon extended to the substitution of groups of elements radicals for other groups or elements in a formula. The recognition that, for instance, one chlorine can be substituted for one hydrogen whereas one oxygen needs to be substituted for two hydrogens was eventually to lead to the notion of valency.
The demand that the symbols in chemical formulae for compounds be arranged in ways that reflect the properties of those compounds had resulted, by around , in a set of formulae that were virtually unique, the main conclusion reached by Rocke Here I give a few examples to indicate some of the key types of argument that led to this result. The simplest empirical formula for acetic acid is CH 2 O as pointed out above. This formula cannot be used to reflect the experimental fact that the hydrogen in acetic acid can be replaced by chlorine in the laboratory in four different ways yielding four distinct chemical compounds.
Three of those compounds are acids similar to acetic acid and in which the relative amounts of chlorine vary as The fourth compound has the properties of a salt rather than an acid. These experimental facts can be captured in a formula by doubling the numbers and rearranging the symbols in the empirical formula so that we have C 2 H 4 O 2 , rearranged to read C 2 H 3 O 2 H.
Another productive move involved the recognition that the action of acids needed to be understood in terms of hydrogen replacement. Polybasic acids became recognised as producing two or more series of salts depending on whether one, two or more hydrogens were replaced. Another insight involved the requirement that rational formulae adequately represent certain asymmetric compounds such as methyl ethyl ether, CH 3 C 2 H 5 O, as distinct from methyl ether, CH 3 2 O and ethyl ether, C 2 H 5 2 O.
These kinds of demands on the connection between rational formulae and the properties of the compounds they represented eventually led, by chemical means, to the solution of the problem of the under-determination of formulae and atomic weights.
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By that time, chemical elements were understood to possess a novel property, valency. I draw attention to the fact that in this section I have been able to describe the emergence of definitive formulae in organic chemistry quite naturally without referring to atoms. By the s organic chemistry had developed to the extent that rational formulae indicative of the properties and composition of chemical compounds had been identified, relative atomic and molecular weights determined and a new property of chemical elements, their valency, fashioned.
Apart from the theoretical implications of these advances, their practical importance was manifest in the emergence of the synthetic chemical industry. To do justice to the subtleties of issues surrounding atomism I introduce a distinction between three positions which I refer to as physical atomism, chemical atomism and agnostic anti-atomism. Physical atomism involves atoms that are embedded in some physical theory such as those of the mechanical philosophers or Newton and possess physical properties such as mass, shape, size and the propensity to attract or repel other atoms. The kinds of properties possessed by physical atoms are determined in advance of chemical research by the physical theory that governs them.
Chemical atoms are the least parts of chemical elements. As well as mass, a property shared by chemical and physical atoms, chemical atoms are presumed to possess chemical properties characteristic of the elements they are atoms of. The kind of property it is necessary to attribute to chemical atoms is to be determined by chemical research.
An example is valency, interpreted as a property of atoms by chemical atomists, which emerged in the course of chemical research as we have seen. Agnostic anti-atomism involves a refusal to commit to atomism. It is not a denial of atomism, which is a claim of similar strength to its affirmation. An agnostic anti-atomist would claim that the practise and success of the chemistry with which we are concerned does not require a commitment to atoms and is compatible with the idea that chemicals retain their properties however much they are divided.
Consequently, the dramatic successes of the enterprise cannot straightforwardly be taken as evidence for atomism. It is clear that both a physical and a chemical atomist is free to use chemical formulae, interpreting the symbols in those formulae as representing physical and chemical atoms respectively. But an agnostic anti-atomist is free to use them too.
The discussion in the previous section of the path that led to unique rational formulae for compounds makes perfect sense if formulae are taken simply as describing chemical properties as well as combining weights and volumes. As already indicated, I deliberately omitted any reference to atoms in that section. The appearance of OH at the end of the formulae for a compound indicates that it has the properties of an alcohol, whilst CO 2 H is indicative of an organic acid and so on.
Further, the substitution of one element for another in a compound in the laboratory is mapped by the replacement of one symbol for another in a chemical formula. The formula of a compound represents some structure of that compound, but it does not have to be an atomic structure.
The compound could possess the structure all the way down, as it were. An analogy will help illustrate the coherence and intelligibility of agnostic-anti-atomism and its assumption that the indefinite divisibility of chemical substances is compatible with their characterisation using formulae. The electric field, E , has the symmetry of an arrow whilst the magnetic field, H , has the symmetry of a spinning disc.
These facts led Maxwell and his followers to assume that E represents a strain in the aether whilst H represents a vortex in that aether.