An example of an historical scientific conception
Phlogiston theory was an early chemical theory of combustion that saw the process of burning as a reaction in which a substance, phlogiston, was released from a combined state.
Phlogiston theory was widely used by early scientists such as Joseph Priestley and James Hutton. Priestley developed a complex chemical scheme to explain the relations of a wide number of substances based on the assumption that some contained phlogiston and that many chemical changes involved combination with, or loss of, phlogiston.
- hydrogen is water deprived of its phlogiston
- inflammable air might be phlogiston
- oxygen is phlogisticated water
- ssimple sulphureous substances contain phlogiston
Phlogiston theory was (slowly) replaced by the 'new' chemical theories of Lavoisier who saw combustion as a reaction with the element oxygen- but not all chemists immediately saw Lavoisier's system (which rejected phlogiston but did include a number of other 'false' elements) as a better framework for developing chemistry.
Sir Herbert Butterfield sketched the development of phlogiston theory:
"the emergence of the phlogiston theory provides a significant moment in the history of chemistry.
This theory, which was to become so fashionable for a time in the eighteenth century, embodied the essential feature of a tradition that went back to the ancient world – namely, the assumption that, when anything burns, something of its substance streams out of it, straggling to escape in the flutter of a flame, and producing a decomposition – the original body being reduced to more elementary ingredients. … Under the system of the Aristotelians it was the 'element' of fire which had been supposed to be released during the combustion of a body. During most of the seventeenth century it was thought to be a sulphurous 'element' – not exactly sulphur as we know it, but an idealised or a mystical form of it -materially a different kind of sulphur in the case of the different bodies in which it might appear. A German chemist, J. J. Becher, who was contemporary with Boyle, said in 1669, that it was terra pinguis – an oily kind of earth; and at the opening of the eighteenth century another German chemist, G. E. Stahl, took over this view, elaborating it down to 1731, renaming the terra pinguis 'phlogiston', and regarding phlogiston as an actual physical substance – solid and fatty, though apparently impossible to secure in isolation. It was given off by bodies in the process of combustion, or by metals in the process of calcination, and it went out in flame to combine with air, or perhaps deposited at least a part of itself in an unusually pure form as soot. If you heated the calx – the residue of a calcinated metal – along with charcoal, the substance would recover its lost phlogiston and would be restored to its original form as a metal. Charcoal was therefore regarded as containing much phlogiston, while a substance like copper was supposed to contain very little."
The reference to the "materially a different kind of sulphur" is consistent with the notion of the four elements which had dominated natural philosophy for many centuries. These were earth, water, air and fire – but these were somewhat idealised notions, such that no one had seen pure 'earth' – but rocky materials and soil were considered to have a high proportion of this element in their composition.
A similar notion was taken up by the alchemists. So Paracelsus proposed three chemical principles: salt, mercury and sulphur. But again there was a distinction between principles and actual material samples -the mercury principle was not identified with the substance quicksilver even if quicksilver would be considered to contain mercury. Indeed all metals were considered to contain both mercury and sulphur, and to be incontrovertible by changing the proportion of sulphur present. (And one of the aims of many alchemists was indeed to prepare gold from less valuable base metals.)
Phlogiston was dicussed as if it was another chemical substance.
- phlogiston refuses aqueous solution (i.e., does not dissolve in water)
Phlogiston was often considered to have levity, that is in effect a negative weight, such that a loss of phlogiston during combustion would lead to an increase in weight.
- phlogiston might have negative weight
- phlogiston weakened the repulsion between substance particles and ether
Work cited:
- Herbert Butterfield (1957) The Origins of Modern Science 1300-1800 (New Edition: Revised and enlarged). G. Bell and Sons Ltd., London.
