|MadSci Network: Science History|
Lindsey, this is a very broad question, and one that I find particularly fascinating. It opens up a lot of interesting history of science. I am going to write quite a long answer. I am not going to provide detailed references: you will need to do your own research on some of the names that I mention. Carmen Giunta's "Classic Chemistry" web pages are a good place to start. "Atom" comes from a Greek word meaning "indivisible". The idea of atoms (as far as we know) started with two Greeks of about 400-500 BC: Democritus and Leucippus. We do not have much of their actual writings. We do have an extensive collection of the writings of Aristotle, who did not like their ideas, and quoted several parts of their works quite extensively in trying to rebut them. The Greeks in the classical period were struggling with the concept of infinity. The argument about atoms went like this: if you have a rock, you can break it into gravel; gravel you can smash into sand; sand you can grind into dust; and so on. But sooner or later you must come to a particle that is so small that it cannot be broken up any further. This smallest- possible particle is an atom. A modern philosopher would simply say "Why? Why couldn't you go on for ever? Why couldn't matter be continuous, or infinitely divisible?" But the understanding of the Greeks in the classical period was different to ours, and this argument carried a lot of weight. Leucippus thought that different materials had different types of atom: he supposed that the atoms of vinegar were sharp and prickly, while those of milk were like smooth spheres. The Romans were fairly methodical in taking over and developing any Greek science or philosophy that was of immediate use, and ignoring the rest (like atoms) which had no direct application. In mediaeval times, the ideas of Plato and Aristotle were hugely influential in Europe, and those of Leucippus and Democritus were therefore in disfavour. The modern atomic theory started at the very beginning of the 19th century, with some work by J.L. Proust (1799 & 1802) showing that pure chemical compounds -- or at least many of them -- have constant composition. Water, for example, always has just 7.94 times as much oxygen as hydrogen (by mass). If matter is continuous, it is hard to find an explanation of why this should be so: it would seem more likely that you should be able to make oxygen-rich water or hydrogen-rich water, just like you can bake an egg-rich or a sugar-rich cake. But IF water is made up of compound atoms, each consisting of an oxygen atom and a hydrogen atom, and an oxygen atom is just 7.94 times as heavy as a hydrogen atom, then it is easy to understand why water must have this constant composition. (We now know that the molecule -- modern replacement for the 'compound atom' concept -- of water contains two hydrogen atoms and an oxygen atom, and that an oxygen atom weighs just 15.88 times as much as a hydrogen atom). John Dalton collected a little extra evidence and published a book in which he described the first modern atomic theory in 1807. The decisive evidence came from the discovery of the law of multiple proportions: copper reacts with oxygen in the proportions 3.97:1 to produce black copper oxide, and 7.94:1 to produce red copper oxide, and 7.94 is just twice 3.97. Easily explained if the oxides contain one and two copper atoms respectively along with a single oxygen atom. Wollaston showed a similar result for the relative proportions of oxalic acid and potash in two potassium oxalate salts -- one and two compound atoms of potash respectively for each compound atom of oxalic acid. It is not too hard to see how this works with atoms, but if we regard matter as continuous, this sort of result can only be seen as a bizarre co-incidence. Throughout the 19th century, the evidence for atoms was this sort of indirect evidence, and many prominent chemists did not 'believe in' atoms, or were agnostic about their existence. Furthermore, there was absolutely no way of determining how large or small atoms were -- except that they were much too small to be seen in the best available microscopes. I cannot resist including one of my favourite quotations. Thomas Thompson had been very critical of the prominent English chemist Humphry Davy, for rejecting Dalton's atomic theory. John Davy, his brother and biographer defends Sir Humphry's point of view. "Dr Thompson has mistaken it, not distinguishing between his sentiments on the hypothetical parts of Mr Dalton's views, the atomic doctrines relating to the weight, size, and number of atoms, and those which he held derived from the results of analysis, the facts and true foundations of the theory. The former are still, as they were at first, and as they ever must be, open to objection, being beyond the scrutiny of the senses and the test of experiment." (John Davy, 1836, Memoirs of the life of Sir Humphry Davy, Vol II, p. 439.) A Chemistry textbook by Alexander Smith, an American chemist, published in 1906, still maintained an agnostic view about the actual existence of atoms, and even suggested that an atom should be thought of as a measure of quantity, like a bushel or a pint. A series of investigations by physicists in the late 19th and early 20th centuries provided the first direct evidence of atoms, subatomic particles, and the internal structure of atoms. These investigations also provided the first opportunity for measuring the actual size of an individual atom. Some of the prominent names in various aspects of this work are Rutherford, J.J. Thompson, Millikan, Moseley, Niels Bohr, and Chadwick. By the 1920s enough more direct evidence had accumulated that no scientist could seriously doubt the physical existence of atoms. In the 1960s new microscopic techniques were developed that eventually produced the first images of individual atoms. (Enter "field ion microscopy" in the MadSci search engine). Other forms of microscopy that could distinguish features down to the atomic size level were developed at the end of the twentieth century. A lot of the material in this answer is drawn from the historical and philosophical researches of Maureen Christie, whose help I acknowledge.
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