The first important effect of Lavoisier's work was the announcement by Dalton about 1808 of his Atomic Theory, which has been the basis of most of the work of chemistry ever since. Dalton's earlier work had been in physics, and its principal result had been "Dalton's Laws" in regard to the evaporation and expansion of gases, announced by him about 1801. These investigations led his mind to the consideration of the various speculations that had been entertained concerning the nature of matter itself, as distinguished from the actions and reactions between material objects that physics studies; and they brought him to the conclusion that there are certain substances or elements which combine together to form compounds that are wholly different from each of the elements (oxygen and hydrogen, for instance, combining to form water); and that those elements are made up of units absolutely indivisible, which combine with each other in absolutely exact proportions. The units he called atoms. He built up a theory wonderfully convincing and coherent, that explained virtually all the chemical phenomena then known, and supplied a stepping-stone following Lavoisier's, from which chemists could advance still further. Dalton classified certain substances as elements which we now know are not elements, because they have been found since to be compounds of two or more elements; but this in itself does not disprove his theory, because he himself pointed out that means might be found later to decompose certain materials that seemed then to be elements, because no means had then been found to decompose them. It may be instructive to note here that Dalton was not the first to imagine that certain forms of matter were elemental, or that matter was indivisible beyond a certain point, or that substances entered into combination with each other in definite proportions. Speculation on all these points had been rife for many years, but it had not produced the invention of any workable law or even theory. Similarly, many men later speculated on the possibility of devising an electrical instrument that would transform the mechanical energy of sound waves into electrical energy, transfer the electrical energy over a wire, and re-convert it into sound; but no one succeeded in producing such an instrument, until Bell invented the telephone in 1876.