This conception dominated the whole scientific world for many years. The experimental work undertaken by Scheele and Priestly, however, finally enabled Lavoisier to discover the true nature of “fire”--combustion. In a famous “Memoir,” published in 1783, entitled “Reflections Concerning Phlogiston,” he showed that all the observed phenomena could be accounted for without the presence of any hypothetical phlogiston; in fact, as he himself says, they “can be better explained without phlogiston than by means of it.” His discovery of oxygen, in the atmosphere, was a fundamentally important step in modern chemical science. Hitherto, the air was thought to be a single gas, or a mixture of various gases; but oxygen was unknown as its most important constituent. Lavoisier’s discovery finally disposed of the phlogiston idea, and ushered in the new era of scientific chemistry. It may be thought that undue space has been devoted to this theory of Phlogiston; but anyone reading the history of Chemistry will realize the extent to which this idea completely dominated the minds of men at that time, and how all chemical researches were perverted by it. The discovery of the true nature of combustion was one of the fundamental turning-points in the history of scientific thought. The material world in which we live is very evidently composed of a variety of substances. At least some of these were soon seen to exist in at least three different states--solid, liquid and gaseous. These seemed to differ radically from one another; ice, water and steam are as different as one can imagine; and yet, somehow, they were the same thing after all; for ice melts and becomes water, and water, when heated, becomes steam. On the contrary, steam cools and becomes water again, and when it is sufficiently cold, will again form ice. There must be some fundamental Thing, therefore, of which water is composed. What is this Thing? How many such Things are there in the world? Are there a limitless number, or only a few? If a certain, limited number, how many? And how discover them? These were questions which naturally occupied the minds of men throughout the ages. No answer was found, however, and it remained for John Dalton to discover and formulate the Law which enabled men to obtain their first glimpse of the nature of the ultimate constitution of matter. JOHN DALTON John Dalton (1766–1844) was born in Eaglesfield, in Cumberland (England), and was the son of a poor weaver. Endowed with natural aptitude and an indomitable will, he utilized all possible opportunities for the study of mathematics and natural philosophy. He taught school, while devoting all his spare time to his beloved scientific researches. In fact, he earned his living as a private teacher to the end of his life, never having enough money to pursue his investigations unhampered by material considerations.