Stahl replied, phlogiston is a "principle of levity;" the presence of phlogiston in a substance causes that substance to weigh less than it did before it received this phlogiston. In criticizing this strange statement, we must remember that in the middle of the seventeenth century philosophers in general were not firmly convinced of the truth that the essential character of matter is that it possesses weight, nor of the truth that it is impossible to destroy or to create any quantity of matter however small. It was not until the experimental work of Lavoisier became generally known that chemists were convinced of these truths. Nevertheless, the opponents of the Stahlian doctrine were justified in asking for further explanations--in demanding that some other facts analogous to this supposed fact, viz. that a substance can weigh less than nothing, should be experimentally established. The phlogistic theory however maintained its ground; we shall find that it had a distinct element of truth in it, but we shall also find that it did harm to scientific advance. This theory was a wide and sweeping generalization from a few facts; it certainly gave a central idea around which some facts might be grouped, and it was not very difficult, by slightly cutting down here and slightly adding there, to bring many new discoveries within the general theory. We now know that in order to explain the process of combustion much more accurate knowledge was required than the chemists of the seventeenth century possessed; but we ought to be thankful to these chemists, and notably to Stahl, that they did not hesitate to found a generalization on the knowledge they had. Almost everything propounded in natural science has been modified as man's knowledge of nature has become wider and more accurate; but it is because the scientific student of nature uses the generalizations of to-day as stepping-stones to the better theories of to-morrow, that science grows "from more to more." Looking at the state of chemistry about the middle of the eighteenth century, we find that the experiments, and especially the measurements, of Hooke and Mayow had laid a firm basis of fact concerning the process of combustion, but that the phlogistic theory, which appeared to contradict these facts, was supreme; that the existence of airs, or gases, different from common air was established, but that the properties of these airs were very slightly and very inaccurately known; that Boyle had distinguished element from compound and had given definite meanings to these terms, but that nevertheless the older and vaguer expression, "elementary principle," was generally used; and lastly, that very few measurements of the masses of the different kinds of matter taking part in chemical changes had yet been made. FOOTNOTES: