Page:The New International Encyclopædia 1st ed. v. 04.djvu/650

CHEMISTRY. 568 CHEMISTRY. ture; the properties of a chemical compound, he liiaiiitained, are (luite difl'erciit from those of its components, while in a mixtire each constituent retains its characteristic properties iiraclically unatTectcd. Above all, he earnestly warned chem- isti* ajjaiiist adopting hypotheses and general theories « priori. Theories are necessary: but unless they are generalizations cautiously made from observed facts, they nuiy be dangerously misleading. Boyle's views are now accepted universally. Had he grasped and succeeded in spreading abroad one more idea — viz. the absolute neces- sity of quantitative investigation — he would have doubtless becomi> the founder of the science of chemistry — that is to say, with him would have commenced the ejioch enlightened by truth and free from fiindamental errors. This he did not accomplish: nor was it possible to accomplish it before the characteristics of gaseous matter came to be known better than they were in his day. And so it came about that chemists failetl to appreciate his great warning against hypothe- ses that are not rigidly correlated with facts, adoptexl a belief in a fiery 'phlogiston,' and thus created a i)eriod of darkness that lasted a century. It must be remembered that the im- portant ])henomena of wh;it we now call oxida- tion engaged the attention of chemists toward the end of the Seventeenth Century and through the entire Eighteenth Century. These ])henom- ena were explained by the supjiosed existence of phlogiston, a substance whieli may have been first produced in the erring mind of some al- chemist, but the first clear reference to which, imder the name of terra pi)7i/iiif<, we find in the works of Beeher (lfi3.5-82). ' Stahl { KJOO-lTrU) named it jjlilogiston, endowed it with certain imaginary properties, and used it as the ba.sis of a doctrine which was soon accepted through- out the civilized world. To give a clear and precise account of this, a.s of any other erroneous doctrine, is a matter of considerable dilliculty. For when ingenious men are dominated by error, they usually mold it into a variety of forms in their efforts to give it the appearance of truth and render it consistent with itself. The phlogistians handled their hy- pothesis with much dexterity. Yet their thought, lacking the eh;iraeter of quantitative precision, was weak : for quantitative conceptions, while already mastered by the physicist, were still in a state of confusion in the mind of the chemist. Distinguishing clearly between the absolute weight of bodies and their specific gravity, we have no dilTiculty in imderstanding that although water vapor is lighter than air. its addition to a given body must increase the weight of the lat- ter, because water, whether liquid or vaporized, has weight. Stahl believed that the conversion of a 'calx' — i.e. a metallic oxide — into metal was caused by the addition of phlogiston. He knew that the conversion was accompanied by a dimi- nution of weight; but from this fact he only de- duced that phlogiston must be "lighter than air,' failing to grasp that such an addition may make a body Ut/litcr in the sense of producing a sys- tem of lower specific gravity, but must neces- sarily make it henrier in the sense of increasing its absolute weight. It is more probable, how- ever, that Stahl understood this in a general way, but thought that the metals had a lower specific gravity than their calces. At least, .Tunc- ker, a pupil of Stahl's, asserts this about met-als and c:ilces as a matter of fact, although Boyle had long since shown experimentally that the specific gravity of metals is really higher than that of their calces, iluch more extraordinary is the conce])tion that we find in the writings of tixiyton dc Mor<':iu, JIacquer, and others, who taught that phlogiston had hss tliun no irciiilit! Stahl conceived of phlogiston as a fiery principle, 'materia aut priiicipiitm ignis, non ipse ignis.' Seeing that charcoal burns up completely, and is capable of producing metals by adding itself, as he thought, to their calces, he considered char- coal as made up almost entirely of phlogiston. Cavendish, knowing that 'iiiManuiiable air' is given oil' when metals are dissiilved in acids, ado])ted the view that that inllaniuKible air (hydrogen) w;is phlogiston, with which metals part on coming into contact with acids. An in- convenient fact in connection with the phlogistic theory wa.s that combustion, including the trans- fonnation of metals into calces, could imly take ])lace in the air. Stahl and his followers referred to this fact as if it were quite natural that if phlogiston was to be absorbed from metals there must be a medium capable of absorliing it. There were thoughtful men, howcer, who would not be satisfied with explanations of this kind. Boerhaave, whose Elementa Vhcmiw (1732) served for many years as the standard text-bot>k of chemistry, taught distinctly that the conver- sion of metals into calces involved the absorp- tion of something from the air. This he deduced by combining the fact that the presence of air was necessary with the fact that th<- conversion involved increase in weight. The latter fact he even freed from an erroneous exjjlanation at- tached to it by Boyle, who had thought that the increase in weight was due to absorption of heat during calcination; by the use of the balance Boerhaave showed that metals have precisely the same weight when glowing hot as when cold, and thus proved that heat has no weight. So near the trutli were some. Yet none rose to combat, the phlogistic theory, and all — even Boerlia;ive — were iloniinated by it more or less. Two things were necessary to make away ■with phlogiston: First, a clear knowledge of some of the ordinary gases; secondly, a clear quanti- tative knowledge of some of the ordinary chem- ical transformations. The gases in question are carbonic acid, oxygen, and air. As to quanti- tative chemical knowledge, it can, of course. 1*0 acquired only by the use of the balance, l^ar- Ionic acid was known since the time of Van Helmont ; yet chemists were not sure but that it might be impure air, until .Toseidi Black iso- lated it and demonstrated its properties in 17.')5. Bergman completed the study of this gas in 1774. The presence and properties of oxygen were suspected by Boyle, Mayow (lOli!)). Boer- haave. and others; but it was first actually iso- lated by Priestley and Schecle in 1774. The nitro- gen of the air was isolated by Rutherford in 177'2. It nuist be remarked here that the ap- ))aratns iind nuinijnilalions of 'pneumatic chem- istry' were gradually perfected by Boyle. Hales, jMoitrel d'l'^lemcnt. Black, and Priestley, the lat- ter having invented the method of collecting gases over mercury, which rendered possible the isolation of gases that are soluble in water. But the precise demonstration of the composition of gases, the introduction of the systematic