a history of science-4-第5章
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even better than in common air。〃
A little later Priestley discovered that 〃dephlogisticated air 。 。 。 is a principal element in the composition of acids； and may be extracted by means of heat from many substances which contain them。。。。 It is likewise produced by the action of light upon green vegetables； and this seems to be the chief means employed to preserve the purity of the atmosphere。〃
This recognition of the important part played by oxygen in the atmosphere led Priestley to make some experiments upon mice and insects； and finally upon himself； by inhalations of the pure gas。 〃The feeling in my lungs；〃 he said； 〃was not sensibly different from that of common air； but I fancied that my breathing felt peculiarly light and easy for some time afterwards。 Who can tell but that in time this pure air may become a fashionable article in luxury？ 。 。 。 Perhaps we may from these experiments see that though pure dephlogisticated air might be useful as a medicine； it might not be so proper for us in the usual healthy state of the body。〃
This suggestion as to the possible usefulness of oxygen as a medicine was prophetic。 A century later the use of oxygen had become a matter of routine practice with many physicians。 Even in Priestley's own time such men as Dr。 John Hunter expressed their belief in its efficacy in certain conditions； as we shall see； but its value in medicine was not fully appreciated until several generations later。
Several years after discovering oxygen Priestley thus summarized its properties： 〃It is this ingredient in the atmospheric air that enables it to support combustion and animal life。 By means of it most intense heat may be produced； and in the purest of it animals will live nearly five times as long as in an equal quantity of atmospheric air。 In respiration； part of this air； passing the membranes of the lungs； unites with the blood and imparts to it its florid color； while the remainder； uniting with phlogiston exhaled from venous blood； forms mixed air。 It is dephlogisticated air combined with water that enables fishes to live in it。〃＇5＇
KARL WILHELM SCHEELE
The discovery of oxygen was the last but most important blow to the tottering phlogiston theory； though Priestley himself would not admit it。 But before considering the final steps in the overthrow of Stahl's famous theory and the establishment of modern chemistry； we must review the work of another great chemist； Karl Wilhelm Scheele （1742…1786）； of Sweden； who discovered oxygen quite independently； although later than Priestley。 In the matter of brilliant discoveries in a brief space of time Scheele probably eclipsed all his great contemporaries。 He had a veritable genius for interpreting chemical reactions and discovering new substances； in this respect rivalling Priestley himself。 Unlike Priestley； however； he planned all his experiments along the lines of definite theories from the beginning； the results obtained being the logical outcome of a predetermined plan。
Scheele was the son of a merchant of Stralsund； Pomerania； which then belonged to Sweden。 As a boy in school he showed so little aptitude for the study of languages that he was apprenticed to an apothecary at the age of fourteen。 In this work he became at once greatly interested； and； when not attending to his duties in the dispensary； he was busy day and night making experiments or studying books on chemistry。 In 1775； still employed as an apothecary； he moved to Stockholm； and soon after he sent to Bergman； the leading chemist of Sweden； his first discoverythat of tartaric acid； which he had isolated from cream of tartar。 This was the beginning of his career of discovery； and from that time on until his death he sent forth accounts of new discoveries almost uninterruptedly。 Meanwhile he was performing the duties of an ordinary apothecary； and struggling against poverty。 His treatise upon Air and Fire appeared in 1777。 In this remarkable book he tells of his discovery of oxygen〃empyreal〃 or 〃fire…air；〃 as he calls itwhich he seems to have made independently and without ever having heard of the previous discovery by Priestley。 In this book； also； he shows that air is composed chiefly of oxygen and nitrogen gas。
Early in his experimental career Scheele undertook the solution of the composition of black oxide of manganese； a substance that had long puzzled the chemists。 He not only succeeded in this； but incidentally in the course of this series of experiments he discovered oxygen； baryta； and chlorine； the last of far greater importance； at least commercially； than the real object of his search。 In speaking of the experiment in which the discovery was made he says：
〃When marine （hydrochloric） acid stood over manganese in the cold it acquired a dark reddish…brown color。 As manganese does not give any colorless solution without uniting with phlogiston ＇probably meaning hydrogen＇； it follows that marine acid can dissolve it without this principle。 But such a solution has a blue or red color。 The color is here more brown than red； the reason being that the very finest portions of the manganese； which do not sink so easily； swim in the red solution； for without these fine particles the solution is red； and red mixed with black is brown。 The manganese has here attached itself so loosely to acidum salis that the water can precipitate it； and this precipitate behaves like ordinary manganese。 When； now； the mixture of manganese and spiritus salis was set to digest； there arose an effervescence and smell of aqua regis。〃＇6＇
The 〃effervescence〃 he refers to was chlorine； which he proceeded to confine in a suitable vessel and examine more fully。 He described it as having a 〃quite characteristically suffocating smell；〃 which was very offensive。 He very soon noted the decolorizing or bleaching effects of this now product； finding that it decolorized flowers； vegetables； and many other substances。
Commercially this discovery of chlorine was of enormous importance； and the practical application of this new chemical in bleaching cloth soon supplanted the； old process of croftingthat is； bleaching by spreading the cloth upon the grass。 But although Scheele first pointed out the bleaching quality of his newly discovered gas； it was the French savant； Berthollet； who； acting upon Scheele's discovery that the new gas would decolorize vegetables and flowers； was led to suspect that this property might be turned to account in destroying the color of cloth。 In 1785 he read a paper before the Academy of Sciences of Paris； in which he showed that bleaching by chlorine was entirely satisfactory； the color but not the substance of the cloth being affected。 He had experimented previously and found that the chlorine gas was soluble in water and could thus be made practically available for bleaching purposes。 In 1786 James Watt examined specimens of the bleached cloth made by Berthollet； and upon his return to England first instituted the process of practical bleaching。 His process； however； was not entirely satisfactory； and； after undergoing various modifications and improvements； it was finally made thoroughly practicable by Mr。 Tennant； who hit upon a compound of chlorine and limethe chloride of limewhich was a comparatively cheap chemical product； and answered the purpose better even than chlorine itself。
To appreciate how momentous this discovery was to cloth manufacturers； it should be remembered that the old process of bleaching consumed an entire summer for the whitening of a single piece of linen； the new process reduced the period to a few hours。 To be sure； lime had been used with fair success previous to Tennant's discovery； but successful and practical bleaching by a solution of chloride of lime was first made possible by him and through Scheele's discovery of chlorine。
Until the time of Scheele the great subject of organic chemistry had remained practically unexplored； but under the touch of his marvellous inventive genius new methods of isolating and studying animal and vegetable products were introduced； and a large number of acids and other organic compounds prepared that had been hitherto unknown。 His explanations of chemical phenomena were based on th
