as discharged its electricity by touching something。 〃On making the experiment related by Otto von Guericke；〃 he says； 〃which consists in making a ball of sulphur rendered electrical to repel a down feather； I perceived that the same effects were produced not only by the tube； but by all electric bodies whatsoever； and I discovered that which accounts for a great part of the irregularities and； if I may use the term； of the caprices that seem to accompany most of the experiments on electricity。 This principle is that electric bodies attract all that are not so； and repel them as soon as they are become electric by the vicinity or contact of the electric body。 Thus gold…leaf is first attracted by the tube； and acquires an electricity by approaching it； and of consequence is immediately repelled by it。 Nor is it reattracted while it retains its electric quality。 But if while it is thus sustained in the air it chance to light on some other body； it straightway loses its electricity； and in consequence is reattracted by the tube； which； after having given it a new electricity； repels it a second time； which continues as long as the tube keeps its electricity。 Upon applying this principle to the various experiments of electricity； one will be surprised at the number of obscure and puzzling facts that it clears up。 For Mr。 Hauksbee's famous experiment of the glass globe； in which silk threads are put； is a necessary consequence of it。 When these threads are arranged in the form of rays by the electricity of the sides of the globe； if the finger be put near the outside of the globe the silk threads within fly from it； as is well known； which happens only because the finger or any other body applied near the glass globe is thereby rendered electrical； and consequently repels the silk threads which are endowed with the same quality。 With a little reflection we may in the same manner account for most of the other phenomena； and which seem inexplicable without attending to this principle。 〃Chance has thrown in my way another principle； more universal and remarkable than the preceding one； and which throws a new light on the subject of electricity。 This principle is that there are two distinct electricities； very different from each other； one of which I call vitreous electricity and the other resinous electricity。 The first is that of glass； rock…crystal； precious stones； hair of animals； wool； and many other bodies。 The second is that of amber； copal； gumsack； silk thread； paper； and a number of other substances。 The characteristic of these two electricities is that a body of the vitreous electricity； for example； repels all such as are of the same electricity； and on the contrary attracts all those of the resinous electricity； so that the tube； made electrical； will repel glass； crystal； hair of animals； etc。； when rendered electric； and will attract silk thread； paper； etc。； though rendered electrical likewise。 Amber； on the contrary； will attract electric glass and other substances of the same class； and will repel gum…sack； copal； silk thread； etc。 Two silk ribbons rendered electrical will repel each other； two woollen threads will do the like； but a woollen thread and a silken thread will mutually attract each other。 This principle very naturally explains why the ends of threads of silk or wool recede from each other； in the form of pencil or broom； when they have acquired an electric quality。 From this principle one may with the same ease deduce the explanation of a great number of other phenomena； and it is probable that this truth will lead us to the further discovery of many other things。 〃In order to know immediately to which of the two classes of electrics belongs any body whatsoever； one need only render electric a silk thread； which is known to be of the resinuous electricity； and see whether that body； rendered electrical； attracts or repels it。 If it attracts it； it is certainly of the kind of electricity which I call VITREOUS； if； on the contrary； it repels it； it is of the same kind of electricity with the silkthat is； of the RESINOUS。 I have likewise observed that communicated electricity retains the same properties； for if a ball of ivory or wood be set on a glass stand； and this ball be rendered electric by the tube； it will repel such substances as the tube repels； but if it be rendered electric by applying a cylinder of gum…sack near it； it will produce quite contrary effectsnamely； precisely the same as gum…sack would produce。 In order to succeed in these experiments； it is requisite that the two bodies which are put near each other； to find out the nature of their electricity； be rendered as electrical as possible； for if one of them was not at all or but weakly electrical； it would be attracted by the other； though it be of that sort that should naturally be repelled by it。 But the experiment will always succeed perfectly well if both bodies are sufficiently electrical。〃＇1＇ As we now know； Dufay was wrong in supposing that there were two different kinds of electricity； vitreous and resinous。 A little later the matter was explained by calling one 〃positive〃 electricity and the other 〃negative；〃 and it was believed that certain substances produced only the one kind peculiar to that particular substance。 We shall see presently； however； that some twenty years later an English scientist dispelled this illusion by producing both positive （or vitreous） and negative （or resinous） electricity on the same tube of glass at the same time。 After the death of Dufay his work was continued by his fellow…countryman Dr。 Joseph Desaguliers； who was the first experimenter to electrify running water； and who was probably the first to suggest that clouds might be electrified bodies。 But about； this timethat is； just before the middle of the eighteenth centurythe field of greatest experimental activity was transferred to Germany； although both England and France were still active。 The two German philosophers who accomplished most at this time were Christian August Hansen and George Matthias Bose； both professors in Leipsic。 Both seem to have conceived the idea； simultaneously and independently； of generating electricity by revolving globes run by belt and wheel in much the same manner as the apparatus of Hauksbee。 With such machines it was possible to generate a much greater amount of electricity than Dufay had been able to do with the rubbed tube； and so equipped； the two German professors were able to generate electric sparks and jets of fire in a most startling manner。 Bose in particular had a love for the spectacular； which he turned to account with his new electrical machine upon many occasions。 On one of these occasions he prepared an elaborate dinner； to which a large number of distinguished guests were invited。 Before the arrival of the company； however； Bose insulated the great banquet…table on cakes of pitch； and then connected it with a huge electrical machine concealed in another room。 All being ready； and the guests in their places about to be seated； Bose gave a secret signal for starting this machine； when； to the astonishment of the party； flames of fire shot from flowers； dishes； and viands； giving a most startling but beautiful display。 To add still further to the astonishment of his guests； Bose then presented a beautiful young lady； to whom each of the young men of the party was introduced。 In some mysterious manner she was insulated and connected with the concealed electrical machine； so that as each gallant touched her fingertips he received an electric shock that 〃made him reel。〃 Not content with this； the host invited the young men to kiss the beautiful maid。 But those who were bold enough to attempt it received an electric shock that nearly 〃knocked their teeth out；〃 as the professor tells it。
LUDOLFF'S EXPERIMENT WITH THE ELECTRIC SPARK But Bose was only one of several German scientists who were making elaborate experiments。 While Bose was constructing and experimenting with his huge machine； another German； Christian Friedrich Ludolff； demonstrated that electric sparks are actual firea fact long suspected but hitherto unproved。 Ludolff's discovery； as it chanced； was made in the lecture…hall of the reorganized Academy of Sciences at Berlin； before an audience of scientists and great personages； at the opening lecture in 1744。 In the course of this lecture on electricity； during which some of the well…known manifestations of electricity were being shown； it occurred to Ludolff to attempt to ignite some inflammable fluid by projecting an electric spark upon its surface with a glass rod。 This idea was suggested to him while performing the familiar experiment of producing a spark on the surface of a bowl of water by touching it with a charged glass rod。 He announced to his audience the experiment he was about to attempt； and having warmed a spoonful of sulphuric ether； he touched its surface with the glass rod； causing it to burst into flame。 This experiment left no room for doubt that the electric spark was actual fire。 As soon as this experiment of Ludolff's was made known to Bose； he immediately claimed that he had previously made similar demonstrations on various inflammable substances； both liquid and solid； and it