a history of science-2-第52章
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t the current could be sent。 Nevertheless； Gray should probably be credited with the discovery of two of the most important properties of electricitythat it can be conducted and insulated； although； as we have seen； Gilbert and Von Guericke had an inkling of both these properties。
EXPERIMENTS OF CISTERNAY DUFAY So far England had produced the two foremost workers in electricity。 It was now France's turn to take a hand； and； through the efforts of Charles Francois de Cisternay Dufay； to advance the science of electricity very materially。 Dufay was a highly educated savant； who had been soldier and diplomat betimes； but whose versatility and ability as a scientist is shown by the fact that he was the only man who had ever contributed to the annals of the academy investigations in every one of the six subjects admitted by that institution as worthy of recognition。 Dufay upheld his reputation in this new field of science； making many discoveries and correcting many mistakes of former observers。 In this work also he proved himself a great diplomat by remaining on terms of intimate friendship with Dr。 Graya thing that few people were able to do。 Almost his first step was to overthrow the belief that certain bodies are 〃electrics〃 and others 〃non…electrics〃that is； that some substances when rubbed show certain peculiarities in attracting pieces of paper and foil which others do not。 Dufay proved that all bodies possess this quality in a certain degree。 〃I have found that all bodies （metallic； soft； or fluid ones excepted）；〃 he says； 〃may be made electric by first heating them more or less and then rubbing them on any sort of cloth。 So that all kinds of stones； as well precious as common； all kinds of wood； and； in general； everything that I have made trial of； became electric by beating and rubbing； except such bodies as grow soft by beat； as the gums； which dissolve in water； glue； and such like substances。 'Tis also to be remarked that the hardest stones or marbles require more chafing or heating than others； and that the same rule obtains with regard to the woods； so that box； lignum vitae； and such others must be chafed almost to the degree of browning； whereas fir； lime…tree； and cork require but a moderate heat。 〃Having read in one of Mr。 Gray's letters that water may be made electrical by holding the excited glass tube near it （a dish of water being fixed to a stand and that set on a plate of glass； or on the brim of a drinking…glass； previously chafed； or otherwise warmed）； I have found； upon trial； that the same thing happened to all bodies without exception； whether solid or fluid； and that for that purpose 'twas sufficient to set them on a glass stand slightly warmed； or only dried； and then by bringing the tube near them they immediately became electrical。 I made this experiment with ice； with a lighted wood…coal； and with everything that came into my mind； and I constantly remarked that such bodies of themselves as were least electrical had the greatest degree of electricity communicated to them at the approval of the glass tube。〃 His next important discovery was that colors had nothing to do with the conduction of electricity。 〃Mr。 Gray says； towards the end of one of his letters；〃 he writes； 〃that bodies attract more or less according to their colors。 This led me to make several very singular experiments。 I took nine silk ribbons of equal size； one white； one black； and the other seven of the seven primitive colors； and having hung them all in order in the same line； and then bringing the tube near them； the black one was first attracted； the white one next； and others in order successively to the red one； which was attracted least； and the last of them all。 I afterwards cut out nine square pieces of gauze of the same colors with the ribbons； and having put them one after another on a hoop of wood； with leaf…gold under them； the leaf…gold was attracted through all the colored pieces of gauze； but not through the white or black。 This inclined me first to think that colors contribute much to electricity； but three experiments convinced me to the contrary。 The first； that by warming the pieces of gauze neither the black nor white pieces obstructed the action of the electrical tube more than those of the other colors。 In like manner； the ribbons being warmed； the black and white are not more strongly attracted than the rest。 The second is； the gauzes and ribbons being wetted； the ribbons are all attracted equally； and all the pieces of gauze equally intercept the action of electric bodies。 The third is； that the colors of a prism being thrown on a white gauze； there appear no differences of attraction。 Whence it proceeds that this difference proceeds； not from the color； as a color； but from the substances that are employed in the dyeing。 For when I colored ribbons by rubbing them with charcoal； carmine； and such other substances； the differences no longer proved the same。〃 In connection with his experiments with his thread suspended on glass poles； Dufay noted that a certain amount of the current is lost； being given off to the surrounding air。 He recommended； therefore； that the cords experimented with be wrapped with some non…conductorthat it should be 〃insulated〃 （〃isolee〃）； as he said； first making use of this term。
DUFAY DISCOVERS VITREOUS AND RESINOUS ELECTRICITY It has been shown in an earlier chapter how Von Guericke discovered that light substances like feathers； after being attracted to the sulphur…ball electric…machine； were repelled by it until they touched some object。 Von Guericke noted this； but failed to explain it satisfactorily。 Dufay； repeating Von Guericke's experiments； found that if； while the excited tube or sulphur ball is driving the repelled feather before it； the ball be touched or rubbed anew； the feather comes to it again； and is repelled alternately； as； the hand touches the ball； or is withdrawn。 From this he concluded that electrified bodies first attract bodies not electrified； 〃charge〃 them with electricity； and then repel them； the body so charged not being attracted again until it has 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 t
