a history of science-2-第57章
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to have the famous letter translated into French； and soon not only the savants； but members of the court and the king himself were intensely interested。 Two scientists； De Lor and D'Alibard； undertook to test the truth of Franklin's suggestions as to pointed rods 〃drawing off lightning。〃 In a garden near Paris； the latter erected a pointed iron rod fifty feet high and an inch in diameter。 As no thunder…clouds appeared for several days； a guard was stationed； armed with an insulated brass wire； who was directed to test the iron rods with it in case a storm came on during D'Alibard's absence。 The storm did come on； and the guard； not waiting for his employer's arrival； seized the wire and touched the rod。 Instantly there was a report。 Sparks flew and the guard received such a shock that he thought his time had come。 Believing from his outcry that he was mortally hurt； his friends rushed for a spiritual adviser； who came running through rain and hail to administer the last rites； but when he found the guard still alive and uninjured； he turned his visit to account by testing the rod himself several times； and later writing a report of his experiments to M。 d'Alibard。 This scientist at once reported the affair to the French Academy； remarking that 〃Franklin's idea was no longer a conjecture； but a reality。〃
FRANKLIN PROVES THAT LIGHTNING IS ELECTRICITY Europe； hitherto somewhat sceptical of Franklin's views； was by this time convinced of the identity of lightning and electricity。 It was now Franklin's turn to be sceptical。 To him the fact that a rod； one hundred feet high； became electrified during a storm did not necessarily prove that the storm…clouds were electrified。 A rod of that length was not really projected into the cloud； for even a very low thunder…cloud was more than a hundred feet above the ground。 Irrefutable proof could only be had； as he saw it； by 〃extracting〃 the lightning with something actually sent up into the storm…cloud； and to accomplish this Franklin made his silk kite； with which he finally demonstrated to his own and the world's satisfaction that his theory was correct。 Taking his kite out into an open common on the approach of a thunder…storm； he flew it well up into the threatening clouds； and then； touching； the suspended key with his knuckle； received the electric spark； and a little later he charged a Leyden jar from the electricity drawn from the clouds with his kite。 In a brief but direct letter； he sent an account of his kite and his experiment to England： 〃Make a small cross of two light strips of cedar；〃 he wrote； 〃the arms so long as to reach to the four corners of a large； thin； silk handkerchief when extended； tie the corners of the handkerchief to the extremities of the cross so you have the body of a kite； which being properly accommodated with a tail； loop； and string； will rise in the air like those made of paper； but this being of silk is fitter to bear the wind and wet of a thunder…gust without tearing。 To the top of the upright stick of the cross is to be fixed a very sharp…pointed wire； rising a foot or more above the wood。 To the end of the twine； next the hand； is to be tied a silk ribbon； where the silk and twine join a key may be fastened。 This kite is to be raised when a thunder…gust appears to be coming on； and the person who holds the string must stand within a door or window or under some cover； so that the silk ribbon may not be wet； and care must be taken that the twine does not touch the frame of the door or window。 As soon as any of the thunder…clouds come over the kite； the pointed wire will draw the electric fire from them； and the kite； with all the twine； will be electrified and the loose filaments will stand out everywhere and be attracted by the approaching finger； and when the rain has wet the kite and twine so that it can conduct the electric fire freely； you will find it stream out plentifully from the key on the approach of your knuckle； and with this key the phial may be charged； and from electric fire thus obtained spirits may be kindled and all other electric experiments performed which are usually done by the help of a rubbed glass globe or tube； and thereby the sameness of the electric matter with that of lightning completely demonstrated。〃＇5＇ In experimenting with lightning and Franklin's pointed rods in Europe； several scientists received severe shocks； in one case with a fatal result。 Professor Richman； of St。 Petersburg； while experimenting during a thunder…storm； with an iron rod which he had erected on his house； received a shock that killed him instantly。 About 1733； as we have seen； Dufay had demonstrated that there were two apparently different kinds of electricity； one called VITREOUS because produced by rubbing glass； and the other RESINOUS because produced by rubbed resinous bodies。 Dufay supposed that these two apparently different electricities could only be produced by their respective substances； but twenty years later； John Canton （1715…1772）； an Englishman； demonstrated that under certain conditions both might be produced by rubbing the same substance。 Canton's experiment； made upon a glass tube with a roughened surface； proved that if the surface of the tube were rubbed with oiled silk； vitreous or positive electricity was produced； but if rubbed with flannel； resinous electricity was produced。 He discovered still further that both kinds could be excited on the same tube simultaneously with a single rubber。 To demonstrate this he used a tube； one…half of which had a roughened the other a glazed surface。 With a single stroke of the rubber he was able to excite both kinds of electricity on this tube。 He found also that certain substances； such as glass and amber； were electrified positively when taken out of mercury； and this led to his important discovery that an amalgam of mercury and tin； when used on the surface of the rubber； was very effective in exciting glass。
XV。 NATURAL HISTORY TO THE TIME OF LINNAeUS Modern systematic botany and zoology are usually held to have their beginnings with Linnaeus。 But there were certain precursors of the famous Swedish naturalist； some of them antedating him by more than a century； whose work must not be altogether ignoredsuch men as Konrad Gesner （1516…1565）； Andreas Caesalpinus （1579…1603）； Francisco Redi （1618…1676）； Giovanni Alfonso Borelli （1608…1679）； John Ray （1628…1705）； Robert Hooke （1635…1703）； John Swammerdam （1637…1680）； Marcello Malpighi （1628…1694）； Nehemiah Grew （1628…1711）； Joseph Tournefort （1656…1708）； Rudolf Jacob Camerarius （1665…1721）； and Stephen Hales （1677…1761）。 The last named of these was； to be sure； a contemporary of Linnaeus himself； but Gesner and Caesalpinus belong； it will be observed； to so remote an epoch as that of Copernicus。 Reference has been made in an earlier chapter to the microscopic investigations of Marcello Malpighi； who； as there related； was the first observer who actually saw blood corpuscles pass through the capillaries。 Another feat of this earliest of great microscopists was to dissect muscular tissue； and thus become the father of microscopic anatomy。 But Malpighi did not confine his observations to animal tissues。 He dissected plants as well； and he is almost as fully entitled to be called the father of vegetable anatomy； though here his honors are shared by the Englishman Grew。 In 1681； while Malpighi's work； Anatomia plantarum； was on its way to the Royal Society for publication； Grew's Anatomy of Vegetables was in the hands of the publishers； making its appearance a few months earlier than the work of the great Italian。 Grew's book was epoch…marking in pointing out the sex…differences in plants。 Robert Hooke developed the microscope； and took the first steps towards studying vegetable anatomy； publishing in 1667； among other results； the discovery of the cellular structure of cork。 Hooke applied the name 〃cell〃 for the first time in this connection。 These discoveries of Hooke； Malpighi； and Grew； and the discovery of the circulation of the blood by William Harvey shortly before； had called attention to the similarity of animal and vegetable structures。 Hales made a series of investigations upon animals to determine the force of the blood pressure；
