A text-book on chemistry : for the use of schools and colleges . ss.—Medicated Tubes. It was obsei-ved, six hundred years before Christ, thata piece of amber, when rubbed, acquired the quality ofattracting light bodies. This fact remained without valuefor more than two thousand years, a striking memorial ofthe barren nature of the philosophy of those times. With-in the last two hundred years it has given birth to an en-tire group of sciences, and established the existence of agreat imponderable principle, which, from the Greekword -rjXeKTpov, signifying amber, has taken the nameElectricity. Th
A text-book on chemistry : for the use of schools and colleges . ss.—Medicated Tubes. It was obsei-ved, six hundred years before Christ, thata piece of amber, when rubbed, acquired the quality ofattracting light bodies. This fact remained without valuefor more than two thousand years, a striking memorial ofthe barren nature of the philosophy of those times. With-in the last two hundred years it has given birth to an en-tire group of sciences, and established the existence of agreat imponderable principle, which, from the Greekword -rjXeKTpov, signifying amber, has taken the nameElectricity. The catalogue of substances in which electric develop-ment can be produced was greatly increased by Gilbert,who showed that glass, resin, wax, and many other bodies,are equally effective as amber. To his successors we owethe electrical machine, an instrument which enables usreadily to demonstrate the properties of electricity. Give an example. What was the first observation made in electricity?From what does the agent derive its name ?I 98 ELECTRICAL Fig. 71. Electrical machines arc of dif- ferent kinds. They may, how-ever, be divided into plate andcylinder machines. These instru-ments are respectively represent-ed in Fig. 71 and Fig. 72. Ineach of them there are three dis-tinct portions. First, a piece ofglass, the shape of which differsin different cases ; in Fig. 71 itis a circular plate, in Fig. 72 acylinder; and from these the in-struments take their name. Sec-ond, the rubbers, made of silk or leather, stuffed with hair : the office of these isto press lightly on the glassas it turns round, and pro-duce friction. Third, abrass body, of a cylindri-cal or rounded shape, butwith points on that portionof it which looks towardthe glass. It is support-ed on glass props, and istermed the prime conduct-or. Some mechanism, such as a winch, is required toturn the glass on its axis; and when it is desired to bringthe machine into activity, all the parts of it having beenma
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