22 CHEMISTRY. 



middle, or rather, between the yellow and green ; the greatest heat- 

 ing power is in the red space or beyond it, varying with the kind of 

 prism used ; the deoxidizing power is greatest in the. violet space, or 

 just beyond it. The red ray is the least refrangible ; the violet ray 

 the most so. 



The chemical effects of light are well marked ; thus a mixture of 

 chlorine and hydrogen may be kept for any length of time in the 

 dark ; but under the influence of light, a combination soon ensues ; 

 so the blackening and the decomposition of the salts of silver occur 

 rapidly in the light. But the most remarkable chemical effect pro- 

 duced by light is upon the leaves of growing plants, which then 

 possess the property of decomposing the carbonic acid of the air, 

 appropriating the carbon to themselves, and giving out the oxygen. 



SECTION III. 

 ELECTRICITY. 



The term electricity is derived from the Greek word signifying 

 amber ^ since this is the substance in which electrical excitement was 

 first discovered. Electrical excitement may be produced by rubbing 

 glass, amber, or resin, which will then attract light bodies. There are 

 two varieties or states of electricity , the vitreous^ or that excited in glass, 

 which is also called positive electricity, — and the resinous^ or that 

 excited in resins, called also negative electricity. The difference 

 between these may be easily shown by suspending a light pith ball, 

 or a feather, by means of a thread of silk ; then on exciting a dry 

 glass tube and presenting it to it, the body will be attracted, but after 

 a few moments it will be repelled, in consequence of receiving elec- 

 tricity from the glass tube ; if now, a stick of resin properly excited 

 be presented to it, it will be attracted ; but after touching the resin 

 and receiving some of its electricity, it will soon be repelled by 

 it ; but, on the other hand, attracted by the glass. This experiment 

 establishes the law " that bodies similarly electrified, repel each 

 other, but dissimilarly electrified, they attract." Electricians divide 

 bodies into electrics^ or non-conductors^ as glass, resin, sulphur, &c., 

 and non-electrics or conductors^ as the metals, water, &c. The 

 essential parts of the electrical machine, are, 1. the electric, which 

 is usually a glass cylinder or plate, contrived so as to be capable of 

 turning by moans of a handle ; 2, the rubber, which is a stuffed 

 cushion covered with an amalgam of tin, zinc, and mercury, against 

 which the electric is rubbed ; 3, the prime conductor, which is a 

 metallic cylinder armed with a number of points for the more per- 

 fect collection of the electricity. Both cushion and conductor should 

 be insulated, that is, supported on a pedestal of glass, which is a 

 non-conductor. The cylinder, as it turns, becomes charged with 



