PHYSICAL SCIENCE. 



xlvii 



where the two plates are soldered together. These 

 plates are inserted into the cells of a porcelain 

 trough, so fixed that the copperplate of one cell 

 is soldered to the zinc plate of the next cell. The 

 [ells are filled with a weak solution of nitric acid. 

 About 200 pairs of plates, thus united, constitute 

 a pretty powerful galvanic battery. 



In such a battery, the plate at one end is 

 copper, and that at the other end zinc. The 

 zinc end is positive, the copper end negative. 

 When the two extremities are joined by a con- 

 ducting wire, a current of electricity passes unin- 

 terruptedly through the battery as long as the 

 nitric acid is dissolving the zinc, which it attacks 

 in preference to the copper. But when the acid 

 is saturated with oxide of zinc, the current is at 

 an end. The quantity of electricity thus set in 

 motion is very great, but its intensity is low. 

 When the conducting wires are iron or platinum, 

 they become red hot, and even undergo fusion if 

 they are not too long or too thick. Pieces of 

 charcoal, attached to platinum wires from each 

 extremity of the battery, when placed in contact, 

 become white hot, and burn, if surrounded with 

 air or oxygen gas, with very great splendour. 



If a platinum wire from the positive extremity 

 of the battery be plunged into a glass of water, 

 and a platinum wire from the negative extremity 

 be plunged into the same glass, taking care that 

 the two wires do not touch each other, the water 

 immediately begins to undergo decomposition ; 

 oxygen gas being evolved from the positive 

 wire, and hydrogen gas from the negative, 

 exactly in the proportions that exist in that 

 liquid. If the positive wire be iron or copper, 

 oxygen gas is not evolved from it in the requisite 

 quantity, but the wires become oxydized. If a 

 salt be dissolved in the water it undergoes de- 

 composition, its acid being attracted towards the 

 positive pole, and its base to the negative pole. 

 All chemical compounds are decomposed by this 

 battery, if sufficiently powerful, and they are 

 decomposed according to this law. Oxygen, 

 chlorine, bromine, iodine, and fluorine, together 

 with all acids, are attracted to the positive pole, 

 while hydrogen, azote, alkalies, earths, and 

 metals, are attracted to the negative pole. 



The same progress has not been made in de- 

 veloping the theory of this curious piece of elec- 

 trical apparatus, as has taken place in investigat- 

 ing the phenomena which it produces. The 

 action of the battery is limited to the time of the 

 chemical action of the nitric acid on the zinc ; 

 hence it is obviously connected with this action. 

 No liquid conductor is found to answer, unless it 

 be capable of acting chemically on the zinc. 

 Nitric acid answers better than any other acid, 

 and it dissolves the zinc more rapidly than any 

 other. 



MAGNETISM. 



THE ore of' iron, called the loadstone, was 

 known to the ancients, and the property which 

 it possesses of attracting iron clings ; but they 

 were ignorant of the directing power, which has 

 been turned to such important account in modern 

 navigation. But this directing power, there is 

 reason to believe, was known to the Chinese at 

 least 2000 years ago, though they did not turn it 

 to so good account as the inhabitants of Europe. 

 Whether the knowledge of the mariner's com- 

 pass was brought from the east is not known. 

 But a citizen of Amalphi, in Italy, whose name 

 is said to have been Flavio Gioyo, first construct- 

 ed the mariner's compass, such as we have it at 

 present, and marked the north end of the needle 

 with a fleur de lis, the armorial bearing of the 

 kingdom of Naples. 



The first person who attempted to generalize 

 the magnetic phenomena was I)r Gilbert of Col- 

 chester, who began his career during the reign of 

 Queen Elizabeth. He found that every magnet, 

 whether natural or artificial, has its powers con- 

 centrated in two opposite points, called the north 

 and south poles ; intermediate between which 

 there is a neutral point: that similar poles repel, 

 whils opposite poles attract, with a force (as was 

 afterwards ascertained by Coulomb) varying 

 inversely as the square of the distance. If a 

 long bar of iron be held in a position nearly ver- 

 tical, its lower extremity is always found to 

 manifest the properties of a north pole. From 

 this fact, Gilbert inferred the magnetism of our 

 globe. 



The variation or declination of the needle was 

 discovered by Columbus during his celebrated 

 voyage of discovery ; since which time it has 

 been regularly observed. In 15SO, the needle 

 in London pointed 11 15' east of north. In the 

 year 1G57, it pointed due north. Its declination 

 increased yearly in a westerly direction till 1814, 

 when it reached its greatest distance west, name- 

 ly 24 21' 10". Ever since 1814 it has been go- 

 ing east at a very slow rate 



The dip or inclination of the magnetic needle 

 was first observed by Norman, in the year 1576. 

 If a steel needle, suspended freely from its centre 

 of gravity, be converted into a magnet, it will no 

 longer hang perpendicularly ; one end of it will 

 incline downwards, while the other will rise 

 above the horizontal position. This is called the 

 dip. Norman found the dip of the north pole at 

 London to be 71 50'. From that time it 

 continued constantly to increase at a very 

 slow rate till the year 1720, when it seems to 

 have reached its maximum; namely, 75 10'. 



