LIGHTNING. 



LIGNIN. 



by the rays of light are greatly promoted by I 

 the presence of air freely circulating, and by | 

 an elevated temperature. Light is the main j 

 cause of perspiration, as in darkness plants 

 perspire little, and is the force which affects 

 the decomposition of carbonic acid, and other 

 matters contained in the cells. In the absence 

 of light, plants have no colour, no strength of 

 tissue, and no natural flavour. 



The influence of coloured light upon the 

 growth of plants is a subject well worthy of 

 investigation, from its bearing on the interests 

 of agriculture. To the farmer it is important to 

 know that, other things being equal, it is upon 

 the quantity of light which his plants receive 

 which regulates the amount of carbon by which 

 their structures are built up, and the quality 

 of their products perfected. Sowing early in 

 the season secures a larger amount of solar 

 light between seed-time and harvest. This 

 accounts for the failure so common where re- 

 planting has to be resorted to, the first growth 

 keeping off so much light from the low re- 

 plant. It also shows the advantage often de- 

 rived from sowing thinly, so that plants shall 

 not injuriously stand in the light of each other, 

 and the benefits often derived from feeding 

 off by sheep in April, any excess of luxuriance 

 in wheat, thus restoring the agency of light 

 to the succulent stems of the young plants. 

 It also shows the evil tendency exerted by 

 shade from trees or excessive growth of hedges. 



The stems of plants rise perpendicularly 

 under the influence of their unerring guide, 

 gravitation, so long as they continue to be 

 concealed beneath the soil; but as soon as 

 they rise above it they are, to a considerable 

 extent, under the control of another agent, 

 light, which inclines them horizontally, or in 

 whatever direction they receive the greatest i 

 quantity of the genial rays. 



LIGHTNING. The identity of lightning 

 with electricity, though previously suspected, 

 was first demonstrated by Dr. Franklin, in 

 the year 1749, in his celebrated experiment 

 of hoisting a kite during a thunder-gust. 

 It has since been proved that, even in the 

 absence of clouds, the atmosphere is gen- 

 erally in an electrical state, and in the 

 clearest weather, electrical currents may be 

 brought down by means of a kite furnished 

 with a wire string and insulated. Since Frank- 

 lin's discovery, electricity has become a sci- 

 ence which has explained most of the appear- 

 ances connected with lightning. There are, 

 however, three phenomena of which no en- 

 tirely satisfactory explanation has yet been 

 given. 



The first is the form of the flash, which is almost 

 always zigzag, or in broken lines, making a 

 greater or smaller angle with each other. The 

 second is xhe frequent repetition of the flashes 

 from the same cloud, which often follow one 

 another in quick succession, contrary to what 

 takes place in the case of electric conductors, 

 which generally recover their natural state, or 

 discharge the whole of their electricity at a 

 single stroka The third is the length of the 

 flash, which sometimes appears to embrace a 

 .arge extent of the sky. This phenomenon can 

 be best observed from the tops of mountains 

 712 



reaching above the clouds from which the light- 

 ning proceeds ; and observers in such cases 

 agree in stating that they have seen flashes 

 certainly extending several miles in length. 



The zigzag form of the flashes is common 

 to lightning and the electric spark : the same 

 explanation should consequently apply to both; 

 but this the theory has not yet been able to give. 



The theory of the electric fluid, and the well- 

 ascertained differences in the conducting power 

 of different substances, suggested the idea of 

 protecting buildings from the destructive effects 

 of lightning by metallic rods. Such rods are 

 usually made of iron about half an inch in dia- 

 meter. The lower portion should descend into 

 the ground some 3 or 4 feet, and in all cases 

 penetrate to the moist earth. It should be 

 made rather larger than the rest of the rod, and 

 have a slant from the foundation, or outwardly. 

 Some recommend that the lower end should be 

 pointed in the same manner as the upper ex- 

 tremity. The top of the rod should have one 

 or more sharp points. As iron is so liable to 

 rust, by which the points become blunted, it is 

 usual to have these either gilt or tipped with 

 platinum, so as to be preserved from oxida- 

 tion. The tops of the rods may be raised some 

 4, 6, or 8 feet above the chimneys or highest 

 points of the buildings they are intended to 

 protect. If the buildings be large, there shoul 

 be more points than one elevated at the diffe- 

 rent parts. Indeed, some persons who have de- 

 voted attention to the subject, say that there 

 should always be several projecting points 

 raised to receive the lightning from different 

 directions. Lightning rods cost but little* as 

 the platinum points can be bought for $1,25, 

 and the conductors may be easily made by any 

 blacksmith. They should have as few joints 

 as possible, and be inspected from time to time 

 to see whether any separation exists, and whe- 

 ther the joint remains in its place. Where, 

 from rust or other causes, any interruption to 

 the course of the electric fluid occurs, a light- 

 ning rod, instead of furnishing protection, is an 

 additional source of danger. 



LIGHTS, NORTHERN, or AURORA BO- 

 REALIS. A luminous meteor, generally ap- 

 pearing in the northern part of the sky, and 

 presenting a light somewhat resembling the 

 dawn or break of day. The appearances which 

 it exhibits, and the forms it assumes, are so 

 proverbially unsteady, that it is not possible to 

 comprehend them under any general descrip- 

 tion. In the Shetland Isles, and other coun- 

 tries in high latitudes, the northern lights are 

 the constant attendants of clear and frosty 

 evenings in winter. They are most frequent 

 :'n autumn. A very interesting account of this 

 meteor, and of the works treatingon this subject, 

 ^vill be found under the head "Aurora Borea- 

 lis," in P.rnnde's Diet, of Science, &c. 



LIGNEOUS (Lat. lignum, wood). In ento- 

 mology, a part so called when it is composed 

 of a hard, inelastic substance like wood. 



LIGNIN (Lat. lignum). The woody fibre. 

 This most important proximate principle of 

 vegetables exhibits itself in a variety of forms, 

 constituting the different textures of hard and 

 soft wood, and various fibrous products, such 

 as hemp, flax, cotton, &c. When by fine me- 



