LIGHTNING. 



resins, during its presence ; and the evolution 

 of carbonic acid gas, or its formation during 

 the night, may be necessary to give greater 

 solubility to certain compounds in the plant. 

 (Agric. Chem. p. 223.) And after giving a va- 

 riety of experiments to elucidate the action 

 of vegetation on the atmosphere (see GASES, 

 THEIR USES TO VEGETATION), he adds, "These 

 facts confirm the popular opinion, and when 

 the leaves of vegetables perform their healthy 

 functions, they tend to purify the atmosphere 

 in the common variations of weather, and 

 changes from light to darkness. It may occur, 

 however, as an objection to these views, that 

 if the leaves of plants purify the atmosphere 

 towards the end of autumn, and through the 

 winter and early spring, the air in our climates 

 must become impure, the oxygen in it dimin- 

 ish, and the carbonic acid gas increase, which 

 is not the case ; but there is a very satisfactory 

 answer to this objection. The different parts 

 of the atmosphere are constantly mixed to- 

 gether by winds which, when they are strong, 

 move at the rate of from 60 to 100 miles in an 

 hour. In our winter, the southwest gales con- 

 vey air which has been purified by the vast 

 forests and Savannas of South America, and 

 which, passing over the ocean, arrives in an 

 uncontaminated state. The storms and tem- 

 pests which often occur at the beginning and 

 towards the middle of our winter, and which 

 generally blow from the same quarter of the 

 globe, have a salutary influence. By constant 

 agitation and motion, the equilibrium of the 

 constituent parts of the atmosphere is pre- 

 served ; it is fitted for the purposes of life : 

 and those events which superstition former- 

 ly referred to the wrath of heaven, or the 

 agency of evil spirits, and in which they saw 

 only disorder and confusion, are demonstrated 

 by science to be ministrations of Divine intel- 

 ligence, and connected with the order and har- 

 mony of our system." (Ibid. p. 230.) 



LIGHTNING. An electric phenomenon, pro- 

 duced by the passage of electricity between 

 one cloud and another, or between a cloud 

 and the earth. The identity of lightning with 

 electricity, though it had been previously sus- 

 pected, was first directly demonstrated by the 

 celebrated Dr. Franklin, in the year 1749, by 

 the experiment of drawing sparks by the elec- 

 tric kite. Since that time the science of elec- 

 tricity has been greatly advanced ; neverthe- 

 less, the cause of some of the appearances 

 connected with lightning is not well explained 

 even at the present day. 



There are three phenomena in particular for 

 which theory fails satisfactorily to account. 

 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 the frequent repetitionof 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 stroke. The third is the length of the 

 flash, which sometimes appears to embrace a 

 large extent of the sky. This phenomenon can 

 be best observed from the tops of mountains, 

 712 



LIGNIN. 



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 should 

 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 jvhe- 

 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 

 in autumn. A very interesting account of this 

 meteor, and of the works treatingon this subject, 

 will be found under the head "Aurora Borea- 

 lis," in Brande'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- 



