Seneca, in passing on to discuss the nature and origin of thunder and lightning, divides the phenomena into three kinds--lightning-flashes, thunderbolts, and thunderings (62). After citing and commenting on the opinions of various philosophers he proceeds to give his own views regarding these appearances. The lightning flash (_fulguratio_) he looks upon as fire widely spread out, the thunderbolt (_fulmen_) as fire condensed and hurled with violence (66). The difference between the two is in force rather than character; a flash is a bolt without strength enough to reach the earth, while a thunderbolt is lightning in its most intense form (69). With regard to the origin of the fire he points out that fire may be artificially produced in two ways: either by percussion, as when stones are struck; or by friction, as when two bits of wood are rubbed against each other. He thinks that probably in both of these ways clouds may emit fire, and that in the violence of storms a source of energy is supplied whereby the warm or smoky exhalations from the earth may be kindled and fall with a fierce glow to the earth (70, 101). These exhalations contain dry and moist bodies, to which heavier elements may be added. A combination of such materials will form a thicker and more solid cloud than one of pure air, and such a cloud may burst with a loud report (78). There can be no peal of thunder unless the hollow clouds are broken up with great violence (76). The characteristic path of the thunderbolt is determined by the oblique current of air in which, while the natural tendency of the fire is upward, the violence of its discharge presses it downwards and compels it to take up a zig-zag course. The peculiar ozone odour noticed during thunderstorms, and long popularly known as the smell of sulphur, is alluded to by Seneca (69, 97) and by Lucretius.[111] [111] Similar views on thunder and lightning are expressed in the _De Rerum Natura_: semina quod nubes ipsas permulta necessust ignis habere.--vi. 206. post ubi conminuit vis eius et impetus acer, tum perterricrepo sonitu dat scissa fragorem.--_Ibid._ 128. ... notaeque gravis halantis sulpuris auras.--_Ibid._ 221.