A. Nitric acid acts very powerfully in _destroying exhalations_, arising from putrid vegetable and animal matters. Q. _Why is LIGHTNING more common in SUMMER and AUTUMN, than in spring and winter?_ A. The heat of summer and autumn produces _great evaporation_; and the conversion of _water to vapour_, always develops _electricity_. Q. _Why does a THUNDER-STORM generally follow very DRY weather, and rarely succeeds continued WET?_ A. The clouds are _always_ charged with electricity; but _dry air_ (being a non-conductor), will not conduct the surplus fluid from the clouds to the earth: so it violently _rends the dry air_ with a flash, in order to relieve the cloud, and reach the earth. Q. _What is the general DIRECTION of a THUNDER-STORM?_ A. Either from east to west; or else from north to south. Q. _Why is ELECTRICITY excited by FRICTION?_ A. Electricity, like heat, exists in _all_ matter; but is often in a _latent state_: friction _disturbs_ it, and brings it into active operation. (see p. 31.) Q. _Why is a TREE sometimes SCORCHED by lightning, as if it had been set on fire?_ A. Lightning scorches it by its own _positive heat_, just the same as fire would. Q. _Why is the BARK of a TREE often ripped quite off by a flash of lightning?_ A. As the lightning runs down the tree, it develops the latent heat so _rapidly_, that it carries the bark of the tree along with it, while it seeks to escape. Q. _Why are BOUGHS of TREES broken off by lightning?_ A. The _mechanical force_ of lightning is very great; and when the flash strikes a tree, it will often break off the boughs by the _force_ with which it strikes against it. Q. _Why is an electric shock felt MOST at the ELBOW JOINT?_ A. Because the path of the fluid is _obstructed by the joint_: and the shock felt at the elbow is caused by the fluid _leaping from one bone to another_. CHAPTER III. Q. _What is the third chief source of heat?_ A. CHEMICAL ACTION. Q. _What is meant by chemical action being the source of heat?_ A. Many things, when their chemical constitution is changed, (either by the abstraction of some of their gases, or by the combination of others not before united,) evolve _heat_, while the change is going on. Q. _Explain by illustration what you mean._ A. Water is cold, and sulphuric acid is cold; but if these two _cold_ liquids be mixed together, they will produce _boiling heat_. Q. _Why will COLD WATER, mixed with SULPHURIC ACID, produce heat?_ A. Because water (being _lighter_ than sulphuric acid), is _condensed_ by the heavier liquid; and its heat is _squeezed out_, as water from a sponge. Q. _Why does COLD WATER, poured on LIME, make it intensely HOT?_ A. The heat is evolved by the chemical action, produced by the cold water combining with the lime. Q. _Where does the heat come from?_ A. It was in the water and lime before; but was in a _latent state_. Q. _Was there heat in the cold water and lime, before they were mixed together?_