During a thunderstorm it is safer to be in the house than out in the open. It is probable that draughts are a source of some danger, and the windows and doors of the room ought to be shut. Animals are more liable to be struck by lightning than men, and a shed containing horses or cows is a dangerous place in which to take shelter; in fact it is better to remain in the open. If one is caught in a storm while out of reach of a house or other building free from draughts and containing no animals, the safest plan is to lie down, not minding the rain. Umbrellas are distinctly dangerous, and never should be used during a storm. Wire fences, hedges, and still or running water should be given a wide berth, and it is safer to be alone than in company with a crowd of people. It is extremely foolish to take shelter under an isolated tree, for such trees are very liable to be struck. Isolated beech trees appear to have considerable immunity from lightning, but any tree standing alone should be avoided, the oak being particularly dangerous. On the other hand, a fairly thick wood is comparatively safe, and failing a house, should be chosen before all other places of refuge. Horses are liable to be struck, and if a storm comes on while one is out driving it is safer to keep quite clear of the animals. When a Wimshurst machine has been in action for a little time a peculiar odour is noticed. This is due to the formation of a modified and chemically more active form of oxygen, called _ozone_, the name being derived from the Greek _ozein_, “to smell.” Ozone has very invigorating effects when breathed, and it is also a powerful germicide, capable of killing the germs which give rise to contagious diseases. During a thunderstorm ozone is produced in large quantities by the electric discharges, and thus the air receives as it were a new lease of life, and we feel the refreshing effects when the storm is over. We shall speak again of ozone in Chapter XXV.