Schönbein wrote sixty papers on ozone—but its real nature was made out by others who succeeded him, chiefly by Andrews, of Belfast, and Tait, of Edinburgh. It turns out that ozone is a condensed form of the elemental gas oxygen—squeezed, as it were, and literally “intensified,” so that three measures of oxygen gas become only two of ozone. It very readily changes back again—two measures of ozone expanding to form three of oxygen. It is produced by the action of an electric discharge upon oxygen gas driven over the discharge and in greatest quantity when that kind of gently-buzzing electric spark which is called “the silent discharge” is used. It can be produced in quantity by passing atmospheric air, or better, pure dry oxygen gas through a glass tube in which such a silent discharge is made to take place. As much as seventeen parts in a hundred of the gas can be thus converted into “ozone,” and some twenty years ago two French chemists succeeded in getting even a larger proportion, and by submitting it to a tremendous pressure at a temperature of 100 degrees below the freezing-point of water, they obtained pure ozone as a transparent liquid. It was of a dark indigo-blue colour, and somewhat dangerous and explosive when the pressure under which it had formed was removed. Ordinary oxygen gas has since then been also liquefied in the laboratory: it is of a paler blue colour. The “smell” which old writers had noticed and Schönbein had named was thus actually obtained as a distinct blue liquid. It is this which, though present only in minute quantities, gives special oxydising activity to fresh air. When pure, or present even to the small extent of 4 per cent. in air, ozone is a destructive agent, a sort of extra-quality oxygen of triple instead of double power. Indiarubber is rotted and destroyed by it in a few minutes—a sort of combustion or quick oxydation taking place—and it is, of course, dangerous to the softer parts of the human body, such as the air passages and lungs and the eyes—when present in more than a minimal proportion. I believe that no one has yet determined exactly how great a percentage of ozone can be tolerated by a human being in the air taken into the lungs. In ordinary fresh country or sea-coast air only one part by measure in 700,000 has been found to be ozone, that is, 1/7000 per cent. But it is quite likely that much more is occasionally present, since it is very difficult to arrange a satisfactory examination of the air of any locality so as to determine how much ozone it contains. It is said that at higher levels the atmosphere contains more ozone than it does at lower levels.