Fifty years ago people were very fond of talking about “ozone,” and the word is popularly used nowadays to signify a mysterious attribute of the air of the sea-coast or moorland without its real significance being generally understood. When Sir Oliver Lodge the other day warned people against hurting their nasal passages by sniffing up an unduly strong dose of ozone produced by a special ozone-making apparatus, I am inclined to think that most people who read what he said wondered what “ozone” might be.

In the eighteenth century it was noticed that the sparks from a frictional electrical machine are accompanied by a peculiar pungent smell in the air. Many years after that, namely in 1840, the great chemical experimenter, SchÖnbein, the friend and correspondent of Faraday and discoverer of gun-cotton, found that the smell in question is produced by a special gas, which is formed in the air when electric discharges take place. He found that this gas was a powerful oxydiser—would, in fact, oxydise iodide of potassium so as to set free iodine—and thus its presence could be detected by means of paper slips coated with a mixture of starch and iodide of potassium. When they were exposed in air which contained even minute traces of this strange gas they became purple-blue, owing to the liberation of iodine and the formation of its well-known blue combination with the starch. SchÖnbein found that in breezy, fresh places his test-papers turned blue, and concluded from that (confirmed by other evidence) that this smelling gas, to which he gave the name ozone—which simply means “the smelling stuff”—is present in good, ordinary atmospheric air, as well as in artificially “electrified” air. It is destroyed when such air is heated above the boiling-point of water, and seems to be, as it were, “taken out” of the air by all sorts of dead organic matter, so that it is not present in the air of large cities. I remember that when I was a boy we used to test the air for ozone with SchÖnbein’s papers (I am aware that their colour change is not absolute proof of the presence of ozone, as other oxydising gases might, if present, act in the same way), and we used to find more ozone when a south-west wind was blowing than in a north-easter!

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.

It is not to be wondered at that ozone should thus have attracted general attention and interest as the distinctive and specially active agent present in the pure air of the sea-coast and the mountain-top. People not infrequently, on arriving at the seaside, sniff up the odours of decomposing seaweed (containing a little iodine), and think they are smelling the “ozone.” It is doubtful whether enough ozone is ever present in the atmosphere under simple natural conditions to affect even a highly-sensitive nose. But it is easy to produce enough by passing air over a silent electrical discharge to fill a large room with its peculiar smell. Whether it really is of benefit to the human being who inhales a properly limited percentage of it seems not to have been clearly decided by experiment, although both in London and the United States of America there are enterprising medical men who are convinced of its value and are using it. It would certainly seem that if the peculiar benefit which is often derived from sea air or from mountain air is due to the presence of this extra oxygen in such air, then nothing can be simpler or more rational than to introduce the proper and useful percentage of ozone into the air of specially-arranged chambers in London and other large towns, so that we can visit or even inhabit them and breathe ozonised air at will without going on a journey for it.

But it is a remarkable fact that, as with various natural so-called “mineral waters,” so with various “airs” which people find beneficial—no one has yet clearly and decisively shown, in the first place, whether they exert any chemical effect of a special kind on the people who seem to benefit by drinking the one or breathing the other; still less has any one shown what is the particular chemical ingredient of the air or of the water of any given resort which exerts the beneficial effect attributed to that air or that water.

The air in different localities differs most obviously and importantly in four particulars, namely, as to whether it is still or windy, whether it is cool or hot relatively to the local surface, whether it is heavy or rarefied, whether it is dry or saturated with moisture. It is also an important fact that the atmosphere consists of layers and currents differing in these qualities, and that the higher layers can be reached by ascending to high-lying lands. At the same time it seems that in a flat country the ascent of a comparatively low hill brings you into a layer or “stratum” of air differing more from that of the plain or valley than would be the case were you to ascend to the same height in a mountainous region. The seaside and the mountain may owe the beneficial character of their air to some of the varying qualities noted above. Chemical differences may or may not be important, and seem hardly to have been as yet brought within the range of accurate knowledge. Ozone may be more or less present, so may perfumes and volatile oils, such as are given off by pine trees, and there may be more or less minute quantities of carbonic acid and of sulphurous acid, and still minuter quantities of the newly-discovered gases—argon and helium—which, for all we know, may have some effect on the human body. There seems to be a great field open for accurate investigations in regard to the action upon human health of all these varying conditions of the air. In the meantime, we proceed by guesswork, and are influenced by tradition and beliefs which are based on a sort of experience, but are of a very vague and unsatisfactory description.

The case is much the same with regard to the natural waters of celebrated resorts. So far as their chemical composition is known, they can be manufactured and applied for drinking or bathing anywhere. But minute quantities of certain gases and other elements may be present in these natural waters and have escaped until now the observation of the chemist, and it is possible, though not demonstrated, that these rare chemical constituents may have some action on those who drink or bathe in the water. Ever since the time of the Romans natural mineral waters have been sought, and the springs which discharge them have been frequented, not because their chemical composition was known, but because experience seemed to show that they produced a beneficial result. It can hardly be doubted that the baths and springs frequented at the present day are not so much themselves the cause of the benefit to the cure-seekers as are the change of scene and diet, and the repose and regular life willingly accepted by those who travel so far to reach these springs.

With regard to ozone, there remains something more to be said, namely, in regard to its application, in a far less diluted form than is possible when it is taken into the lungs, to the destruction of putrefying organic matter and putrefactive and disease-producing bacteria. It is now some five or six years since air containing a high percentage of ozone—produced by the action of the electric discharge—was used for the purification of the water-supply of large towns. It is a fact that river water into which such ozone containing air is pumped becomes pure in the highest degree, in consequence of the destruction by the ozone’s oxydising action, both of the bacterial germs always present in vast numbers in river water and of the organic matter on which the bacteria depend. This application of ozone is in use in several large towns for the purification of drinking-water, for which purpose it has very great advantages. It has also been successfully used by Dr. Allen, the director of the Plymouth Marine Laboratory, for keeping the water of the marine aquarium there in a state of purity and well charged with oxygen gas. A similar use has been made of oxygen containing a considerable percentage of ozone by enterprising surgeons for the cleansing of ulcer and abscess. It is clear that such a gas may present mechanical advantages over any liquid application.

Ozone is not, apparently, in favour or fashion with the general body of medical practitioners at the present day, but possibly further examination and determination of its physiological properties may lead to its receiving more attention in medicine. Already the peroxide of hydrogen—which is more or less correctly described as “ozonised water,” and is used (under the name “Auricomous hair-wash”) to change dark hair by its oxydising action to a golden tint—is used by surgeons for washing out purulent wounds and abscess. Those who use the gas itself only go a step further. Some day we may see a more general use of ozone; on the other hand, it remains to be seen by direct and accurate experiment whether its properties are as valuable to man as we may hope they will prove to be.