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.