Still another form of breathing appliance may well be taken on these rescue expeditions, and that is the reviving apparatus, for use upon those who have apparently ceased to breathe. In this case a mask is put over the sufferer's mouth and nose, and then the turning of a lever into a certain position causes oxygen to escape from a cylinder in such a way as to cause a suction which empties the man's lungs of the bad gases which have laid him low. That done, another movement of the lever and a deep breath of oxygen flows into his lungs in their place. Thus by alternating the positions of the lever an artificial respiration is set up far more effective than can possibly be attained by the ordinary method of moving the man's arms and pressing his chest. Indeed there are cases, such as when his arms or ribs are injured, when the ordinary method is impossible, but it is hard to imagine an instance when this beneficent apparatus could not be used, and so long as there be any spark of life left in the poor fellow there seems to be every reason to expect a complete revival as the result of its use. Of course there are many other places where poisonous gases are likely to be met with, such as gas-works, chemical-works, limeworks, and so on, where this apparatus may be kept with advantage, in case of accident. Indeed all that has been described above has its use apart from colliery explosions, although they are the outstanding opportunities for its employment. Old workings, tunnels which have been empty for a time, sewers--all these have, on occasion, to be entered, not to mention houses full of smoke, or factories full of chemical fumes, all of which form cases in which the rescue apparatus would find useful employment. CHAPTER XIX HOW SCIENCE HELPS TO KEEP US WELL One branch of science--medical science--concerns itself almost entirely with health, but it would be out of place to refer to such matters here, even if the present writer were capable of doing justice to the subject. A new medicine or a new method of operating upon a suffering patient would be quite correctly described as a scientific marvel, but it is not of such that this chapter deals, but rather with those great works by which the engineer, often taught by the medical man, promotes the health of a whole community. Most important of these, perhaps, is the provision of pure water. Some places are more fortunately situated than others in this respect, being near streams flowing down from mountains clear and unpolluted, which can be drunk after the minimum of purification. Others have to make use of the waters of a moderately clean river, as London does those of the Thames and Lea, in which cases the greatest care has to be exercised in the filtration of the liquid before it can be sent out through the mains for domestic consumption.