POISON IN THE AIR WE BREATHE. There are few words in the English language which produce a more painful impression upon the popular mind than does the word Poison, and there are at least two valid reasons why it should be so. In the first place we find poison waging war against human life, sometimes openly, sometimes insidiously, often successfully; and in the second place, from remote ages down to the present day, we are accustomed to see poison going hand in hand with crime as its chief companion. But we must remember that poison is by no means an evil-doer, only as the agent of the assassin or of the suicide. These are doubtless its most hideous aspects, but there are many others where its effects are produced as surely, though often very insidiously, without any evil intention, and, like a wolf in sheep’s clothing, clad in garments which are intended for our good. It is to such cases that we wish to direct attention, by pointing out a few of the modes in our every-day life in which poison may enter the human body without our knowledge, and wherein its presence is unnoticed until disease or death makes it manifest. Let us, then, consider in the first place the composition of atmospheric air, and how it may become so vitiated that, instead of supporting life and health, it may carry disease or death to those who breathe it. It was long thought that air was spiritual, that it was like the life, and that it was the soul of the world; but we now know that it is just as material as a piece of iron, and that it will weigh down the scales of a balance in the same way; and the time may yet come when by means of immense pressure and intense cold it may be condensed into a liquid, as carbonic acid and other gases have already been. We find air present everywhere. There is scarcely a solid, however compact it may appear, which does not contain pores, and these pores are filled with air. It is to be found in abundance in the soil; indeed were it not so, numberless worms and insects which inhabit the latter would cease to exist. The most compact mortar and walls are penetrated by it, and water in its natural state contains a large quantity of air in solution. The atmosphere was formerly believed to extend no higher than five miles above the earth’s surface, but meteorological observations have since shown that it extends to a height of more than two hundred miles. Owing to the force of gravity the air is much denser near the earth, and gets thinner, layer by layer, as you ascend. If then the atmosphere were possessed of color it would be very dark just round the globe, and the tint would gradually fade into space. There is no absolutely normal condition of the air we breathe; or, if there be, it is not at present known. It contains, however, in all cases, unless under purely artificial conditions, two _essential_ elements, which are nearly invariable under normal circumstances, namely, oxygen and