at home and abroad, the results of inquiries as to the prevalence of this disease amongst the picked men of the armies and navies of the world, the reports of hospitals for consumption, and of commissioners and committees appointed to make special investigations of jails, workhouses, and schools—all these point to poisoning by impure air as the most fertile source of consumption and many allied diseases. _Putrefaction._—We now pass on to the second source of foulness of the air, viz.: putrefaction. Putrid emanations have from the earliest times been held to be capable of producing injurious effects on the human system. In the Bible we read of the great care taken to disinfect or clean vessels which may have contained any putrid matter, and in ancient Rome measures were adopted for the efficient cleansing of the sewers and streets of that city. Our present method of disposing of our refuse is by means of water, which washes it through channels called “sewers” to the sea. But meanwhile the organic portions are undergoing decay, and certain gases are thus evolved, which, mixing together, form what we term “sewer gas.” The principal gases thus given off are carbonic acid, nitrogen, and sulphuretted hydrogen, and although this mixture if breathed is injurious to health, it can not be regarded as poisonous. Thus sulphuretted hydrogen (similar to the odor given off by rotten eggs) although a deadly poison when inhaled in large quantities, is so diluted in sewer gas that its poisonous properties are in a great measure neutralized. There is still, however, sufficient sulphuretted hydrogen in sewer gas to render it very injurious by lowering the tone of health, and by gradually diminishing vitality to such an extent that disease ensues. What, however, is of far greater importance as a poisonous agent, is the organic matter which is held in suspension by these gases. The composition of this organic matter is by no means uniform. It is composed of particles from all kinds of decomposing matter, sometimes containing minute living organisms, and sometimes without doubt the germs of disease. The exact nature of these germs of disease is still a matter of uncertainty, and the question as to whether they may appear spontaneously during the progress of decay, or whether they are merely wafted by sewer gas, just as the ripe seeds of many plants are scattered by the atmosphere, is equally unsettled. This, however, has been sufficiently established, that when diseases do come amongst us they take root with most effect in those places where decomposing matter is found, and that the germs of these diseases find in the organic element of sewer gas a congenial soil, in which they can increase and multiply indefinitely, and by which they can be carried from the dead to the living. That typhoid fever depends, to a great extent, upon the polluted air of sewers, cesspools, and of the soil, is proved by very strong evidence. In some cases the disease has