But we have a living matter in the atmosphere. We often notice in the summer, after a rain, that the ground is yellow. On gathering up the yellow powder and examining it under the microscope, we find that it consists of pollen. The pollen of rag weed and other plants is supposed to be the cause of hay fever. But we also have something far more important in the germs of certain classes of vegetation. The effects are familiar. If food is put away, it becomes mouldy. This mould is a peculiar kind of vegetation which is called a fungus, and the plants fungi. In order for this mould to develop a certain temperature and a certain degree of moisture are necessary. Our food, we say, decays. Now, what we call decay is really the growth of these fungi. Animal and vegetable substances which these fungi seize upon are destroyed. All ordinary fermentations and putrefactions are due to mould fungi, yeast plants, or bacteria, and liquids undergoing these processes carry these fungi and their germs wherever they go. The refuse of the city pollutes the air. You have only to pass along any street to find more or less rubbish. That furnishes the nidus for the growth and development of these germs, and until we adopt better methods of getting rid of that refuse, we never shall have the air of this city in the condition that it should be. One of the most constant sources of the pollution of the air in inhabited localities is the decomposition that takes place in the ground. Refuse of every kind gets into it. Our sewers are leaky, and putrefaction is constantly going on. The soil down to the limit of the ground water contains a large amount of air. This air, when the atmospheric pressure in the house is diminished, is drawn in with such organic impurities as it contains. A cement floor in the cellar is not a protection against this entrance of the ground air, for the cement is porous to the passage of air, but a remedy may be found by laying on the cement a covering of coal tar pitch, in which bricks are set on edge, the spaces between the bricks are filled with the melted pitch, and the bricks then covered with coal tar pitch. When the house is building, the foundation walls should also be similarly coated, outside as well as inside. Such a cellar floor was considered to be absolutely impervious to ground air and moisture. The lecturer had recently laid this floor in his own house with the greatest success. The atmosphere of the entire house is improved, and the expense is very moderate. Another source of the contamination of the air of houses is the heating apparatus. Stoves and furnaces, however well constructed at first, will, from the contraction and expansion of the metal, soon allow the escape of coal gas, and this danger is greatly increased by the use of dampers in the stove-pipe. When, to regulate the fire, the damper in the pipe is closed, the gases, having their passage to the chimney cut off, will