The air contains, besides the gases of oxygen and nitrogen, carbonic acid, ammonia, accidental impurities, and volatile organic matter, which latter is the most important, as it probably influences to a great extent the healthfulness of the special situation. We become aware of the existence of such organic putrescent substances when entering a crowded room from the fresh air; and it is probable that the well-known unhealthiness of marshy and other districts is owing to the presence of some organic impurities.
We may have occasion to refer to this when speaking of the deluge, etc.
CHAPTER III.
THE CHEMICAL ASPECT.
By the word chemistry we understand the science which investigates the composition of all material substances, taking them apart or separating them, by a chemical process, and discovers the nature and properties of the minutest particle. These small particles have received the name, elements or elementary substances; that term is applied in chemistry to those forms of matter which have hitherto resisted all attempt to decompose them.
“We know that we have earth, air, water, and we have seen in Chapter II that the earth’s crust is made up of many substances, rocks, coral reefs, clay, marl, feldspar, quartz, limestone, granite, etc., etc. These substances are composed of small particles, or elements, and are called minerals or inorganic substances. There is another class of substances, called organic, that are derived from living things or beings. These are also taken apart or separated into their elementary substances. As plants or animals, all such elementary substances have received the name organic substances because plants and animals have organs of reproduction, hence the name.
The taking apart of any substance into its constituent elements is called analysis by chemists.
The same elements can also be put together to produce various substances; that is termed synthesis.
Chemists have adopted a name for each of the elements, and these names are represented by symbols, or letters.