The nitrogen in the air is of more importance to plant life than to animal life. It is of little significance from a hygienic standpoint, yet it is an important constituent of all matter containing protein. Nitrogen tends to regulate or influence the rate of combustion by diluting the oxygen in the air. Respiration does not seem to affect the amount of nitrogen in the air as there is no noticeable difference between the amount of nitrogen in the inspired and expired air. Argon (A) In 1894 Lord Rayleigh and Prof. William Ramsay discovered a gaseous substance in the atmosphere which had no chemical affinity. They gave this element the name argon which means in the Greek, “inactive.” So far as is known argon will not combine with any other element. The atmosphere contains about 0.94% argon, but so far, according to Rosenau, it has not been demonstrated in the body. Argon has no hygienic significance. Helium, krypton, xenon, neon and argon form what is known as the =argon group=, since they will not unite with other elements to form compounds. These elements are all found in very small quantities in the atmosphere. Carbon Dioxide (CO_{2}) Carbon dioxide is a gas, a very small amount of which is found in the atmosphere. About 0.03% is ordinarily expressed. This means there are three parts of carbon dioxide in 10,000 parts of atmosphere. This is a very small percentage, but when we consider the enormous bulk of atmosphere we appreciate the fact that the total amount is beyond our comprehension. It is claimed that there is more carbon in the atmosphere in the form of carbon dioxide than there is in all other forms on the earth. In densely populated areas there is a slight increase in the amount of carbon dioxide. It may reach 0.04% or even 0.05%. There is also likely to be more carbon dioxide in the air close to the soil than there is a few feet above. This is because the processes going on in the soil tend to produce this gas and hence the soil air contains a larger percentage. The air receives its carbon dioxide from such sources as respiration, fermentation, from chemical action in the soil, from mineral springs, and from oxidation of organic matter.