In order to understand better a problem encountered at depth by a diver, nitrogen narcosis, it is necessary that we understand first the basic components of air.

Air is not a single gas, but is rather a combination of gases. Air is a mixture of primarily two gases, oxygen and nitrogen. It is composed of approximately 80% Nitrogen and 20% Oxygen, the other gases being unimportant amounts and inert.

Dalton's Law

Coupled with this knowledge of the composition of air, we must now take into consideration two laws of physics which deal with the reaction of these gases under pressures. The first of these laws we will consider is DALTON'S LAW OF PRESSURES.

Essentially, Dalton's Law states that the partial pressures of gases in a mixture remain constant and act independently of each other. In other words, let's apply this to our air. We have said that air is a mixture of gases, 80% Nitrogen, and 20% Oxygen. According to Dalton's Law, this means that at surface pressure, (14.7 psi) we have in our air 11. 76 psi of Nitrogen (80% of 14. 7) and 2.94 psi of Oxygen. (20% of 14. 7). Now suppose we increase the pressure of this air to 100 psi. As we apply Dalton's Law, we see that now we have 20 psi of Oxygen and 80 psi of Nitrogen.

Henry's Law

Now let us consider the second of these laws, Henry's Law, which states that the amount of gas that a fluid will absorb under pressure varies in direct proportion to the pressure and the partial pressure of the gases.

What this means is that in our air at surface pressures, the blood of the diver will absorb in solution a given amount of the component gases and as we increase the pressure from the surface (14. 7) to two atmospheres, (29. 4 psi) it will absorb twice as much of these gases in proportion to their partial pressures.
This can be illustrated if the reader will think for a moment of a bottle of carbonated beverage, still sealed in the bottle. Take for instance champagne, a graphic example. As long as the cork is in place, keeping the fluid and the gas (carbon dioxide in this case) under a high pressure, the gas remains in solution in the wine. However, as soon as the cork is removed, the pressure is reduced dramatically, and the gas, no longer able to be contained in solution in such large quantity at the reduced pressure, boils or fizzes out in the form of bubbles

Nitrogen Narcosis

Because the diver, as he descends, is continually increasing the pressure of the gases he is breathing in accordance with ambient pressure, he is also increasing in his blood stream the quantity of Nitrogen in solution. Nitrogen, in quantity in the blood stream acts as an anesthetic and as the pressure and quantity of nitrogen in the blood stream increases, so does its anesthetic affect upon the diver, until he comes into a condition known as Nitrogen Narcosis, or Rapture of the Depths.

This condition makes itself known somewhere in the vicinity of the fifth atmosphere absolute, or fourth atmosphere, gauge, (approximately 125 feet.) However, this effect is extremely variable, and one diver may be affected at one depth, and another at a different depth, or even the same diver at different depths on different occasions.

The individual divers reactions to Narcosis may also differ; for some it is a joyous, free feeling; to others it brings on depression, melancholy, and unreasoning fear. In all cases it reduces logical thinking. The important concept here is to realize that Nitrogen Narcosis is a progressive thing, becoming gradually more pronounced as the partial pressure of nitrogen increases in the blood stream.

Treatment for a person suffering from Nitrogen Narcosis consists of bringing the victim up to a level where the concentration of nitrogen in his blood stream is no longer great enough to create this narcotic effect. Likewise, prevention of Narcosis is simply do not dive below 120 feet.

So it can be seen that a little common sense can eliminate this dangerous situation.