Christmas Course II

Take another example—the point of view derived from chemistry is that oxygen is necessary in the air but that nitrogen is not necessary to the same extent. From what is commonly thought about oxygen and nitrogen, we might imagine that it does not matter so much to the breathing when there is too little nitrogen in the air, provided there is enough oxygen. But the truth is, that when air contains too little nitrogen, the human being gives off nitrogen in order to replace it in the air around him. The human being is so constituted that there must be a certain relation between his own nitrogen content and the nitrogen content of the surrounding air quite apart from the breathing process.