4 PSYCHOTHERAPY : SCIENTIFIC AND RELIGIOUS chap.
in temperature and humidity, which have a deleterious effect on the body. A human being, for instance, may find that in a cold place his extremities become numb and even dead, while his subjection to extreme heat, especially if it is accompanied by humidity, often results in a decrease in the capacity for work, and sometimes in actual exhaustion.
That light has an important influence upon health is well known, though it is not clear how this effect is exercised. Diffused sunlight is one of the factors which maintain mass relationships of organs and functional activity. Progressive advances in the production of radiations since the discovery of X-rays, the isolation of various rays for study, the difficulty of establishing units of dosage, together with research in this and in the estimation of biological reactions have resulted in an accumulation of data which cannot be precisely evaluated. Noise is of importance in so far as it may lead to, or increase the intensity of, psycho-neurosis. In such occupations as boiler-making it is said to induce deafness by producing a chronic fibrosis of the tympanic membrane. As regards atmosphere, it is important that there should not be any serious alteration in pressure ; changes may lead after a time to fatigue, while very rapid variations produce what is known as caisson disease.
Analysis of the geographic distribution of the influenza epidemics of 1918 points to the necessity of further study of how the human subject and invading organisms are influenced by the atmosphere. Apparently the weather was the main factor in causing variations in virulence of the epidemic in different localities. Water, apart from its effect on the atmosphere already referred to, is a potent carrier of the germs of certain diseases such as typhoid fever.