The Clinical Application of Heat 63 



The visible range of electromagnetic waves represents roughly 

 the size of quanta below which no specific action is produced in 

 the body tissues. In other words, the action of infrared radia- 

 tion is distinguished by the fact that it produces no specific reac- 

 tions at all, and its absorption merely causes a rise of temperature 

 in the tissues. The short wave radiations, such as X-rays or 

 ultraviolet rays, are limited in their application by the harmful 

 efifects which are produced by an excess of the specific changes 

 for which they are responsible. Infrared radiation, on the other 

 hand, can be applied at a strength which is limited only by the 

 capacity of the tissues to withstand heating. It is for this reason 

 that infrared radiation has become known under the name of 

 ''radiant heat" for, in contrast to the shorter wave length radia- 

 tions, it offers a safe method of pumping heat energy into the 

 body. 



To produce any sensible effect with infrared, large doses, in 

 some cases as much as 200,000 gram calories at a treatment, are 

 used, but this infrared radiation must not be accompanied by 

 more than a minute proportion of ultraviolet radiation, which, in 

 this case, would produce unwanted specific effects, and would 

 severely limit the total energy which could be pumped into 

 the patient without injury. 



\\' hen a body is heated it emits electromagnetic radiation, and 

 the total quantity of energy emitted from one square centimeter 

 of its surface, as well as the wave lengths in which this energy 

 is emitted, depend on the temperature of the body. A body at 

 2000° K * emits 256 times as much energy from each square 

 centimeter of its surface as a similar body at 500° K. For the 

 first, the greatest intensity of radiation is in a wave length of 

 about 2[{ ; for the second, the wave length of maximum intensity 

 is four times as great. . Even a body at 4000° K, which is sixteen 

 times as efficient an emitter of radiation as one of 2000° K, emits 

 the greater part of its energy in the infrared, but now there 



* It is convenient to give temperatures in degrees Kelvin or "absolute," which 

 means the centigrade temperature plus 273°. The total energy radiated from a 

 blaclv surface is proportional to the fourth power of the absolute temperature, and 

 other characteristics of the radiation are all most simply expressed in this tem- 

 perature scale. 



