The Clinical Application of Heat 65 



energy from all directions. It is possible to calculate the energy 

 received on the skin from the strengths, temperatures, and 

 positions of the various parts of the source,^ but it is a somewhat 

 severe mathematical problem, and is clearly an impossible method 

 for ordinary clinical use. Reliance must be placed on direct 

 measurement, and what is needed is some simple method of 

 measuring the energy actually received on the patient's skin. 



In X-ray work, with beam therapy, a suitable standard of 

 measurement would be the energy falling in one minute on a 

 surface of one square centimeter placed normal to the beam. 

 That was suggested by IMayneord and TuUey ^ as suitable also 

 for infrared work, but, in fact, a slight amplification of their 

 definition is necessary. A more suitable specification would be 

 the total energy coming from all directions which impinges in one 

 minute on a surface of one square centimeter placed in the 

 position to be occupied by the skin of the patient. For a unit 

 incident energy flux of one gram calorie per minute, we have 

 suggested the name pyron} 



This unit specifies the total incident energy without regard 

 to wave length (color, quality), but as the effect of all wave 

 lengths of infrared radiation is simply to heat the tissues, the 

 consideration of the range of wave lengths used in any given 

 circumstance is of an importance secondary to the consideration 

 of the total energy received in all wave lengths. There are, of 

 course, problems connected with the difference in penetrating 

 power of different wave lengths, but the first task is to provide 

 convenient methods of determining the total flux. 



Special Problems of Measurement 



Before discussing practical methods of measurement we must 

 first consider a theoretical point. As explained above, all hot 

 bodies radiate energy, and cease to do so only if cooled to the 

 absolute zero of temperature (zero on the absolute scale, see 

 footnote, page 63. Thus, all our surroundings continually 

 radiate energy, and energy is being continually radiated from 

 our skins to our surroundings. What we must measure, there- 



