The Clinical Application of Heat 61 



and even here it may be necessary to consider also conduction 

 through the air, and radiation from the heated walls of the 

 cabinet. Methods relying mainly on conduction are met with 

 more frequently, examples being hot baths, electric blankets, 

 and hot-water bottles. All these methods of conveying heat to 

 the patient are admittedly convenient, but they present consid- 

 erable difficulties from the point of view of quantitative control 

 of administration. It is extremely difficult to discover how much 

 heat the patient actually receives, for example, from a hot bath. 

 The increase in body temperature produced can serve only as 

 a very rough indication of the amount of heat received, for it 

 must be remembered that as soon as heat is administered, the 

 processes of heat removal are also speeded up. In addition, the 

 ability to excrete heat may differ very considerably from patient 

 to patient, and even in one and the same patient there may be 

 changes according to the state of health. 



A further difficulty in the application of electric blankets 

 and heating pads arises from the time factor. As has been 

 pointed out by Brown and Mendelssohn,- it takes more than 

 an hour for an electric blanket to deliver heat at full strength. 



Heat Transfer by Radiation 



The administration of heat by radiation has proved to lend 

 itself better than either convection or conduction to accurate 

 measurement and quantitative dosage. It is for this reason 

 that attention has been turned to this method of clinical heat 

 application. 



X-rays, ultraviolet rays, visible light, and infrared rays are all 

 of a similar nature, and can all be classified under the heading 

 of electromagnetic radiation. All represent a transport of energy, 

 and when any of these rays is absorbed in a perfectly absorbing 

 or "black" body, this energy appears as heat. The difference 

 between these various types of radiation is solely that of differ- 

 ence in wave length : the wave length of X-rays is from several 

 thousand to several hundred times shorter than that of visible 

 yellow light ; ultraviolet rays are intermediate in wave length 



