Physical Problems in Study of Conditions for Health 27 



Sometimes an increased loss of heat from the body with the resultant 

 impression of cooler conditions occurs when both the temperature and 

 the relative humidity have risen. This is again quite understandable 

 if we keep the general problem in mind. It may be ascertained that 

 there has been a sufficient increase in the rate of air circulation to account 

 for it; or there may have been a decrease in the incident heat radiation 

 {e.g. as may often be found in passing from a room heated by good 

 radiators to one heated by a hot-air system and indicated by a thermo- 

 meter as apparently the warmer) , or it may be found that the internal 

 metabolism has been disturbed, so that less heat is conducted to the 

 surface from within. 



Another fruitful source of misinterpretation occurs in the direct 

 consideration of hygroscopic surfaces. The temperature coefficient 

 of evaporation of water from a hygroscopic surface is not the same as 

 from a free water surface; there is usually a difference in temperature 

 on each side of any hygroscopic surface which is not in equilibrium with 

 its surroundings; and there is a simple sufficient condition that evapora- 

 tion or condensation will take place until the absolute vapour pressures 

 across the surface (or more correctly, across the equivalent thin heter- 

 ogeneous layers) are equal in each direction. These three points have 

 been overlooked or misrepresented to a surprising degree. 



In illustration of this a case is known of a manufacturer who went 

 to a great expense on supposedly expert professional advice in the 

 duplication of the atmospheric conditions which were stated to be 

 favourable for a particular process. The average external temperature 

 and humidities were duplicated successfully, but it so happened that 

 the average temperature of the substance during the drying and other 

 processes was not duplicated; neither was the incident heat radiation 

 nor the mode of air circulation. The result was that the artificial 

 conditions of controlled evaporation and internal temperature differed 

 nearly as much from those sought as did the previously uncontrolled 

 arrangements. 



Before going further,— although it is not the intention at this time 

 to review either original work or the work of others, — attention should 

 be called to the report of Dr. Leonard Hill in 1919, to the Medical 

 Research Committee for National Health Insurance (published by 

 H. M. Stationery Office, London) entitled "The Science of Ventilation 

 and Open Air Treatment." In this extensive monograph there is 

 presented a mass of original and also of collected data, which have 

 been analyzed from the point of view of what has been called the main 

 problem and which constitute one of the most advanced contributions 

 on the subject. The work of the New York State Commission on Ven- 



