some Experiments in Climatological Physiology. 329 



of results, corresponding to progression in the conditions, is to 

 be expected. For this reason secure deductions must perforce 

 be based upon means of a large number of results. 



With regard to any attempt at a mathematical solution 

 from first principles, it may be pointed out that the spatial 

 form of the surface of the body is too complex to admit of 

 definite geometrical representation. Thus any attempt to 

 deduce theoretically the complete form of the function repre- 

 senting the evaporation, when the discharge of the aqueous 

 vapour into the air may be assumed to be by diffusion, and to 

 quantitatively estimate it, will be hopeless. The only practical 

 hope is to base a view of evaporation from a human body on 

 the observed phenomena of evaporation from a water surface, 

 or better still, from the observation of evaporation-losses from 

 a porous surface kept continually moist by a copious supply 

 of water beneath it. In fact the observation of such loss 

 carried on simultaneously with experiments on the human 

 body would doubtless be of high value, since it would reveal 

 :any departure arising from the physiological reactions super- 

 vening with increasing heat. 



In considering evaporation-losses either from the body or a 

 porous surface, it is necessary to distinguish the conditions 

 under which the aqueous vapour formed finds its way into 

 the atmosphere regarded as of unlimited receiving capacity. 



If wholly by diffusion, the process would be relatively very 

 slow and the rate would be profoundly affected by the equipres- 

 sural surfaces and the orthogonal trajectories thereto indicating 

 the lines of maximum flow. But aqueous vapour has a density 

 of only 0*6221, air being taken as unity. Hence it will 

 tend to rise at a velocity depending on this difference of 

 gravitational pressure. Further, the body itself sets up 

 •convectional currents, and movements in air : hence the 

 diffusion tends to be accelerated, and the characteristics of 

 the distribution of the aqueous vapour into the air tend to be 

 less dependent on the form of the surface. That is to say, 

 other things being equal, each unit of surface becomes more 

 nearly identical in contributing to the total evaporation. 



It is important further to distinguish the conditions ob- 

 taining in regard to the losses from the respiratory apparatus, 

 and through the skin. By the mechanism of the respiratory 

 apparatus, air being taken into the lungs is warmed or cooled, 

 as the case may be, approximately to the temperature of the 

 body, say to about 37° C. Hence its capacity for absorbing 

 water would appear to depend sensibly on the difference 

 between the total capacity of air at that temperature and 

 on the original defect from complete saturation. Now the 



