The Necessity of Pure Air for Health. 247 



science in its widest aspe6l in all parts of the world, and 

 to which busy workers in other walks of life not having 

 noted what has been laid down from time to time, have 

 not attached much importance. There have been 

 many attempts to define "health" and all are open to 

 various objeftions, but the definition that is perhaps the 

 simplest for my purpose and open to least objeftion is 

 '* That state of body which enables it to perform every 

 " fun6lion which can reasonably be required of it, to 

 *' accomplish each ordinary task and be equal to some 

 " exertion of brain and muscles without painful sense of 

 " fatigue." 



It is how to maintain this condition from early child- 

 hood to old age that we have to study. It is readily 

 understood that though the general principle is the same 

 all through life yet the details vary somewhat according 

 to age. For example, an infant, say two or three months 

 old, requires perhaps as much as twenty hours sleep each 

 day, while an ordinary man, even working hard, will only 

 require something like nine hours at the most, and many are 

 able to enjoy perfe6l health with only seven or eight. 

 My remarks will be confined to the conditions for healthy 

 living for a man between 25 and 35 years of age princi- 

 pally, though I shall have to speak of special conditions 

 required on either side of the limit. 



The body of such a man is one large wonderful 

 chemical laboratory for the produ6lion of energy, either 

 mental or muscular, which can ultimately be resolved 

 into heat, one of the commonest forms of motion. 

 Given such a body free from hereditary defeft, what are 

 the conditions of the environment necessary for it, 

 to maintain it in its most perfeft working order (health), 



