CHAPTER II. 

 THE INORGANIC FOODSTUFFS. 



WATER AND SODIUM CHLORIDE. 



We can readily understand how the need for the organic foodstuffs 

 arises: the fats, carbohydrates and proteins. For they are fuels which 

 in the course of combustion give up a certain number of heat-units 

 which can be utilized in the performance of all the work which an 

 animal daily accomplishes. We can also readily understand how the 

 need for Water arises. Protoplasm consists very largely of water. 

 Over 70 per cent, of our body-weight is water and consequently we 

 living animals are reservoirs or sacks of water which are at the same 

 time porous. Just as an earthenware jar containing water will gradu- 

 ally but continuously lose the water by evaporation from the outer 

 surface of the jar, so we also lose water continually, by evaporation 

 from the skin and from the respiratory epithelium in the lungs, apart 

 from the water which is daily lost in urine and which serves the purpose 

 of flushing the excreta out of the conduits of the body. Consequently 

 a need for water arises, a need of the cells and tissues which is expressed 

 in our consciousness by that indefinite sensation which we call " thirst." 



But it is not so clear why we should require Mineral Salts. \Ve do 

 not decompose them. They can yield us no energy. It is not at once 

 evident why we should lose them as we cannot help losing water. Yet 

 we do lose them daily and that daily loss must be replaced. We daily 

 take in sodium chloride and it reappears as sodium chloride in the 

 urine. At the end of its passage through the tissues it appears unal- 

 tered, yet it has unquestionably performed a function and indeed many 

 functions during its sojourn in our bodies. 



The probable nature of some of these functions will be more clearly 

 apprehended at a later stage, when we take up the consideration of the 

 relationship between the properties of living matter and those of its 

 constituents. But having regard at present only to the beginning and 

 the end of the cycle of processes in which the mineral salts of the diet 

 take part as they pass through the body, the question presents itself: 

 what is the daily loss of mineral salts and what must be the daily intake 

 to recoup the body for this loss? 



The mineral a^lts which are found in our tissues are, for the most 

 part, supplied in abundance in our diet. We do not consciously seek 

 for them as desirable in themselves. A remarkable exception to this 

 rule is afforded by common salt, sodium chloride, of which we feel 



