340 PLEASANT WA YS IN SCIENCE. 



important Returning to our comparison of the human 

 system to a steam-engine we have seen how the heat-form- 

 ing and flesh-forming constituents of food correspond to the 

 supply of fuel and water ; but an engine would quickly fall 

 into a useless state if the wear and tear of the material of 

 which it is constructed were not attended to and repaired. 

 Now, in the human frame there are materials which are con- 

 tinually being used up, and which require to be continually 

 restored, if the system is to continue free from disease. 

 These materials are the mineral constituents of the system. 

 Amongst them we must include water, which composes a 

 much larger portion of our bodies than might be supposed. 

 Seven-ninths of our weight consists simply of water. Every 

 day there is a loss of about one-thirtieth part of this con- 

 stituent of our system. The daily repair of this important 

 waste of material is not effected by imbibing a corresponding 

 supply of water. A large proportion of the weight of water 

 daily lost is renewed in the solid food. Many vegetables 

 consist principally of water. This is notably the case with 

 potatoes. Where the water supplied to a district is bad, so 

 that little water is consumed by the inhabitants at least, 

 without the addition of some other substance it becomes 

 important to notice the varying proportion of water present 

 in different articles of food. As an instance of this, I may 

 call attention to a remarkable circumstance observed during 

 the failure of the potato crops in Ireland. Notwithstanding 

 the great losses which the people sustained at that time, it 

 was noticed that the amount of tea imported into Ireland 

 exhibited a remarkable increase. This seemed at first sight 

 a somewhat perplexing phenomenon. The explanation was 

 recognized in the circumstance that the potato a watery 

 vegetable, as we have said no longer formed the chief 

 portion of the people's diet. Thus the deficiency in the 

 supply of water had to be made up by the use of a larger 

 quantity of fluid food ; and as simple water was not palatable 

 to the people, they drank tea in much larger quantities than 

 they had been in the habit of taking before the famine. 



