4 BIOLOGICAL PROBLEMS 



Before doing so, a word may be said as to the 

 relative value of animal and vegetable protein. There 

 is no difference. Especially should it be kept in 

 mind that there is no special virtue in butcher's meat. 

 At one time it was thought that vegetable proteins 

 were less digestible. While this may be true for 

 those in certain nuts, the work done in the laboratory 

 of Professor Hopkins at Cambridge, of which an 

 account will be found in the second lecture of this 

 series, shows that the protein in the present "war- 

 bread ' is practically utilized in full. 



Carbohydrates. Familiar instances of carbohy- 

 drates are starch and sugar. Potatoes consist in 

 large proportion of the former. 



In chemical composition they may be regarded as 

 being formed of carbon and water in equal propor- 

 tions, but differing as to the total number of atoms 

 in their molecules. Since they contain no nitrogen, 

 they cannot serve for growth or for replacement of 

 wear and tear. What then is their use? A similar 

 question applies to fat, the third constituent given in 

 diet tables. We are thus led to a discussion of the 

 meaning of the word " calorie", which is now 

 familiar to everyone as expressing the value of a diet 

 in a particular aspect. So much sugar, for instance, 

 is said to be equivalent to so many "calories". 



Calorie. Perhaps the most striking characteristic 

 of living beings is their perpetual change. Now 

 changes, as we all know, cannot be brought about 

 without work being done, both in the physical and 

 the social spheres. In the language of science, the 

 capacity of doing work is called "energy", and what 

 is important for our present purpose is that the name 

 is used in no metaphorical manner, but for something 

 that can be measured, so that different amounts of 

 energy can be exactly compared. Probably why 



