332 Biological Chemistry. 



gas is passed over calcium carbide it forms calcium 

 cyanamide 



which substance has been largely employed as a nitro- 

 genous manure. 



Concluding Remarks. 



In the foregoing pages attention has been drawn to 

 the methods of determining the structure of organic com- 

 pounds, and examples have been given of some of the 

 experimental processes which have been employed for 

 ascertaining the changes they undergo within the living 

 organism. It must be emphasized, however, that reference 

 has been made only to a limited number of aspects of 

 the branch of science included in the term biological 

 chemistry. There are many other problems which have 

 received, at the outside, the merest passing mention in this 

 book, which entail a knowledge of that branch of chemical 

 science which is generally known as physical chemistry, 

 A few examples will suffice to illustrate this statement. 



The animal cell contains as a rule, in addition to other 

 substances, a mixture of fats, proteins, and carbohydrates, 

 together with relatively large amounts of water. Such a 

 collection cannot form a simple solution, but constitutes a 

 complex heterogeneous system. Now it is an experimental 

 fact that a cell can carry out all its functions in a normal 

 manner only when its various chemical constituents exist 

 together in a certain definite state of aggregation ; certain 

 oxidative processes, for example, will only take place when 

 the cell is more or less intact. A study of the state of 

 aggregation in the complex heterogeneous system existing 

 in a cell entails some knowledge of the physics of surface 

 action, of viscosity, of osmosis, and other factors. 



Again, in the higher animals, the metabolites are 



