NATUBE AND OEIGIN OF LIFE. 11 



tions of the three elements carbon, hydrogen, and oxygen, 

 the protein molecule always contains in addition nitrogen 

 and sulphur, and hence can attain a far higher degree 

 of complexity endowing it with many new properties. 

 Small quantities of other elements, such as phosphorus, 

 iron, chlorine, potassium, sodium, calcium, and mag- 

 nesium are also found combined with the five essential 

 elements mentioned above in the protein molecule. So 

 complex are the proteins that the exact chemical formula 

 of even the simplest varieties has not yet been made 

 out. We know, however, that the molecule is very 

 large, containing some hundreds or even thousands of 

 chemical atoms, with certain characteristic groupings 

 of C, H, O, and N. Further, the proteins display 

 several physical properties whicn play an important part 

 in the processes of life. 



Correlated with this complexity of chemical composi- 

 tion is the most striking and important of all the physico- 

 chemical properties of living matter, namely a capacity 

 for perpetual change. It involves both an exchange of 

 material and a transformation of energy. This funda 

 mental process, the basis of all vital activities, is called 

 metabolism. Briefly to explain the process it will be 

 best perhaps to take first the case of animals. What is 

 said of metabolism in animals applies in principle to 

 living organisms in general. An animal is perpetually 

 taking in food and oxygen, and perpetually giving off 

 carbon dioxide and waste products. Hence the neces- 

 sity for respiration (oxygen being derived from the air) 

 and nutrition. Now the food consists chiefly of fats, 

 carbohydrates, and proteins ; and since energy has been 

 consumed in the building up of these compounds, it will 

 again be freed if they are broken down or split up into 

 simpler compounds. During their whole life animals 

 perform work, and give off heat ; and the energy so 

 spent is derived from the food. The stored-up potential 

 energy of the food is continually being converted into 



