io INTRODUCTORY 



able for its intensity : it acts very largely in 

 a contrary direction to material forces. These 

 tend to dissipate energy : the action of the 

 sun, the working of our machines, alike result 

 in the conversion of energy into heat, which, if 

 not lost by radiation into space, diffuses itself 

 about the earth, tending towards a condition 

 of uniform temperature in which it is useless for 

 human purposes. Coal contains energy because 

 its carbon is held in a stressful disunion from 

 oxygen : when the coal is burnt, these elements 

 become united and the energy resulting from the 

 separation is dissipated. The furnaces which turn 

 the wheels of modern industry involve, then, a 

 continuous loss of energy. Life, on the other 

 hand, is always building up energy by uniting or 

 disuniting substances which naturally tend to fly 

 apart or come together. Conspicuous in this 

 matter is the activity of plants : they form out 

 of mineral substances organic tissues which, 

 directly or indirectly, provide the whole of the 

 animal world with its food. The most character- 

 istic ingredient of these tissues is nitrogen. This 

 abounds in the air ; but plants cannot directly 

 annex it. A small amount, in combined form, is 

 brought down by rain. But vegetation mainly 

 depends for it upon the action of the minute 

 bacteria that have been already mentioned. These 

 have the power of absorbing it from the air, and 

 they yield it up to the plant roots which ramify 

 in the soil around them. The peculiar capacities 

 of these minute embodiments of life ultimately 

 provide the living world with the nitrogen that is 

 essential for its subsistence. How they originated 

 remains the greatest of problems. We may fancy, 

 if we please, that Life pervades the Universe, 

 and under certain conditions joins itself with 

 Matter and commences its earthly evolutionary 



