298 BIOLOGY 



energy in its coiled spring into the motion of its pendulum 

 and hands; a sailboat is a machine for converting the energy 

 of the wind into the motion of the boat. So one might illus- 

 trate indefinitely. In no case is there any creation of energy 

 by the machine, simply the conversion of one form into an- 

 other. Not only is there no creation of energy, but there is 

 an actual loss of available energy, inasmuch as heat always 

 develops, and after energy has assumed the form of heat, as 

 we have just seen, it is difficult to get it back into another 

 form. While there is no actual destruction of energy when it 

 is converted into heat, there is, in every form of machinery 

 with which we are acquainted, a loss of available energy. Some- 

 times this loss is very great. For example, in an ordinary 

 electric lamp about 95% of the electrical energy that is put 

 into the bulb is lost; only 5% of it appears as light. The effi- 

 ciency of a machine is indicated by the percentage of the energy 

 supplied which we can get back in the form that we desire. 

 Machines differ much in their efficiency in this respect. It is 

 quite easy to get very efficient machines for converting motion 

 into heat, but very difficult to get an efficient machine for con- 

 verting heat into motion. The most efficient machines that 

 we have for this latter purpose are gas engines, some of 

 which give back 25% or 30% of the energy put into them. 

 Most engines give a far smaller proportion than this. Many 

 steam engines give back as motion not more than 5% to 10% 

 of the energy furnished. This matter of efficiency is one of 

 interest as we come to study the power of living organisms to 

 convert one type of energy into another. 



THE LIVING ORGANISM AS A MACHINE 



From the definition above given it is very easy to see that 

 the living organism, either animal or plant, is a machine, since 

 it is a mechanism which transforms one type of energy into 

 another. This may best be understood by considering first 

 the life of plants and then that of animals. 



