The processes involved in the plant organism differ still more in their _direction_ from those of a “purely physical” train. To see this clearly we must consider the imaginary mechanism known as a Carnot heat-engine.[15] This is a system in which we have (1) a heat-reservoir at a constant high temperature, (2) a refrigerator at a constant low temperature, and (3) a working substance which is a gas. Energy is drawn from the reservoir in the form of heat, and this heat expands the gas, doing work. The gas contracts, and its heat is then given up to the refrigerator. The work done is equal to the difference between the amount of heat taken from the reservoir and the amount given to the refrigerator. [15] See appendix, p. 363. This series of operations is called a direct Carnot cycle. But the mechanism can be worked backwards. In this case heat passes from the refrigerator into the working substance, which was at a lower temperature. The working substance, or gas, is then compressed, as the result of which operation it is heated to just above the temperature of the reservoir. The heat it thus acquires is then given up to the reservoir. In the direct Carnot cycle, therefore, energy passes from a state of high potential to a state of low potential and work is done _by_ the mechanism. In the reversed Carnot cycle energy passes from a state of low potential to a state of high potential and work is done _on_ the mechanism. The Carnot engine is thus perfectly reversible. No energy is dissipated in its working. It is, of course, a purely imaginary mechanism.