The philosophy of biology . e made of some substancewhich is a perfect non-conductor ofheat, but the bottom of the cylinderis made of a substance which con-ducts heat perfectly. There is apiston in the cylinder which fits itclosely, but which moves up and downwithout friction. At the bottom ofthe latter is a valve which can beturned so as to place the bottom ofthe cylinder, and therefore the gas,in thermal contact with a reservoirof heat (+), or a refrigerator ( - ).But when the valve is turned so that the non-con-ducting part 0 fills the bottom, the gas is perfectlyinsulated, and heat can nei
The philosophy of biology . e made of some substancewhich is a perfect non-conductor ofheat, but the bottom of the cylinderis made of a substance which con-ducts heat perfectly. There is apiston in the cylinder which fits itclosely, but which moves up and downwithout friction. At the bottom ofthe latter is a valve which can beturned so as to place the bottom ofthe cylinder, and therefore the gas,in thermal contact with a reservoirof heat (+), or a refrigerator ( - ).But when the valve is turned so that the non-con-ducting part 0 fills the bottom, the gas is perfectlyinsulated, and heat can neither enter nor leave it. Such an engine is, of course, an imaginary one, sincethere can be no mechanism in which there is not acertain amount of friction between moving parts, andthere are no substances which conduct or insulateheat perfectly. The engine is, in fact, the limit to aseries of engines each of which is supposed to be moreperfect than the last one. It is a fiction which is ofconsiderable use in theoretical FIG. 31. 366 THE PHILOSOPHY OF BIOLOGY THE CARNOT POSITIVE CYCLE We have therefore a substance which can be heatedby contact with a hot body, and which can thenexpand, doing mechanical work by raising a piston,and perhaps turning a flywheel, and on which workis then done so that it returns to its original is a cycle of operations. If we consider only thechanges which occur in the working substance we canrepresent these changes by a diagram. (J/7//5 ofheat enTer A >.c o
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