THE RELATION' OF LIFE TO OTHER FORCES. 315 



the conditions under which the functions 01 life are discharged are the 

 principal examples of the necessity for this unceasing and mingled de- 

 struction and renewal. They are the chief, but not the only instances 

 of this curious conjunction. 



A theoretical case will make this plain. Suppose an instance of some 

 permanent structure, say a marble statue. If we imagine it to be placed 

 under some external conditions by which each particle of its substance 

 should waste and be replaced, yet with maintenance of its original size 

 and si i ape, we obtain no idea of life. There is waste and renewal, with 

 preservation of the individual form, but no vitality. And the reason is 

 plain. With the waste of a substance like carbonate of calcium whose 

 attractions are satisfied, there would be no evolution of force; and even 

 if there were, no structure is present with the power to transform or 

 manifest anew any power which might be evolved. With the repair, 

 likewise, there would be no storing of force. The part used to make 

 good the loss is not different from that which disappeared. There is 

 therefore neither storing of force, nor its transformation, nor its expendi- 

 ture; and therefore there is no life. 



But real examples of the preservation of an individual substance under 

 the circumstances of constant loss and renewal, may be found, yet with- 

 out any semblance in them of life. 



Chemistry, perhaps, affords some of the neatest and best examples 

 of this. One, suggested by Shepard, seems particularly apposite. It is 

 the case of trioxide of nitrogen (N 2 3 ) in the preparation of sulphuric 

 acid. The gas from which this acid is obtained is sulphur dioxide, and 

 the addition of an equivalent of oxygen and the combination of the re- 

 sulting sulphur trioxide (S0 8 ) with water (H 2 0) is all that is required. 

 Thus: 



SO, + + H 2 = H a S0 4 

 Sulph. dioxide : Oxygen : Water = Sulphuric Acid. 



4 



Sulphur dioxide, however, cannot take the necessary oxygen directly 

 from the atmosphere, but it can abstract it from trioxide of nitrogen 

 (N a 3 ), when the two gases are mingled. The trioxide, accordingly, by 

 continually giving up an equivalent of oxygen to an equivalent of sulphur 

 dioxide, causes the formation of sulphuric acid, at the same time that it 

 retains its composition by continually absorbing a fresh quantity of oxy- 

 gen from the atmosphere. 



In this instance, then, there is constant waste and repair, yet without 

 life. And here an objection cannot be raised, as it might be to the pre- 

 ceding example, that both the destruction and repair come from without, 

 and are not dependent on any inherent qualities of the substance with 

 which thejr have to do. The waste and renewal in the last-named ex- 

 ample are strictly dependent on the qualities of the chemical compound 



