722 HANDBOOK OF PHYSIOLOGY. 



not at the change, but the want of it. Yet, in both cases alike, the in- 

 dividuality is preserved, not by the same individual elements throughout, 

 but by a succession of them. 



Again, concurrent waste and repair do not imply of necessity the ex- 

 istence of life. It is true that living beings are the chief instances of tha 

 simultaneous occurrence of these things. But this happens only because 

 the conditions under which the functions of 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 sub- 

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

 size and shape, 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 calci- 

 um whose attractions are satisfied, there would be no evolution of force ; 

 and even if there were, no structure is present with the power to trans- 

 form 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 ex- 

 penditure ; 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 

 without 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 

 resulting sulphur trioxide (S0 3 ) with water (H 2 0) is all that is required. 

 Thus : 



SO, + + H 2 = H 2 S0 4 

 Sulph. dioxide : Oxygen : Water = Sulphuric Acid. 



Sulphur dioxide, however, cannot take the necessary oxygen directly 

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

 (N 2 3 ), when the two gases are mingled. The trioxide, accordingly, by 

 continually giving up an equivalent of oxygen, to an equivalent of sul- 

 phur dioxide, causes the formation of sulphuric acid, at the same time 

 that it retains its composition by continually absorbing a fresh quantity 

 of oxygen from the atmosphere. 



