CONVERSION OF CHEMICAL INTO MECHANICAL ENERGY 441 



be insurmountable. The objection, for instance, that living matter is 

 destroyed at the height of temperature required was met by Engelmann 

 by supposing that the elements so heated only form a very small pro- 

 portion of the whole contractile tissue. If this be so, then they cannot 

 be the doubly refractile elements which we perceive under the micro- 

 scope, for these form a very large proportion of the whole. The foun- 

 dation of Engelmann's analogy between muscular tissue and catgut 

 or caoutchouc therefore falls to the ground. Furthermore even the 

 small proportion of the structural elements of muscular tissue which 

 Engelmann assumes to be subjected to heating, having been destroyed 

 thereby, would have to be decomposed and the products excreted. 

 Muscular work should therefore consume muscle-tissue and the nitrog- 

 enous excretion should increase. This, however, on a normal mixed 

 diet, does not occur. Again, the intense local heating which Engel- 

 mann assumes implies difficulty in the distribution and dissipation of 

 the heat which results in muscular combustion, yet the swift relaxation 

 which succeeds normal muscular contraction implies just the reverse. 

 A direct transformation of heat into work through the agency of 

 expansion or contraction is therefore an improbable explanation of 

 muscular contraction. 



Other observers have sought to attribute the phenomena of mus- 

 cular contraction to the Swelling or shrinkages of semisolid elements 

 through the imbibition or giving up of water, as a jelly absorbs water 

 from or parts with it to the surrounding medium. The known proc- 

 esses of this kind are, however, relatively slow and gradual in develop- 

 ment, whereas muscular contraction and relaxation may in the muscles 

 of the insect's wing alternate no less frequently than 300 times per 

 second. 



The only physical displacements which are capable at the same time 

 of such rapid alternation, of the performance of so much mechanical 

 work in a non-rigid system, and of transforming so large a propor- 

 tion of energy into mechanical work as a living muscle, are the dis- 

 placements which result from changes in Surface-tension. These are 

 excessively rapid because the forces involved are of great magnitude 

 and the frictional resistances which oppose them may be, under favor- 

 able conditions, very small. The amount of energy stored up in a 

 fluid surface is very great and the release of this energy by chemical 

 or resultant electrical changes affecting the tension of a large surface 

 would suffice to permit the performance of a large quantity of me- 

 chanical work. The maximal attainable Efficiency of a surface-tension 

 engine as Brunner and Wolf have shown, is fifty per cent., i. e., the 

 heat absorbed in extending the surface of water is equivalent to one- 

 half of the mechanical work done in producing the surface-extension. 

 This is also the maximal efficiency which has ever been observed in the 

 performance of muscular work. 



The earliest theory to regard a muscle as a surface-tension engine 

 was that proposed by Imbert who assumed that the individual Fibrils 



