440 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



oxidizable, and it has been shown that this action is due to the forma- 

 tion of peroxides which catalyze oxidations, including the oxidation of 

 the spontaneously oxidizable material itself. In the case of metals the 

 process is ultimately brought to a close by the thickness of the covering 

 of oxide which excludes the air. To preserve metals from spontaneous 

 oxidation, therefore, one of two methods should be adopted: Either 

 they should constantly be kept clean and polished to avoid the accumu- 

 lation of catalyzers, or they should be allowed to become so com- 

 pletely tarnished that air can no longer penetrate to the underlying 

 metal. The intermediate policy of sporadic infrequent polishing leads 

 to maximal loss of the metal by oxidation. 



Very many instances of autocatalysis are afforded by the spontaneous 

 oxidation of fats and oils, and particularly by the oxidation of the 

 "Drying Oils" which are employed in paints and varnishes. 



It is a general characteristic of the processes of Autocatalysis that they 

 begin relatively slowly, progressively increase in velocity to a maximum, 

 and then fall off in velocity again until the reaction finally ceases. 

 The cessation of the reaction may be due to the exhaustion of the 

 Substrate or material undergoing transformation, as for example in the 

 hydrolysis of cane-sugar, or it may be due to the back-pressure of the 

 accumulated products, as in the case of the hydrolysis of methyl 

 acetate. In general the autocatalyzers, like other catalysts, accelerate 

 the attainment of equilibrium from either direction. 



The underlying combustion which releases the heat and mechanical 

 energy of muscular contraction is therefore an example of a large class 

 of chemical transformations which produce their own catalyzers. Of 

 the various stages of the process only a few are known, but among the 

 known products lactic acid and carbon dioxide are capable of identifica- 

 tion as direct or indirect catalyzers of the combustion. 



Our knowledge of the second phase of the problem which is presented 

 by the genesis of muscular work and heat is still more fragmentary and 

 much more conjectural. No machines of the ordinary type with the 

 details of which we are familiar, such as those which operate by gaseous 

 or liquid pressures and mechanical thrusts, will even approximate in 

 characteristics and behavior to the motile mechanisms of living proto- 

 plasm. The low and only very slightly fluctuating heat of combustion 

 precludes any explanation attributable to alternate expansions and 

 contractions due to heating and cooling. Engelmann, indeed, has 

 proposed such an explanation!, based upon the supposition that intense 

 heating of minute particles in the muscle-substance may occur in a 

 number of circumscribed foci. He has pointed out that a number of 

 Doubly Refracting Substances, such as catgut or India-rubber, have the 

 unusual property of contracting when they are heated, and he assumes 

 that the heat-energy of combustion in muscular tissue is directly trans- 

 formed into mechanical work by transient intense heating of localized 

 doubly refracting elements. Many objections have been urged against 

 this hypothesis and they appear in our present state of knowledge to 



