276 GENERAL CONSIDERATIONS [Ch. IX 



The limiting conditions at which inaction occurs are of two 

 sorts. These may be termed respectively structural and dynami- 

 cal. These two sorts of limiting conditions may be illustrated 

 by comparing the protoplasmic mass to a factory, with many 

 boilers and engines, much shafting and belting, and countless 

 machines doing the most varied work. The amount of energy 

 developed in the boilers and the efficiency of the engines and 

 machines varies with certain conditions, such as the amount of 

 heat applied to the former, and the friction and waste in the 

 latter. The limiting mechanical conditions are reached when' 

 tlie boiler is rent by the steam pressure, a break-down is caused 

 by friction, or a part rusts through and crumbles away. The 

 limiting dynamical conditions are reached when the heat no 

 longer suffices to form steam in the boiler, or the power is 

 insufficient to run the machines. In either case, at the structu- 

 ral, or at the dynamical limit, work ceases. It may be the 

 work of a small part of the factory, so that the cessation is 

 hardly noticed ; or it may involve all the machines, producing 

 complete cessation of activity. 



To return to the protoplasm : the structural limiting condi- 

 tions are of two main sorts, — mechanical and chemical. The 

 mechanical limiting conditions are those in which the gross 

 structure becomes broken down, while the chemical limiting 

 conditions are those in which the composition becomes changed. 

 To the mechanical group belongs the breaking down of the 

 plasma films, either by drawing out the water of the protoplasm 

 (by osmosis or by drying) or by the expansion due to the 

 freezing of the chylema. To the chemical group belong, for 

 example, the reactions upon protoplasm of the halogen salts 

 of the heavy metals, and of complex nitrogenous organic com- 

 pounds in whose molecules hydrogen is unstably joined to 

 nitrogen, also the coagulation of the plasma by high tempera- 

 tures and the destruction of molecules by contact, by the 

 electric current, or by light. The dynamical limiting condi- 

 tions, on the other hand, are the absence of oxygen or other 

 food-stuffs, the absence of the water necessary to the solution 

 and circulation of the food, absence of light, in the case of 

 chlorophyllaceous organisms, and a temperature much below 

 0° C. Thus, the conditions essential to metabolism are the 



