156 C. M. CHILD. 



directly with, though not necessarily proportional to the general 

 physiological activity of the protoplasm. To a certain lower 

 range of concentrations or intensities, also experimentally deter- 

 mined for each species, the rate and degree of acclimation or 

 acquirement of tolerance vary directly, though not necessarily 

 proportionally, to the general physiological activity of the proto- 

 plasm, and the rate and degree of recovery after temporary ex- 

 posure to the action of the agent also vary in the same way. 

 The metabolic reactions and particularly the oxidations are im- 

 portant factors in the general physiological activity of proto- 

 plasm and many lines of evidence show that susceptibility may 

 be used within certain limits and with certain precautions as a 

 rough comparative measure or indicator of the rate of oxidation. 

 This of course does not mean that all the agents used in deter- 

 mining susceptibility act directly on the oxidations. Undoubtedly 

 different agents act in very different ways upon living proto- 

 plasm but the general non-specific character of the susceptibility 

 gradients indicates the interrelation of different processes and 

 conditions in the protoplasmic system. The difference between 

 a region of high and one of low rate of oxidation unquestionably 

 involves not simply the oxidative reactions but many other 

 factors, e.g., colloid dispersion, permeability of limiting surfaces, 

 active mass of enzymes, etc., and within physiological limits 

 change in the physiological state involves changes in all these 

 factors. Apparently the general relation between susceptibility 

 and physiological state is primarily an expression of the fact 

 that the dynamic equilibrium of the more active state is, on the 

 one hand, more readily upset by any extreme external action 

 than that of the less active state and, on the other, undergoes 

 more rapid and more complete recovery from temporary altera- 

 tion. In short, so far as susceptibility is non-specific and quan- 

 titative, it is apparently an indicator of the quantitative aspects 

 of physiological state in protoplasm. Susceptibility to the higher, 

 directly lethal concentrations and intensities can be determined 

 by survival time and in many cases loss of motor activity, swell- 

 ing or shrinkage of cells or other changes preceding death can 

 be used as a check on survival time. To lower concentrations, 

 which are not directly lethal, susceptibility can be determined by 



