April 22, 1921] 



SCIENCE 



393 



of the anion concentration by means of con- 

 centration cells. A further increase in the 

 amount of acid will now serve to decrease the 

 concentration of protein ions by increasing the 

 concentration of the common anion. The con- 

 centration of ionized protein will therefore 

 pass through a maximum which should coin- 

 cide with the maximum for the rate of 

 digestion. If the ordinary theory of chemical 

 kinetics, on the basis of the law of mass 

 action, be applied to the above system, it may 

 be predicted that: 



I. The optimum hydrogen ion concentra- 

 tion for the digestion of the protein must 

 coincide with the hydrogen ion concentration 

 at which the concentration of protein ions 

 and therefore the conductivity due to the 

 protein is at a maximum. 



II. The limiting pH for the activity of 

 pepsin on the alkaline side must dejjcnd on 

 the isoelectric point of the protein, since this 

 is the point at which the protein first begins 

 to react with the acid. 



III. The addition of a salt with the same 

 anion as the acid to a solution already con- 

 taining the optimum amount of acid will 

 have the same depressing effect on. the 

 digestion as the addition of the same amount 

 of anion in the form of acid. 



IV. The pepsin should combine with the 

 protein only when the latter is ionized, i.e., 

 pepsin should behave the same as the in- 

 organic anions studied by Loeb. 



These predictions have been tested quanti- 

 tatively and found to be fulfilled. It has also 

 been found by direct experiment that neither 

 the influence of the acidity on the destruction 

 of the enzyme, nor the viscosity of the protein 

 solution can account for the influence of the 

 hydrogen ion concentration on the rate of 

 digestion. 



It will be seen that from this point of view 

 pepsin digestion is a chemical reaction in 

 which the pepsin as well as the protein takes 

 part. It is therefore not a catalytic reaction 

 at all in the classical sense. The specificity 

 of the reaction is therefore probably governed 

 by the same conditions that determine the 

 specificity of any chemical reaction, since 



from a quantitative standpoint each chemical 

 reaction is specific. It may be added that a 

 very similar mechanism was proposed by 

 Stieglitz and his collaborators for the hydro- 

 lysis of the imido esters by acid. 



It is, of course, impossible at present to 

 apply these results directly to the activities 

 of the living organism since conditions there 

 are much more complex. It is probable, how- 

 ever, that much of the apparent complexity 

 is due to the fact that several processes, each 

 simple in itself, occur simultaneously and 

 thus lead to a complicated result. Dernby's* 

 experiments render it probable that the phe- 

 nomenon of autolysis may be explained in 

 this way. 



John H. Northrop 



eockefeller institute toe 

 Medical Eeseakch 



knipp's singing tube 



My colleague. Dr. C. T. Knipp, when con- 

 structing a piece of apparatus, found that one 

 of the parts — a glass tube intended for a 

 mercury traj) — gave forth a musical sound 

 under the heating action of a gas flame. Fol- 

 lowing tliis clue he constructed various modi- 

 fications of the tube and described them with 

 the interesting results obtained.^ Inquiry has 

 been expressed concerning the explanation of 

 its action. It occurred to the writer that this 

 explanation might be found in the theory ad- 

 vanced for similar cases where sounds are 

 maintained by heat.^ 



Fig. 1 pictures one type of the tubes tested. 

 It is a resonator with a loop at A and a node 

 at N, so that the distance ABCN constitutes 

 approximately one fourth of the wave-length 

 of the sound given out by the tube when 

 operating.^ The air surges back and forth at 

 A with the greatest velocity and displacement. 

 From this point the to and fro motion of tlie 



sDernby, K. G., Biocliem. Z., 1917, LZXXI., 

 198. 



iPhys. Bev., Vol. 15, p. 155, 1920; and other 

 publications. 



2 Eayleigh, ' ' Theory of Sound, ' ' Sec. 322. Bar- 

 ton, ' ' Text-Book of Sound, ' ' Sec. 265-277. 



8 Fhys. Mev., Vol. 15, p. 336, 1920. 



