420 DILATOMETRIC METHODS 



medium, and slowly raise the tip of the needle out of the liquid to 

 disengage the drop. 



4. Repeat preceding step with control drop containing heat- 

 inactivated enzyme. 



5. Record positions of drops, relative to standard drops, at inter- 

 vals of about 15 min., until the rate of change in position of both 

 active and control drops is equal, indicating that the enzymatic 

 process is complete. 



6. Plot results to determine the slope relating density change 

 with time. When the size of the drops and experimental conditions 

 are kept constant, the millimoles of alanylglycine split per unit time, 

 R, is given by the equation: 



R = {Sv/Kd) 



where S represents the slope, y the drop vol., K the contraction 

 constant under the experimental conditions, and d the drop density. 



DENSITY AND "REDUCED WEIGHT" 



Aside from its use to follow the course of reactions, the density 

 gradient technique can be employed as a very convenient method 

 for the measurement of the density of very small amounts of mate- 

 rial. Linderstr0m-Lang, Jacobsen, and Johansen (1938) applied the 

 technique to the measurement of the deuterium content in mixtures 

 of water and deuterium oxide. Jacobsen and Linderstr0m-Lang 

 (1940) modified the apparatus for more rapid determinations of 

 specific gravity by the use of a 200 ml. graduated cylinder as a 

 gradient tube, omitting thermostatic control and the use of the 

 traveling microscope. This simplification is well adapted to the 

 measurement of the specific gravity of biological fluids with an 

 accuracy of 0.1%. Lowry and Hunter (1945) employed a similar 

 apparatus for the determination of serum protein concentration. 



It is of particular interest that the gradient tube has been adapted 

 to the determination of "reduced weight" ( Linderstr0m-Lang and 

 Holter, 1940), since the latter value affords a measure of the size of a 

 tissue sample independent of its water content. In some respects this 

 circumvents one of the chief difficulties encountered in histochemical 

 and cytochemical investigations, i.e., obtaining a quantitative defini- 

 tion of structural elements in tissues or cells to which quantities of a 



