DENSITY' AND REDUCED WEIGHT 421 



constituent measured can be referred. The "reduced weight" is the 

 weight of the sample minus the weight of an equal volume of water, 

 or expressed mathematically: 



gr = g — (vg dj 



where Qr is the "reduced weight," g the weight of the sample, Vg the 

 volume of the sample, and dy, the density of water at the experimental 

 temperature. When the gradient tube is employed to determine the 

 "reduced weight," it is only necessary to place the sample in a small 

 drop of water, measure the specific gravity of the unit as a whole, 

 obtain the diameter of the water drop with the aid of the micrometer 

 microscope and from this value calculate the volume, and finally 

 apply the equation : 



Qr = (d — dj V 



where d is the specific gravity of the drop containing the sample, 

 and V is the volume of this drop. For standardization of the gradient, 

 drops are used consisting of mixtures of double-distilled water and 

 deuterium oxide ; and they have density differences of about 1 X 10~^. 



The accuracy of the method depends on the size of the drop. For 

 0.1 lA. drops it is about 5 X 10"'^ mg. and for 1.0 lA. drops it is about 

 5 X 10-^ mg. 



The tissue sample employed for "reduced weight" measurement 

 can be used subsequently for enzymatic determination, provided the 

 kerosene-bromobenzol medium has no influence on the activity. 

 The drop containing the tissue sample is removed from the gradient 

 tube by means of a retriever consisting of a thin glass rod the tip of 

 which is fused to a piece of cover glass, 2X2 mm., at an angle a 

 little greater than 90°. Once removed from the tube, the material can 

 be transferred to the medium appropriate for the enzyme measure- 

 ment. 



