GENERAL APPARATIS AND MAXIPlLATIOISr 191 



The samples to be weighed are held in quartz fiber hooks (G) 

 1 cm. long with loops at each end 2 mm. in diameter. These are 

 made from 3-4 cm. lengths of solid fiber weighing about 30 /xg./cm. 

 One end of a piece of the fiber is held in a small oxygen flame so 

 that the force of the flame bends the tip as it softens. By proper 

 manipulation a complete circle can be made. The weight is now 

 adjusted by clipping off the straight end with a scissors until the 

 desired deflection of the balance is obtained with it. When a number 

 of these hooks have been brought to the same weight within a few 

 mm. deflection, the second loop is made in each one. The hooks are 

 stored on a glass rack (H) which has a series of pegs (J) 0.5 mm. 

 in diameter projecting from the large horizontal tube at 3 cm. 

 intei-vals. The fine glass springs (K) prevent hooks from falling or 

 blowing off. The hooks are handled by a glass rod (L) about 1 mm. 

 in diameter drawn out at the end to 0.2 mm. A 5 mm. bend (M) is 

 made in the end of this rod to slip into the loop for transfer. During 

 attachment or removal of hooks at the end of the fiber, a straight rod 

 (N) is used to hold the fiber. 



After the case has been closed for 1.5-2 min., readings may be 

 taken; successive observations have been found to agree to 0.03 mm. 

 One hook is kept as a standard weight. Calibration of the balance 

 is carried out by accurately pipetting 3-10 /xl. of standard salt 

 solution into the lower loop of a hook (if 1-2 fx\. of distilled water is 

 placed in the loop first, the transfer of the salt solution is easier), 

 drying the solution, and observing the deflection given by the known 

 weight of salt. Deflections given by various weights are plotted to 

 form a calibration curve. 



In order to obtain the dry weight of microtome sections of tissue, 

 Lowry places a 3-5 (A. drop of water in the lower loop of a weighed 

 hook with a fine-tipped pipette, and then places the section in the 

 drop with a fine rod. Hooks with sections are put on the rack, dried 

 at 100° for 30 min., and reweighed. For the measurement of neutral 

 fat, the hooks with the dried sections may be kept in ethyl or 

 petroleum ether for 30 min., redried in the oven, and reweighed. 



Quartz Torsion Balance. A quartz torsion balance was devised 

 by Lowry (1944) that has a capacity of 50-100 mg. and a sensi- 

 tivity of ±0.1 fig. The instrument is shown diagrammatically in 

 Figure 75. The beam (A) is a quartz tube 25 cm. long and about 1 

 mm. in diameter suspended between the horizontal quartz fibers (C) . 



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