2 9 2 



PHYSIOLOGICAL CHEMISTRY 



be as free as possible from imperfections and striations. After the 

 tubes are made and fitted into place the jackets are moved up on 

 each tube by means of the rack and pinion until the indicator on the 

 scale is exactly at zero. Marks are made on each tube at the point 

 reached by the top of the jacket and the portion of the tube above that 

 point is made opaque by a ring Bl of black paper or paint. Tubes 

 and jackets are then marked right and left and always used on the 

 same side. Since it is rare to find two tubes which when filled with the 

 same solution give exactly the same readings it is necessary to take this 

 fact into account and correct accordingly. 



The jackets C are made of tubing (metal or glass) a little larger 

 than the tubes and about the same length (they should clear the 

 mirror when it is turned horizontal), closed at the bottom and made 



PATH OF LIGHT 



PARTITION 



FIG. 84. NEPHELOMETER IN POSITION, SHOWING RELATION TO SOURCE OF LIGHT. 



light tight by black paint or paper. The collars D supporting the 

 jackets may be made of cork or more permanently of metal. A little 

 cotton wool in the bottom of the jackets will prevent breakage if the 

 tubes should fall into the jackets. 



The openings in the prism case, particularly the lower ones, should 

 be protected against accidental splashing by thin glass plates (thick 

 cover slips) which are held in place by a little glue. 



Artificial light is necessary and the lamp should be enclosed in 

 a tight box into one end of which the nephelometer fits snugly. A 

 partition extending part way up the box as shown in the diagram 

 (Fig. 84) serves the double purpose of shutting off the light from 

 the lower part of the instrument and of providing a stop against 

 which the instrument is pushed, so that its distance from the light 



