296 



PHYSIOLOGICAL CHEMISTRY 



The colorimeter cups are replaced by the jackets C which project 

 through the holes in the cup supports F and are supported on them 

 by the collars D. They move when the cup supports move. The 

 mirror is turned to the horizontal position so that it reflects no light. 

 The light in the nephelometer comes from in front and not from below 

 (see Fig. 95). The nephelometer tubes are small test-tubes 100X15 

 mm., preferably made from the same sample of colorless glass tubing 

 so that they are of exactly the same bore. The flanges at the top should 

 be well made so that the tubes rest firmly and evenly in the slots. The 

 glass should be as free as possible from imperfections and striations. 



st 



* -^ 



PATH OF LIGHT 



,J 



PARTITION 



F-f$ 



CURTAIN 



FIG. 95. NEPHELOMETER IN POSITION, SHOWING RELATION TO SOURCE OF LIGHT. 



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 BI 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 

 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. 



