386 PROCEEDINGS OF THE AMERICAN ACADEMY. 



could discover at all, makes a very evident cloudiness. By sliding 

 the shades up and down a point may be found where the two tubes, 

 containing solutions of different cloudiness, appear equal in depth of 

 tone. The reason of this is that only the portion of the opalescence 

 is visible upon which light is allowed to fall. Of course the intensi- 

 ties of the opalescence, and hence the quantities of precipitate, are 

 then inversely as the length of the lighted portions of the two tubes. 



If care is taken to direct the light horizontally upon the tubes, 

 considerable accuracy may be obtained with the apparatus, especially 

 if the columns are nearly equal in cloudiness. 



A pointed blackened roof with a small hole in the top for the eye 

 is useful in excluding light from the surface of the liquid, thus ren- 

 dering the comparison easier. The chief advantages of the apparatus 

 lie in the facts that the two disks of light to be compared remain equal 

 in size thi-oughout the comparison, and that the eye is not confused 

 by bright surface reflections. Two typical test series are given below. 

 In each case one shade was adjusted at ten centimeters, and the 

 other was run backward and forward until apparent similarity was 

 obtained. 



(a.) One tube contained 0.010 milligram of silver, and the other 

 0.0125 milligram, measured by means of a very dilute standard solu- 

 tion. Both amounts were made up to twenty-five cubic centimeters, 

 and one cubic centimeter of hundredth normal hydrochloric acid was 

 added to each. The opalescence in each was then compared after a 

 thorough stirring and a short delay. 



Heights of Columns appearing Alike. 



{b.) In a similar experiment one tube contained 0.025 milligram of 

 silver, the other 0.0225 milligram. 



