68 



The Precipiti7is : MetJiods 



The test-tube racks (Fig. 4) were made in the laboratory, and consisted 

 of a flat strip of wood upon which three equidistant blocks of wood were 

 nailed to support a strip of cardboard perforated with holes of suitable 

 size 2 cm. apart. The holes were bored by means of a red-hot iron rod 

 pointed at one end. Each rack contained holes for 20 tubes, the strip 

 below being indented by means of a nail-punch with holes correspond- 

 ing to those above in the cardboard; these indentations received the 

 convex bottoms of the small tubes and fixed them in place. Each rack 

 was lettered according to the alphabet at one end, and numbers 1 to 20 

 were written in ink upon a strip of white celluloid which was tacked 



Fig. 4. 

 Showing the test-tube racks on the stand, and a single rack in the foregi-ound. The 

 black riband is shown at the top, forming a background for the top row of tubes. For a 

 further description see the text, p. 70. Every part of the stand and racks, except the 

 celluloid strip bearing the numbers, is blackened. 



(The figure is reduced to rV natural size.) 



