216 ELEMENTARY CHEMICAL MICROSCOPY 



set of two exactly alike. Thick slices cut from one-hole rubber 

 stoppers (or carefully bored compact corks) cemented to glass 

 object slides serve to hold the tubes in a vertical position. The 

 tubes are forced into the holes in the rubber or cork cells with a 

 twisting motion and pushed down tightly until in contact with 

 the glass slide. The apparatus should then be turned upside 

 down and the contact between tube and glass slide examined 

 with a hand magnifier to make sure of perfect contact and that 

 no particles of the cell have been scraped off and lie between 

 the ends of the tubes and the slide. Liquids for comparison 

 may be introduced into the tubes by means of glass tubes drawn 

 down fine enough to enter the colorimeter tubes. Air bubbles 

 may be removed by means of a glass rod drawn down to the 

 dimensions of a hair or by means of a platinum wire. 



It will be found desirable to blacken both the upper and the 

 lower ends of the tubes and to wrap around the tubes black paper 

 so as to avoid the entrance of side light. In the author's labor- 

 atory it is the custom to loosely coil around the tubes several 

 thicknesses of dull black paper, glueing the last layer; when the 

 glue has hardened a black paper tube results which may be 

 slipped on the colorimeter tubes when observations are to be 

 made and removed for cleaning the tubes. 



The method of procedure is the same as that followed when 

 working with an ordinary colorimeter. A solution containing 

 a known concentration of the colored substance is placed in 

 one tube. The other tube contains the solution of the substance 

 of unknown percentage composition. The tubes are so placed 

 on the stages of the microscopes that their axes substantially 

 coincide with the optic axes of the microscopes, the instruments 

 having been previously focused and illuminated. Liquid is 

 carefully removed from the tube which yields the darker field 

 in the comparison eyepiece, until the colors of the halves of the 

 field are of the same intensity. The depths of the columns of 

 liquid are then determined (conveniently with a pair of dividers, 

 a strong hand lens and a finely divided steel scale). The com- 

 putations are made as usual. 1 



1 Andrews' Cells may also be advantageously used. See Fig. 69. 



