498 BELL SYSTEM TECHNICAL JOURNAL 



measurement by micromodifications of the conventional i^ravimetric 

 and volumetric methods, the microscope sometimes may be ingeniously 

 applied to quantitative determinations. Colorimetric and turbidi- 

 metric measurements may thus be made on a few thousandths of a 

 cubic centimeter of solution contained in a capillary tube. The rela- 

 tive quantities of two or more components of a mixture may frequently 

 be approximated from area determinations made on the individual 

 particles in the microscope field. Instead of weighing the mercury 

 recovered by capillary distillation, the condensed globules may be 

 united by centrifuging and the mass of the resulting single globule 

 estimated from diameter measurements under the microscope. 

 Similarly the analysis of a minute amount of gas may be carried out 

 by measuring the shrinkage in diameter of a single microscopic bubble 

 as the absorption reagents in which it is immersed are changed. 



A striking example of a quantitative determination so contrived 

 that the final measurement is performed microscopically is the micro- 

 molecular-weight method of Barger. Two solutions, one known and 

 the other containing the unknown, are placed in a capillary with a 

 small air bubble separating them. The ends of the capillary are 

 sealed and the lengths of the two liquid columns measured on a microm- 

 eter scale. After several hours the measurement is again made, and 

 repeated at intervals until the column lengths become constant. 

 When this occurs, the vapor pressure of the two solutions wall be 

 identical and since vapor pressure is a function of the molar concen- 

 tration, the latter may also be assumed to be the same in each solution. 

 Knowing the original weight concentrations and the molecular weight 

 of one of the substances, that of the other may be calculated from the 

 change in the volumes of the two solutions necessary to bring about 

 equilibrium. 



The application of quantitative microtechnique to engineering and 

 research problems at Bell Telephone Laboratories has required a con- 

 siderable amount of development work directed toward the improve- 

 ment of apparatus and the creation of new types of technique. Thus, 

 in order to carry out micrometallurgical analyses, several forms of 

 electrolysis cells were developed which permit the determination of 

 metals such as copper, zinc, nickel, lead, cadmium, tin and others. 

 With these cells, using five milligram samples, the same accuracy is 

 attained as in ordinary analysis on half a gram. One of these cells, 

 designed for the analysis of extremely dilute solutions, permits the 

 qualitative detection of one part of copper, zinc, or lead in 100,000,000 

 parts of water and is partic ularlx- useful in isolating minute fjuantities 



