490 BELL SYSTEM TECHNICAL JOURNAL 



sublimed directly on the slide. CryslalloKraphic constants and other 

 optical properties as well as the melting point and solubility may be 

 readily determined on the sublimate even though only a fraction of a 

 milligram is available. 



Another advantage to be gained by reactions carried out under the 

 microscope is that of performing tests in situ. In practice, it is 

 sometimes difficult to isolate physically the particles of material to be 

 studied. For example, it may be desired to learn something of the 

 nature of a tiny inclusion embedded in a metal or a thin film of cor- 

 rosion product present on its surface. Here again, the microscope is 

 of great service, both in guiding the physical manipulations necessary 

 to restrict the action of the reagents to a localized area and in observing 

 the actual identification reaction (usually chosen to yield an intensely 

 colored product or bubbles of gas, rather than a precipitate). 



When, instead of the commonly used qualitative reagents, com- 

 pounds are employed which are capable of more specific and sensitive 

 reactions and which yield intensely colored products rather than 

 precipitates, such products may be instantly recognized, even in a 

 single drop of solution. Within recent years, many organic com- 

 pounds have been developed for this purpose. The use of these, and 

 of a number of inorganic compounds giving highly characteristic color 

 reactions, constitute the basis for a new technique combining rapidity, 

 simplicity, and certainty of identification without the use of the micro- 

 scope. The drop to be examined is placed on a white background such 

 as a porcelain plate. A drop of the reagent is added and the color 

 change, which may involve a sequence of changing shades, is observed. 

 In cases where turbidity is also significant, a black porcelain plate is used. 

 Because of the highly specific nature of these reagents, it is often 

 unnecessary to resort to a preliminary group separation. Because of 

 its simplicity, the technique is particularly useful for field investi- 

 gations. 



A modification of the drop analysis procedure described above 

 consists in bringing the drop under examination together with the 

 reagent onto loose-textured paper such as filter paper. The colored 

 product which forms is adsorbed on the fibers of the paper at the center 

 of the drop, while the solution spreads out due to capillarity. The 

 capillary action of the paper fibers is sometimes utilized to de\ise 

 rapid separations, thus enabling the analyst to detect two or more 

 elements simultaneously. In a typical case, a solution contains 

 copper and nickel. A drop of this solution, acidified with acetic acid, 

 is brought onto the paper which is impregnated with hxdrorubeanic 

 acid. The bluish-black copper compound is insoluble and therefore 



