ANALYSIS IX C().\f.\n'.\nATl()N RESEARCH 491 



forms first, near the tvnlcr of the drop. The nickt-l coinpoimd, Ix-iiij,^ 

 more soluble, does not form until the solution has spread out to a 

 considerable extent and most of the acid has evaporated, when it is 

 visible as a violet zone near the periphery of the drop. 



Papers which have been impregnated with specific reagents and 

 preserved in the dry condition are extremely useful both in the lal)ora- 

 tory and the field. In the analysis of gaseous substances or materials 

 readily volatilized, these dry test papers have been very satisfactory. 

 Arsine, stibine, hydrogen sulphide, sulphur dioxide, hydrocyanic acid 

 and other objectionable gases present in minute quantities in the 

 atmosphere, may be detected and their approximate concentrations 

 determined by passing a stream of the air through the fibers of a 

 suitable test paper. 



In order to obtain solutions to which identification tests may be 

 applied, some preparatory chemical treatment is necessary. The 

 initial solution and concentration of the sample, its recovery from 

 inert material as well as its separation into convenient analytical 

 groups require laboratory operations capable of dealing with a few- 

 drops of liquid and often with a fraction of a milligram of solid. 

 A variety of types of apparatus and special processes have been 

 developed to facilitate these operations. In a few cases, reduction 

 in size has alone sufficed; more often such reduction, with retention 

 of the original form of the apparatus, results unsatisfactorily and new 

 principles must therefore be followed in the microdesign. 



For example, it is obvious that the usual folded paper cone cannot 

 be satisfactorily reduced in size to permit the removal and recovery 

 of suspended matter from a few drops of liquid. Microfiltration may 

 be accomplished in a number of ways but the most convenient is to 

 draw the liquid into a capillary pipette provided with a retaining well 

 which holds a very small pellet of the filtering medium. In this way 

 a single drop may be filtered and completely washed in a few seconds, 

 the residue being concentrated in the tiny filter plug from which it 

 may readily be redissolved in a trace of acid. Practically all of the 

 common analytical operations have been reduced to a microscale and 

 may be carried out with rapidity and precision in equipment of appro- 

 priate design. 



It may be of interest to note here a few of the more ingenious 

 devices and processes that have been developed to aid qualitative 

 microanalysis. The electrolytic cell of H. Brenneis provides for the 

 precisely controlled electrolysis of a drop of solution, at the same time 

 permitting continuous observation of the electrode surfaces under the 

 microscope. By its use, O.OOl mgm. of copper ma\- readily be recog- 



