ANALYSIS IN COMMUNICATION RESEARCH 489 



few tenths of a cul)ic centimeter. Idcntificatifjn reactions are then 

 carried out in sint^Ie small ch(){)s of the solution. To accomplish this, 

 a numl)er of procedures are available. One which is very j^eneraily 

 applicable, yielding information of a particularly specific and positive 

 character, is that of carrying out the reaction directly under the 

 microscope. By this means, quantities of elements ranging between 

 a thousandth and a few hundredths of a milligram may be detected. 

 The drop to be e.xamined is placed on a glass slide, the reagent intro- 

 duced from a capillary pipette and the progress of the transformation 

 watched under magnifications of between fifty and three hundred 

 diameters. 



In addition to revealing the presence of minute amounts of reaction 

 products separating from the solution, the use of the microscope 

 facilitates study of the individual particles composing such precipi- 

 tates. A number of properties are thereby brought into analytical 

 significance which might not otherwise be observed or utilized. In 

 ordinary practice, the analyst is guided in his conclusions as to the 

 presence of an element simply by the bulk formation of a precipitate, 

 specific recognition of which is based only on characteristics readily 

 apparent to the unaided eye, such as color and gross structure. Under 

 the microscope, however, the crystal structure and similar distinctive 

 morphological features, color, transparency, index of refraction, be- 

 havior toward polarized light, characteristics of growth and other 

 specific properties may all be studied and employed to give greater 

 certainty to the identification. Further, it is frequently possible to 

 identify the individual components of a mixed precipitate, thereby 

 obviating the necessity of separation. Thus, the double salt potassium 

 mercuric thiocyanate gives insoluble compounds with a large number 

 of the bivalent metals. To apply this reagent to a solution containing 

 several metals would result in the formation of a precipitate which to 

 the unaided eye would yield very little specific information. Under 

 the microscope, the experienced analyst, in a single observation, can 

 often tell from the known habits of the crystals produced by various 

 combinations of metals, the nature of the mixture. 



Recognition of substances is not always confined to the precipita- 

 tion of insoluble reaction products, but soluble salts, when the solu- 

 tion is carefully evaporated, sometimes possess sufficiently distinctive 

 morphology to permit direct identification. In this way minute 

 amounts of sodium chloride have been detected in dust deposits 

 collected near the sea-coast. In the identification of traces of organic 

 material, valuable information is frequently obtained from microscopic 

 studies of the crystalline deposits produced when the substance is 



