THE MICROSCOPE IN CHEMISTRY. 107 



The purity of all reagents should be carefully estab- 

 lished, and they should be kept in hard German glass 

 bottles, and only distilled water used in all our researches. 



The true nature of a reaction that is common to several 

 substances may often be determined with the microscope. 

 Thus 'a solution of nitrate of silver becomes covered with 

 a white film when exposed to several different vapors, but 

 hydrocyanic acid is the 'only one which is crystalline. 

 This will detect 100,000th of a grain of the acid. A slip 

 of clean copper boiled in a hydrochloric acid solution of 

 arsenic, mercury, antimony, etc., becomes coated with the 

 metal, but when heated in a reduction-tube, arsenic only 

 yields a sublimate of octahedral crystals, and mercury 

 only will furnish metallic globules. 



A solution of iodine produces distinct reaction with 

 100,000th of a grain of strychnine in solution in 1 grain 

 of water, but as this is common to other alkaloids, other 

 tests are needed. Yet the absence of such a reaction 

 shows the -absence of the alkaloid. 



The degree of dilution is important. Thus bromine with 

 atropin yieldsHa crystalline deposit from 1 grain of a 

 20,000th or stronger dilution, but not with diluter solu- 

 tions. A limited quantity of sulphuretted hydrogen 

 throws down from corrosive sublimate a white deposit, 

 while excess produces a black precipitate. 



Blue and Reddened Litmus Paper are used as tests for 

 acids and alkalies. It is a bibulous paper dyed in infu- 

 sion of litmus. The red is made by adding a little acetic 

 acid to the infusion. Dry substances and vapors require 

 the paper to be moistened with distilled water. If the 

 acid reaction depends on carbonic acid, warming the paper 

 on a slide over a lamp will restore the color. So if a 

 volatile alkali, ammonia or carbonate of ammonia, have 

 made the red paper blue, its color will be restored by a 

 gentle heat. Sometimes the infusion of litmus is more 

 convenient than the paper. 



