362 MICROCHEMISTRY OF PLANT PRODUCTS 



insoluble in all the acids excepting hydrofluoric acid. Silicious 

 skeletons may be removed from diatoms by placing the latter 

 in hydrofluoric acid which is contained in a platinum vessel. 

 The vessel should be kept on a water-bath, and the diatoms 

 should remain in the acid for 24 hours. At the end of this time 

 the acid should be thoroughly washed out from the diatoms. 

 On examination with the microscope, the diatoms will then be 

 found to have lost their silicious skeletons. In some instances 

 a thin exterior membrane which is stained brown by iodine is 

 to be observed; but in other instances this membrane has been 

 a too insignificant part of the skeleton to retain its identity 

 after the removal of the silica. 



Sinapine, C^gHgjNO.. — This is an alkaloid occurring in 

 the seeds of the white mustard. When sections of these seeds 

 are placed in a concentrated solution of potassium hydrate, 

 they assume a yellow color, which changes to orange on warm- 

 ing. This reaction loses some of its value, however, from the 

 fact that a glucoside called sinalbine also occurs in the seeds 

 of the white mustard and turns yellow on the application of 

 potassium hydrate. 



Solanin, CjgH^^NOii + HaO.— This glucoside occurs in 

 the tissues of Solanum tuberosum, in the berries of Solanum 

 nigrum and S. Dulcamara, and in many other species of the 

 Solanaceae. To demonstrate its presence, sections should be 

 placed in a mixture of i part of ammonium vanadate and 1,000 

 parts of a mixture of 98 parts of sulphuric acid with 36 parts 

 of water. This produces with solanin a yellow color, which 

 changes successively into orange, purple-red, brown, red-orange, 

 carmine-red, raspberry-red, and blue-violet. The color then 

 passes into a grayish-blue and disappears. With concentrated 

 sulphuric acid solanin assumes a yellow color, which changes 

 to red, and then violet, and then passes into gray and disappears. 



Starch, CgHjoOg. — A solution of potassium iodide-iodine 

 stains starch from pale violet to purple, depending on the strength 

 of the iodine solution. Chloroiodide of zinc stains starch-grains 

 purple, and at the same time swells them. A solution of chloral 



