MICROCHEMISTRY, 89 



garlic-oil, is a relatively simple compound, allyl sulphide (cf 

 126). 



Since we have no microchemically applicable reactions for 

 most of these compounds, one must be content in most 

 cases to recognize them as members of this group, which 

 may conveniently be called the group of the terpene-like 

 compounds, except where macroscopic studies of the sub- 

 stances contained in the plant concerned afford points of 

 vantage which may be turned to account microchemically. 

 In many cases even this recognition cannot be made with 

 certainty, for lack of a completely trustworthy reaction for 

 the group. 



143. If we omit caoutchouc and gutta percha, most of 

 these compounds are characterized by being strongly refrac- 

 tive and quite or almost insoluble in water. They are, how- 

 ever, soluble in the solvents of fats, like ether, chloroform, 

 carbon bisulphide, benzol, and ethereal oils. They are also 

 mostly readily soluble in cold alcohol. Like the fats, they 

 are deeply colored by alcannin, and the process is just the 

 same as for the fatty oils ( 109). 



Besides these, the following special reactions may be 

 described : 



a. Ethereal Oils. 



144. The ethereal oils are characterized microchemically 

 by the fact that they may be obtained from the plants by 

 distillation with water vapor, and that they leave on paper 

 a greasy spot which disappears after a time ; and for their 

 microchemical recognition their volatility may be utilized. 

 To test this, sections of the parts concerned may be boiled 

 in water for a time and examined with the microscope before 

 and after the boiling. 



According to A. Mayer (II), it is sufficient for the removal 

 of all ethereal oils to heat the uncovered sections for ten 

 minutes in a drying oven up to 130 C., since the fatty oils 

 remain unchanged under this treatment. Otherwise the 

 ethereal oils agree with the fatty oils in being browned or 



