RESINS. 45 



the plant may vary very radically from that obtained 

 from another. The chief chemical constituents of resins 

 are ester-like unstable resene (esters of resins, tannols), 

 very stable resine, and in some instances aromatic acids, 

 such as cinnamic and benzoic acids, in which instances 

 the resins are frequently termed Balsams, although this 

 is not a valid distinction. The balsam fir of the Adi- 

 rondacks that yields the so-called Canada Balsam, so 

 widely used in microscopy, does not contain any of these 

 aromatic acids, but is nevertheless termed a balsam. 



Hard resins and soft resins, or those that will powder 

 and those that will not, was a distinction made by the 

 older chemists, but inasmuch as the differences are really 

 due to the percentages of volatile oil contained, it is 

 evident that such a distinction is not of permanent value. 



In general, pharmacognosy distinguishes three kinds 

 of resins: (i) Ordinary resins, (2) gum resins, and (3) 

 balsams. The gum resins are characterized solely by 

 their containing a large percentage of gums. By bal- 

 sams is meant either ordinary resins which, like turpen- 

 tine or Canada balsam, are rich in ethereal oils, holding 

 the resin for the most part in solution, making syrup- 

 like compounds, or those bodies which are really poor 

 in resins, but are made up of resin-like substances with 

 aromatic acids, like benzoic or cinnamic acids — such as 

 is seen in Balsam of Tolu, Balsam of Peru, Styrax, etc. 



Color, transparency, hardness, fracture, tenacity, and 

 streak are all features of pharmacognostic interest. The 

 solubility of resinous bodies is of importance. The 

 resins are insoluble in water; the gum resins, especially 

 those very rich in gums, make emulsions when mixed 

 with water. Solubility in alcohol, ether, carbon disul- 

 phide, turpentine oil, benzol, petroleum, ether, and 

 acetone varies widely, but these are the best solvents. 

 The reaction of resins and gum resins to chloral hydrate 

 is of chemical interest. Many are completely soluble in 



