THEIR MODE OF ACTION. 379 



powder and moistened with water. Hence it hap- 

 pens that bitter almonds pounded and digested in 

 alcohol, yield no oil of bitter almonds, containing 

 hydrocyanic acid, by distillation with water; for 

 the substance which occasions the formation of 

 those volatile substances, is dissolved by alcohol 

 without change, and is therefore extracted from the 

 pounded almonds. Pounded bitter almonds contain 

 no amygdalin, also, after having been moistened 

 with water, for that substance is completely decom- 

 posed when they are thus treated. 



No volatile compounds can be detected by their 

 smell in the seeds of the Sinapis alba and S. nigra. 

 A fixed oil of a mild taste is obtained from them 

 by pressure, but no trace of a volatile substance. 

 If, however, the seeds are rubbed to a fine powder, 

 and subjected to distillation with water, a volatile 

 oil of a very pungent taste and smell passes over 

 along with the steam. But if, on the contrary, the 

 seeds are treated with alcohol previously to their 

 distillation with water, the residue does not yield a 

 volatile oil. The alcohol contains a crystalline 

 body called sinapin, and several other bodies. 

 These do not possess the characteristic pungency 

 of the oil, but it is by the contact of them with 

 water, and with the albuminous constituents of the 

 seeds, that the volatile oil is formed. 



Thus bodies regarded as absolutely indifferent 

 in inorganic chemistry, on account of their pos- 

 sessing no prominent chemical characters, when 



