76 GROWTH AND SECRETIONS. 



these oils are in general strongly odorous. The 

 fixed, or fat oils, as they are called, are formed 

 in cells, from which they never escape by any 

 natural process, but must be artificially extracted. 

 The caoutchouc (India rubber) is an instance 

 of a milky secretion, as are also our common 

 spurge, and opium, the well known product of 

 the white poppy. Most of the juices to which 

 the name of milky has been applied are tchife, 

 but not all of them, for instance, the lactic secre- 

 tion of our English celandine is of a brilliant 

 orange colour. Of the resinous juices one ex- 

 ample, common resin, is familiar to every one. 

 Of this class are the true balms, Gum Benzoin, 

 &c. Examples of volatile or essential oils, as 

 they are otherwise called, such as those of the 

 rose, &e. will readily occur to every one's recol- 

 lection ; and the fixed oils, those, for instance, of 

 the nut, the almond, linseed oil (the product of 

 the seed of the flax), olive oil, so useful for both 

 food and light to the inhabitants of the south of 

 Europe, with many others, are too well known 

 to need more particular notice here. The prin- 

 cipal chemical distinction between the volatile 

 and fixed oils is that the former are powerfully- 

 odorous, slightly soluble in water, with which 

 they pass over in distillation, communicating 



