214 VEaETABLE COLORS. 



table colors are soon bleached by sulphurous acid, 

 and rapidly destroyed by chlorine. The art of the 

 dyer consists in staining wool, linen, and cotton of 

 various colors ; some of the colors he employs are 

 metallic, but the greater number are of vegetable or 

 animal origin. Dyes are divided into those which at 

 once permanently stain the fabric to be dyed, such 

 as indigo ; and those which require the use of some 

 mordant or substance to fix them. The most import- 

 ant mordants are alumina, oxide of iron, and oxide 

 of tin. These substances have a remarkable attrac- 

 tion for coloring matters, and also for the fibres of 

 cotton, wool, and silk ; they consequently assist in 

 dyeing those materials of various colors. 



543. Lastly, there are a number of peculiar veget- 

 able substances, to the presence of which in plants 

 their medicinal properties are in many cases due. 

 Chemists have obtained from various plants a great 

 number of substances called " active principles," 

 some of which are highly poisonous, whilst others in 

 small quantities are valuable medicines. Amongst 

 this class of substances are the vegetable alkalies, as 

 they are called ; these are compounds of oxygen, 

 hydrogen, carbon, and nitrogen, which possess the 

 power of combining with acids and forming neutral 

 salts. They are, of course, destroyed by heat, and, 

 whilst burning, give off the usual results of the com- 

 bustion of organic matter — water, carbonic acid, and 

 ammonia. 



514. All plants contain a small quantity of inor- 



