PREPARATION OF WOAD CAKES. 297 



yellow spots, which turn brown and acquire the appear- 

 ance of rust : this seems to be occasioned by the sudden 

 changes which sometimes occur in the atmosphere ; the 

 rays of a hot sun darting immediately upon plants after a 

 mist or rain, often produces a rustiness of the leaves and 

 stalks. 



It often happens, that, in consequence of a great degree 

 of heat accompanied by drought, the plants are not fully 

 developed ; the leaves acquire not more than one third 

 of their usual. size, yet exhibit all the other characteristics 

 of perfect maturity ; the harvest however is lost, for if the 

 leaves be cut in that imperfect state, the plants either perish 

 or languish without yielding any product. 



The isatis is not exempt from the ravages of insects . 

 there is one called the flea, which often destroys the first 

 and second harvests of leaves; another, known by the 

 name of the louse, attacks the last leaves, but does less in- 

 jury than the other, because Uie first harvests are the most 

 important. The snail and the cabbage-worm likewise com- 

 mit some depredations upon woad. 



ARTICLE II. 



Preparation of Woad Cakes. 



The manufacturer of woad cakes should avoid cutting 

 the leaves of the plant, till the period when they are richest 

 in indigo; this substance is, to be sure, contained in the 

 leaves of the isatis, during all the periods of its vegeta- 

 tion ; but the coloring principle does not present itself at 

 all times in the same quantity or of the same quality. In 

 the young leaves the coloring principle is of a delicate 

 blue, in those of a middle age the color is deeper, and in 

 the ripe leaves it approaches to black. It has likewise 

 been proved by observation, that the coloring principle is 

 obtained from the young leaves with more difficulty than it 

 is from those advanced towards maturity. 



It appears, then, that the most advantageous time for 

 gathering the leaves of woad, is when they have acquired 

 their full growth. But by what marks is this to be deter- 

 mined ? 



