MISUSE OF OPIUM. COLOURING PRINCIPLES. 245 



through the day, a quantity sufficient to poison any one unac- 

 customed to its use. The practice of taking opium often com- 

 mences with the occasional use of it, for the purpose of allaying 

 pain or procuring sleep ; and those who are obliged to have 

 occasional recourse to it for this purpose, should be on their 

 guard against taking it more frequently than is absolutely neces- 

 sary. For such persons, Morphia is the most desirable medicine; 

 since it produces more completely the effects they desire, with- 

 out that excitement to the nervous system, which leads to the 

 employment of it as a source of pleasure. The quantity of this 

 drug annually consumed in England, may be stated at about 

 35,000 Ibs. ; whilst that which has been introduced into China, 

 in spite of the laws which prohibit it, has for some years 

 averaged more than 3^ millions of pounds, the value of which 

 considerably exceeded that of the tea exported. The quantity 

 seized by the Chinese government, in March 1839, was upwards 

 of three million pounds. 



386. We shall next notice some of the principal Colouring 

 matters secreted by plants. On these are dependent the varied 

 hues so beautifully and abundantly distributed through the 

 vegetable kingdom ; of which some at once delight the eye of 

 man, whilst gazing upon the garden, the meadow, or the forest ; 

 whilst others, extracted from the interior, even of plants of the 

 dullest aspect, contribute to his comfort and luxury in various 

 ways. The colouring secretion most universally diffused through 

 plants, is that termed chromule, on which the colour of all green 

 parts depends. It is found in little grains, which adhere to the 

 inside of the cells beneath the cuticle ; and the formation of it is 

 due, as formerly stated (. 288), to the influence of light, in fixing 

 carbon from the atmosphere. The brightness of this green colour 

 soon disappears after the death of the part ; and it is not unfre- 

 quently seen to alter its hue, whilst vital actions are going on in 

 it. Thus the leaves of many trees, as the Lombardy Poplar, 

 change to yellow in autumn, long before their fall ; whilst others, 

 as the Berberry, Sumach, &c., turn to red. This alteration is 

 due to an increased absorption of oxygen, which is no longer 

 given out by day; and the chromule may be artificially converted, 



