116 M. Humboldt on Vegetable Milk. [August, 



season in which the cow-tree yields the greatest quantity of 

 milk. 



When this fluid is exposed to the air, perhaps, in consequence 

 of the absorption of the oxygen of the atmosphere, its smface 

 becomes covered with membranes of a substance that appears 

 to be of a decided animal nature, yellowish, thready, and of a 

 cheesy consistence. These membranes, when separated^ from 

 the more aqueous part of the fluid, are almost as elastic as 

 caoutchouc ; but at the same time they are as much disposed to 

 become putrid as gelatine. The natives give the name of cheese 

 to the coagulum, which is separated by the contact of the air ; 

 in the course of five or six days it becomes sour. The rnilk, 

 kept for some time in a corked phial, had deposited a little 

 coagulum, and still exhaled its balsamic odour. If the recent 

 juice be mixed with cold water, the coagulum is formed in small 

 quantity only; but the separation of the viscid membranes occurs 

 when it is placed in contact with nitric acid. 



This remarkable tree seems to be peculiar to the Cordilliere 

 du Littoral, especially from Barbula to the lake of Maracaybo. 

 There are likewise some traces of it near the village of San 

 Mateo ; and, according to the account of M. Bredmeyer, in the 

 valley of Caucagua, three days' journey to the east of the 

 Caraccas. This naturalist has likewise described the vegetable 

 milk of the cow tree as possessing an agreeable flavour and an 

 aromatic odour : the natives of Caucagua call it the milk tree. 



M. Humboldt offers some general observations upon the milky 

 juices of plants, and concludes with some particular observations 

 upon the fluid which is procured from the carica papaya; 

 this has been analyzed by M. Vauquelin;* but the specimen 

 which he examined had had its properties altered by having 

 been conveyed to a great distance, and kept for a long time. 



The younger is the fruit of the papaw, the more milk does it 

 yield ; in proportion as the fruit ripens, the milk, which is less 

 abundant, becomes more watery : there is then less of that 

 animal matter which is coagulable by acids and by the absorp- 

 tion of oxygen. When nitric acid is poured drop by drop into 

 the milky juice procured from a very young fruit, a very extraor- 

 dinary phenomenon is observed. In the centre of each drop 

 there is formed a gelatinous pellicule, divided by greyish striae ; 

 these striae are merely the juice which is rendered more watery, 

 because the contact of the acid has caused it to lose its albumen. 

 At the same time the centre of the pellicule becomes opaque, 

 and of the colour of the yolk of the egg ; while it increases in 

 bulk by the prolongation of the diverging fibres. The whole 

 fluid at first has the appearance of an agate with milky clouds ; 

 and it appears as if organic membranes were produced under the 



* Ann. de Cliim. xliii. 207. 



