54 ANIMAL AND VEGETABLE MILK. 
hevea, I mixed a solution of carbonate of soda with the 
milk of the papaw. Ao clot la formed, even when pure 
water is poured on a mixture of the milk with the alkaline 
solution. The membranes appear only when, by adding an 
acid, the soda is neutralized, and the acid is in excess. I 
made the coagulum formed by nitric acid, the juice of 
lemons, or hot water, likewise disappear by mixing it with 
carbonate of soda. The sap again becomes milky and liquid, 
as in its primitive state ; but this experiment succeeds only 
when the coagulum has been recently formed. 
On comparing the milky juices of the papaw, the cow-tree, 
and the hevea, there appears a striking analogy between the 
juices which abound in caseous matter, and those in which 
caoutchouc prevails. .All the white and newly prepared 
caoutchouc, as well as the waterproof cloaks, manufactured 
in Spanish America by placing a layer of milk of hevea 
between two pieces of cloth, exhale an animal and nauseat- 
ing smell. This seems to indicate that the caoutchouc, in 
coagulating, carries with it the caseum, which is perhaps 
only an altered albumen. 
The produce of the bread-fruit tree can no more be 
considered as bread than plantains before the state of 
maturity, or the tuberous and amylaceous roots of the cas- 
sava, the dioscorea, the Convolvulus batatas, and the potato. 
The milk of the cow-tree contains, on the contrary, a 
caseous matter, like the milk of mammiferous animals. 
Adv ancing to more general considerations, we may regard, 
with M. Gay-Lussac, the caoutchouc as the oily part, — 
the butter of vegetable milk. AYe find in the milk of 
plants caseum and caoutchouc; in the milk of animals, 
caseum and butter. The proportions of the two albuminous 
and oily principles differ in the various species of animals 
and of lactescent plants. In these last they are most fre- 
quently mixed with other substances hurtful as food ; but of 
which the separation might perhaps be obtained by chemical 
processes. A vegetable milk becomes nourishing when it is 
destitute of acrid and narcotic principles ; and abounds less 
in caoutchouc than in caseous matter.* 
* The milk of the lactescent agarics has not been separately analysed ; 
it contains an acrid principle in the Agaricns piperatus ; and in other 
•pedes it is sweet and harmless. The experiments of MM. Braconnot, 
