418 



\vhen the suckling does not excite the sensation in which they feel plea* 

 sure. It is thought, in some countries, that serpents know how to tickle 

 the teats of cows, and that these animals enjoy this excitement, and allow 

 themselves to be sucked by these reptiles. 



CCXXII. Of t fie physical firofierties of milk y and of the chemical nature of 

 this fluid. The^ quantity of milk is, in general, proportioned to that of 

 the aliments, to the degree of their nutritious qualities, to their moist 

 and farinaceous nature. Though it equals, in weight, about one-third of 

 the quantity of food taken by the nurse, it may exceed that proportion, 

 or may not come up to it. Its specific gravity, even when the' milk is 

 .lightest, is greater than that of distilled water, and is always proportioned 

 to its consistency. The latter quality is in an inferior degree, in woman, but 

 is greater in the cow, the goat, the ass, and the ewe. Its fluidity is interme- 

 diate between that of aqueous and oily liquids, its colour, its smell and fla- 

 vour have something very peculiar, and by which it is easily recognized; 

 in the last place, it is not exactly alike, at different periods of the same 

 milking. This is proved by the work of Messrs. Deyeux and Parmentier 

 on milk, a work abounding in valuable observations, and which may be 

 considered as the complete history of this animal fluid. They observed, 

 that the milk first drawn from the cow is serous, that its consistency gra- 

 dually increases, and that the richest milk is that which is obtained to- 

 wards the end of milking, as if the fluid contained in the udder were af- 

 fected by the laws of gravitation*. 



The milk, when exposed to the open air, in a vessel, becomes decom- 

 posed, like the blood, and separates into three parts ; the serum, the 

 curd or cheesy part, and the fatty part or cream. The latter, which is 

 lighter than the others, is always on the surface, and its quantity depends, 

 not only on the richness of the milk, but also on the extent of the surfaces 

 by which it is in contact with the air; and this proves, as was first ob- 

 served, by Fourcroy, that the oxygen of the atmosphere has some influ- 

 ence on its separation. The caseous part, which coagulates spontane- 

 ously, appears albuminous, and abounds in oxygen. MM. Parmentier 

 and Deyeux consider it as the colouring matter of milk, and as giving to 

 it its most characteristic properties. Lastly, the serum or whey, which 

 alone constitutes the greatest part of this fluid, contains, besides a pecu- 

 liar acid, (the lactic acid'] which is formed when this substance is allowed 

 to remain for some time, a saccharine matter, which may be obtained by 

 evaporation, and which, when chrystallized in rhomboidal parallepipeds, 

 constitutes the sugar of milk, whose purity depends on the degree of care 

 with which the process has been carrkd on. This sugar of milk con- 

 tains, as Schele first "ascertained, while endeavouring, by means of the 

 nitric acid, to convert it into the oxalic, a peculiar acid, in the form of a 

 powder, difficult of solution, and to which he gave the name of saclatic 

 acid. Milk may be considered as one of the most compound of the ani- 

 rnal fluids, whose qualities are very valuable, and whose parts have but 

 an imperfect affinity to each other. So that it is liable to spontaneous 

 decomposition, and this takes place very easily. This kind of emulsion 

 contains but a small quantity of asote, so that it retains its vegetable charao 



* The author seems to forget that he is speaking of the cow, and that her uddci' 

 hangs in a situation unfavourable to this hypottcsis.--~7y7#!?'. 



