FAMILIAR LETTERS ON CHEMISTRY. 27 



In that endless series of compounds, which begins with carbonic acid, ammonia, 

 and water, the sources of the nutrition of vegetables, and includes the most 

 complex constituents of the animal brain, there is no blank, no interruption. The 

 first substance capable of affording nutriment to animals is the last product of the 

 creative energy 01 vegetables. 



The substance of cellular tissue and of membranes, of the brain and nerves, 

 these the vegetable cannot produce. 



The seemingly miraculous in the productive agency of vegetables disappears in 

 a great degree, when we reflect that the production of the constituents of blood 

 cannot appear more surprising than the occurrence of the fat of beef and mutton 

 in cocoa beans, of human fat in olive-oil, of the principal ingredient of butter in. 

 palm oil, and of horse fat and train-oil, in certain oily seedsu 



LETTER IX, 



MY DEAR SIR : 



The facts detailed in my last letter will satisfy you as to the manner in which 

 the increase of mass in an animal, that is its growth, is accomplished ; we have 

 still to consider a most important question, namely, the function performed in the 

 animal system by substances destitute of nitrogen ; such as sugar, starch, gum, 

 pectine, &c. 



The most extensive class of animals, the graminivora, cannot live without these 

 substances ; their food must contain a certain amount of one or more of them, and 

 if these compounds are not supplied, death quickly ensues. 



This important inquiry extends also to the constituents of the food of carnivorous 

 animals in the earliest periods of life ; for this food also contains substances which 

 are not necessary for their support in the adult state. The nutrition of the young 

 of carnivora is obviously accomplished by means similar to those by which the 

 graminivora are nourished ; their development is dependent on the supply of a fluid, 

 which the body of the mother secretes in the shape of milk. 



Milk contains only one nitrogenised constituent, known under the name of 

 caseine ; besides this, its chief ingredients are butter (fat), and sugar of milk. The 

 blood of the young animal, its muscular fibre, cellular tissue, nervous matter, and 

 bones, must nave derived their origin from the nitrogenised constituent of milk 

 the caseine ; for butter and sugar of milk contain no nitrogen. 



Now, the analysis of caseine has led to the result, which, after the details I have 

 given, can hardly excite your surprise, that this substance also is identical in com- 

 position with the chief constituents of blood, fibrine and albumen. Nay more a 

 comparison of its properties with those of vegetable caseine has shown '-that these 

 two substances are identical in all their properties ; insomuch, that certain plants, 

 such as peas, beans, and lentils, are capable of producing the same substance 

 which is formed from the blood of the mother, and employed in yielding the blood 

 of the young animal. 



The young animal, therefore, receives in the form of caseine which is distin- 

 guished from fibrine and albumen by its great solubility, and by not coagulating 

 when heated the chief constituent of the mother's blood. To convert caseine into 

 blood no foreign substance is required, and in the conversion of the mother's blood 

 into caseine, no elements of the constituents of the blood have been separated. 

 When chemically examined, caseine is found to contain a much larger proportion 

 of the earth of bones than blood does, and that in a very soluble form, capable of 

 reaching every part of the body. Thus, even in the earliest period of its life, the 

 development of the organs, in which vitality resides, is, in the carnivorous animal, 

 dependent on the supply of a substance, identical in organic composition with the 

 chief constituents of its blood. 



What, then, is the use of the butter and the sugar of milk ? How does it happen 

 that these substances are indispensable to life ? 



Butter and sugar of milk contain no fixed bases, no soda or potash. Sugar of 

 milk has a composition closely allied to that of the other kinds of sugar, of starch, 

 and of gum ; all of them contain carbon and the elements of water, the latter 

 precisely in the proportion to form water. 



There is added, therefore, by means of these compounds, to the nitrogenized 



