320 EXTRACTIVE MATTERS. 



whether they are chemically produced, or exist preformed in an 

 animal fluid, exhibit few distinguishing properties (that is to say, 

 are uncrystallisable, incapable of entering into any crystallisable or 

 stoichiometrically constituted combinations with other substances, 

 are not volatile at a certain degree of temperature, &c.,) and 

 cannot therefore be separated, or exhibited in a pure state. 

 Modern science has indeed made considerable advance, by learning 

 on the one hand to avoid as far as possible the formation of such 

 substances, and on the other, to separate some of them, and render 

 them more accessible to accurate chemical investigation. We will 

 here observe that substances such as albuminate of soda, Mulder's 

 binoxide and teroxide of protein, creatine, the inosates, &c., have 

 been reckoned among the extractive matters; and as many better 

 known substances (as urate of soda, hippurate of soda, and others) 

 are impeded in their crystallisation, and are enveloped or con- 

 cealed as it were by the extractive matters, they also have been 

 embraced under the same head, and have likewise been regarded in 

 the light of extractive matters, and have been calculated as such 

 in analyses. When we consider that the matters circulating in the 

 blood are, on physiological grounds, engaged in an almost constant 

 metamorphosis, we shall easily comprehend the difficulties that 

 beset the chemist in his attempt to seize them at any definite 

 stage of their metamorphosis, especially as they only circulate 

 through the blood in small quantities for the purpose of being 

 deposited in some tissue, or of being eliminated from the organism 

 by the organs of excretion. 



The extractive matters must, therefore, be likewise regarded as 

 important factors in the metamorphosis of animal tissue. In 

 accordance with the views of Berzelius, these bodies were con- 

 sidered for the most part as products of the metamorphosis of 

 tissues which, having become unfitted for further purposes, after ful- 

 filling their function, are elaborated in the blood in the better known 

 form of excrementitious matters. But to regard these substances 

 as of a purely excrementitious nature, was taking too circumscribed 

 a view of their importance. Since the blood contains the products 

 of the metamorphosis of the tissues no less than the elements 

 necessary for their formation, it is not only possible but probable 

 that plastic and useful matters, as well as the products of re- 

 gressive formation, may have been comprehended under the head 

 of extractive matters ; for, as we have already observed (p. 27,) the 

 idea of the progressive and regressive metamorphosis of matter 

 cannot be followed through an unbroken series of sequences. 



