108 CHEMICAL COMPONENTS OP THE HUMAN BODY. 



heat, deprive even metallic oxides of their oxygen. Further, it is by the alka- 

 linity of the blood, that the metamorphosis of the malic, citric, tartaric, and 

 other organic acids used as food, is promoted ; and the same influence is exerted 

 even over uric acid, which, when introduced into the system from without, is 

 speedily resolved into urea and oxalic acid. If on the other hand, there be not 

 an adequate supply of alkali in the blood, some of the vegetable acids (such as 

 the gallic and tartaric) pass through it unchanged, and make their appearance 

 in the urine ; this being especially the case in carnivorous animals, whose blood 

 (according to Prof. Liebig) contains more of the alkaline phosphates, and less of 

 the carbonates, than that of herbivorous or omnivorous animals. 1 It was at one 

 time maintained by Prof. Liebig, that the presence of carbonate of soda in the 

 serum of the blood promotes the absorption of carbonic acid by the circulating 

 fluid, this being displaced by oxygen in the lungs. He has latterly urged, how- 

 ever, that this action must be very insignificant, in comparison with that which 

 is performed by phosphate of soda. 3 



84. The ordinary analyses of the blood and of other fluids of the living body 

 indicate the presence of a considerable amount of the Alkaline Phosphates, soda 

 being the predominating base ; and much ingenious speculation has been put 

 forth concerning their special uses in promoting the metamorphoses of tissue, 

 arising out of the remarkable variety in the combining proportions of phosphoric 

 acid and its bases. But it has been rendered doubtful, to say the least, by the 

 recent analyses of Rose, whether the phosphates do exist as such in the blood, 

 c., and whether they are not rather formed during the incinerating process, by 

 the oxidation of phosphorus, and the combination of the phosphoric acid thus 

 formed with the alkaline bases previously combined with carbonic or with or- 

 ganic acids. Moreover, as it appears from the researches of Prof. Liebig just 

 cited, that the relative amount of the alkaline phosphates and carbonates in the 

 blood of different animals, is subject to great variation in accordance with the 

 nature of their food (the former being in largest proportion in the blood of car- 

 nivorous, and the latter in that of purely herbivorous animals), it may be sus- 

 pected that substances, whose quantity seems to be so much a matter of indif- 

 ference, must either be of secondary importance in the vital economy, or must 

 be in some degree vicarious with each other. This last idea is perhaps the 

 nearest to the truth ; for, according to Prof. Liebig, the alkalinity of the blood 

 in carnivorous animals is due much rather to the presence of the basic phosphate 

 of soda than to that of the carbonate. Moreover, the remarkable power which 

 the serum of the blood possesses for the absorption of carbonic acid (nearly twice 

 its volume of that gas being taken up by it at the ordinary temperature, which 

 is double the amount which water will absorb under the same circumstances), 

 is mainly due to the presence of phosphate of soda ; a solution of 1 part of 

 which in 100 parts of water is found to take up twice as much carbonic acid as 

 an equal bulk of water will absorb at the same temperature, two-thirds of this 

 being readily yielded up when the liquid is agitated with air, or the atmospheric 

 pressure is diminished. This property is not possessed by a solution of phos- 

 phate of potash j and the constant presence of a certain amount of phosphate of 

 soda in the blood, even when none exists in the food, which is very significant 

 of its importance in the economy, is explained by Prof. Liebig by the fact, that 

 when phosphate of potash is brought in contact with chloride of sodium, a double 

 decomposition takes place, of which phosphate of soda is one of the products, 

 this remaining in the blood, whilst the potash-salt is appropriated by the muscu- 

 lar substance ( 85). s The alkaline phosphates find their way very readily into 



1 See Prof. Liebig's "Familiar Letters on Chemistry," letter xxviii. ; and his "Re- 

 searches on the Chemistry of Food," pp. 93, et seq. 



See his " Researches on the Chemistry of Food," pp. 113-6. 

 Ibid, pp. 104-118. 



