4S5 SECOND CLASS OF MINERAL CONSTITUENTS. 



After this general view of the occurrence and uses of salt in the 

 animal economy, it is hardly requisite to allude to the sources 

 from which the animal body receives its due supply. Chloride of 

 sodium is so generally distributed throughout nature, that this 

 necessary quantity is conveyed into the organism with the ordinary 

 food and with the water. 



The habits of civilized life have elevated salt to the rank of a 

 positive necessary, but we must by no means conclude from this 

 circumstance that the salt contained in ordinary food is not suffi- 

 cient for the support of the animal functions. A simple comparison 

 of the quantity of salt contained in the animal body, with that 

 which we are daily taking with the food, at once shows that we 

 use more salt than is requisite : and if, on the one hand, as several 

 travellers narrate, certain negro tribes in the interior of Africa 

 exchange gold-dust for an equal weight of salt, and in want of it 

 have recourse to the most disgusting substitutes ; we know, on the 

 other hand, that whole races in the South Sea Islands, and in 

 South America, flourish without even the knowledge of this sub- 

 stance. Further, as Liebig has shown, tempests carry salt from 

 the ocean far into the interior, and thus supply the spring water 

 with it. A glance at the results of the analyses of the ashes of 

 plants, is sufficient to show that the ordinary articles of vegetable 

 food are perfectly sufficient to supply the necessary quantity of salt 

 to the animal body. 



CARBONATE OF SODA. 



This salt not unfrequently occurs in the ash of burned animal 

 matters, but in most cases it is merely the product of the com- 

 bustion of combinations of soda with organic acids or protein- 

 compounds. Investigations deserving of the greatest confidence 

 prove however that carbonate of soda, together with other soda- 

 compounds, exists in the blood and in the lymph. It is also con- 

 tained, together with large quantities of the carbonate of potash 

 and lime, in the urine of herbivorous animals. 



The earlier observers assumed the presence of carbonate of 

 soda in the blood as a recognised fact ; and indeed it was believed to 

 take an active part in the excretion of carbonic acid ; but certain 

 later investigations seemed to leave it very doubtful whether 



