1888-89.] Mr Irvine & Dr Woodhead on Carbonate of Lime. 339 
rather larger proportion of the phosphoric acid will be replaced by 
the carbonic acid in the lime salts. 
Additional evidence may be drawn from irregular formation and 
diseased conditions of bone. In exostosis, and in the new bone of 
a callus, there is always a larger proportion of the carbonate. One 
point, however, should be borne in mind ; after the process of bone 
formation is completed, or when it is interfered with in any way, 
there will always be a slightly greater proportion of carbonate, 
because in the process of absorption the phosphate once formed 
is slightly more soluble, and is therefore more readily removed. 
Phosphoric acid is present in larger proportion in the tissues 
and fluids in old people, where the tissue changes and the forma- 
tion of carbonic acid are not going on so rapidly. Once laid 
down in bone the carbonates are more permanent than the phos- 
phates, i.e ., it is more difficult to remove them. This is most notice- 
able in the bones of old people, where the proportion of carbonate to 
phosphate of lime is always slightly higher than in adults. 
Liebig laid great emphasis* on the fact that the carbonates of 
the alkaline earths are soluble in water containing free carbonic 
acid, whilst the earthy phosphates are also soluble in water con- 
taining free phosphoric acid. As a matter of fact, both sets 
of salts are soluble in solution of either acid. As we have 
already shown, if carbonic acid be passed into a solution in which 
phosphate of lime and an alkaline carbonate are present, as in the 
blood, there is immediately a deposition of that salt, partly as a 
pure phosphate of lime but partly also as a salt of lime in which 
carbonic acid has taken the place of one, or perhaps even more, of 
the phosphoric acid molecules. It does not matter then, whether 
the lime in the blood is combined in solution with phosphoric acid 
as in the Carnivora, or partly with carbonic acid as in the 
Herbivora, the addition of carbonic acid from the tissue and in the 
nascent condition will cause a deposition of the phosphate of lime 
and of double phosphate and carbonate, as the carbonic acid gradually 
and partially takes the place of the phosphoric acid. It might be 
urged, that because there is more carbonic acid in the blood of 
herbivorous animals, we should therefore find less or greater deposi- 
tion of the carbonate or phosphate of lime on the evolution and 
* Letters on Chemistry , p. 414.. 
