IX] OF SOLUBILITY IN COLLOID MEDIA 435 



sulphate is only very little more soluble in presence of albumin 

 than in pure water, and the rarity of its occurrence within the 

 organism is so far accounted for. On the other hand, the bodies 

 derived from the breaking down of the albumins, their "catabolic" 

 products, such as the peptones, etc., dissolve the calcium salts to 

 a much less degree than albumin itself; and in the case of the 

 phosphate, its solubility in them is scarcely greater than in water. 

 The probabihty is, therefore, that the actual precipitation of the 

 calcium salts is not due to the direct action of carbonic acid, etc. 

 on a more soluble" salt (as was at one time believed) ; but to cata- 

 bolic changes in the proteids of the organism, which tend to throw 

 down the salts already formed, which had remained hitherto in 

 albuminous solution. The very slight solubility of calcium phos- 

 phate under such circumstances accounts for its predominance 

 in, for instance, mammalian bone*; and wherever, in short, the 

 supply of this salt has been available to the organism. 



To sum up, we see that, whether from food or from sea-water, 

 calcium sulphate mil tend to pass but little into solution in the 

 albuminoid substances of the body : calcium carbonate will enter 

 more freely, but a considerable part of it will tend to remain in 

 solution : while calcium phosphate will pass into solution in 

 considerable amount, but will be almost wholly precipitated 

 again, as the albumin becomes broken down in the normal process 

 of metabolism. 



We have still to wait for a similar and equally illuminating 

 study of the solution and precipitation of silica, in presence of 

 organic colloids. 



From the comparatively small group of inorganic formations 

 which, arising within living organisms, owe their form solely to 

 precipitation or to crystallisation, that is to say to chemical or other 

 molecular forces, we shall presently pass to that other and larger 

 group which appear to be conformed in direct relation to the forms 

 and the arrangement of the cells or other protoplasmic elements |. 



* Which, ill 1000 parts of ash, contains about 840 parts of phosphate and 

 76 parts of calcium carbonate. 



T Cf. Dreyer, Fr., Die Principien der Geriistbildung bei Rhizopoden, Spongien 

 nnd Echinodermen, Jen. Zeitschr. xxvi, pp. 204-468, 1892. 



28—2 



