738 JOHN JOHNSTON AND E. D. WILLIAMSON 



of CO3 directly, 1 the net result in either case being the precipita- 

 tion of an amount of CaC0 3 which could readily be calculated by 

 means of the equilibrium equations if the amount of C0 2 abstracted, 

 or of ammonia produced, were known. But by this we do not imply 

 that an organism cannot secrete calcium carbonate except from a solu- 

 tion already saturated with it. Nevertheless the possibility is open 

 that the effects just considered may sometimes be in reality examples 

 of the changes in conditions to be next considered — that the organism 

 may be merely the agency which localizes the process, the mechanism 

 which occasions the precipitation. It may even be that certain 

 bacteria are abundant where CaC0 3 is being precipitated because 

 there they can easily secure material — particularly C0 2 — needed for 

 their life-processes; on this basis they would be concomitants, 

 rather than causes, of the deposition of the carbonate. 



3 . Change of conditions. — The important physico-chemical factors 

 are temperature, and concentration of free C0 2 , of the water; in com- 

 parison with these two all other such factors are entirely subsidiary. 

 As an illustration of the magnitude of the effect producible by change 

 of these factors, a change in the proportion of C0 2 in the air in actual 

 contact with the solution from 3 . 3 down to 3 . o parts per 10,000 — a 

 change 2 which may occur at the present time — of itself decreases the 

 solubility from 65 to 63 parts per million, and so will cause the ulti- 

 mate precipitation of the corresponding quantity of CaC0 3 from a 

 solution already saturated with it. A similar amount will be 

 deposited 3 if the temperature of the saturated solution rises about 

 2 C, the proportion of C0 2 in the air remaining constant; under 

 these circumstances the concentration of free C0 2 in the water falls, 

 and its diminution is responsible for the larger part of the diminished 



1 The production of free ammonia causes an increase in the concentration of OH - 

 and therefore an increase in [CO^], since the quotient [OH~]V[CO^] is constant when 

 P is constant, in accordance with the equation already given. It may be noted that 

 the production by the decaying organisms of an ammonium salt such as NH 4 N0 3 or 

 (NH 4 ) 2 S0 4 would tend to increase the solubility of CaC0 3 and hence would not favor 

 its deposition. 



2 The range of variation in the course of geologic time has in all probability been 

 very much greater than this with correspondingly larger possible consequences. 



3 This of course implies that supersaturation does not take place; but in the sea 

 supersaturation is highly improbable by reason of the great abundance of appropriate 

 nuclei always present. 



