the Formation of the Phosphate Nodules. 405 



These experiments having demonstrated the solubility of the 

 normal phosphate in water in the presence of decaying vegetable 

 matter, such conditions as are found in the bottom of swamps 

 and at the same time in soils containing vegetable mould, the 

 question presents itself as to how the solution is brought about, 

 that is whether it is owing to the conversion of the tri- into 

 the mono-calcium phosphate by the acid action of the mixture 

 (humus acids) or to the formation of certain absorption com- 

 pounds with the so-called humus acids as described by Bem- 

 mellen,* or to both. I think it likely that both processes enter 

 into the reaction, and it is intended to carry out further 

 investigation in the direction of solving this problem. 



On neutralizing the solution with ammonia, the phosphate 

 was partially precipitated, while neutralization with calcium 

 hydroxide caused a complete precipitation. 



To determine whether water then charged with phosphate 

 would give it up in coming in contact with marl, 1 allowed 

 specimen to stand in vessels over pure calcium carbonate in 

 some instances, and marl in others ; and in each case all trace 

 of phosphoric acid was removed in the course of twenty-four 

 to forty-eight hours. 



It is highly probable that carbonic acid plays an important 

 part in the formation of the phosphate nodules, so that it is 

 important to consider its action in this connection. 



Carbon dioxide passed into water containing normal calcium 

 phosphate dissolves it under the formation of mono-calcium 

 phosphate and acid calcium carbonate, so that the solution con- 

 tains both of these substances. 



This solution remains unaltered on standing, but if it be 

 boiled or allowed to stand over normal calcium carbonate or 

 marl, all of the phosphate will be precipitated, as was demon- 

 strated by a number of experiments, just as in the case of solu- 

 tion of phosphate by means of swamp water mixture. This 

 experiment shows that phosphates may be transported in hard 

 waters, but on standing on calcareous beds would tend to be 

 given up. 



From these experiments it certainly seems probable that 

 both carbonic acid and the humus substances in fresh water 

 swamps play an important part both in the accumulation and 

 the concentration of calcium phosphate, and the formation of 

 nodules in this way. The marls in the first place contain a 

 small percentage of phosphate, which percentage would be 

 gradually increased as the carbonate is gradually removed. 



Calcium phosphate, held in solution from whatever source as 

 the diacid salt, or in combination with the so-called humus 



* Landswirtschaf fcliclie Versuchs-Stationen, xxxv, p. 180. 



Am. Jour. Sci.— Third Series, Vol. XLIII, No. 251.— Mat, 1892. 

 27 



