524 Causes of the Presence of Phosphates 



they were washed out of their matrix in the London clay, from 

 the dejections and decaying bodies of fishes and molluscs and 

 worms^ that inhabited the tertiary sea, at whose bottom they 

 were for a long time rolling, while the gravel of the crag was in 

 process of formation. 



The bodies of the molluscous creatures that inhabited the very 

 shells of which the crag is composed, must have given out phos- 

 phoric compounds during their decay, which may, by absorption, 

 have supplied additional phosphoric matter to that which these 

 concretions brought with them from their matrix in the London 

 clay. Experiments are wanting to show whether concretions of 

 other fragments of marlstone, and of marl, and chalk, and soft 

 limestone, bathed in sewage water, mixed with salt enough to 

 equal that in sea-water, and with peroxide of iron, and burnt clay, 

 and at a temperature approaching the probable warmth of the 

 shallow sea-water in which the crag was formed, may absorb 

 and form similar phosphoric compounds from the ingredients of 

 sewage. I earnestly commend such experiments to the care of 

 the many accomplished chemists who are now directing their 

 attention to sanitary and agricultural improvements. 



There seems to be little doubt as to the power of the chemical 

 agents I have spoken of, the chief difficulty lies in the time 

 required to effect the desired combinations. Experiments on this 

 subject are at the present time of pressing importance, with a 

 view to the grand desideratum of turning to a profitable use the 

 noxious contents of our sewers. The great difficulty seems to lie 

 in discovering a method of expediting the processes of com- 

 bination,* 



In many of the red marl districts, from Devonshire to Durham, 

 through the entire centre of England, it is the practice to lay 

 quicklime in long heaps parallel to the hedges of fields under 

 preparation for wheat, and to mix lime with the vegetable rubbish 

 and rough soil from the ditch and foreland margin of the field ; 

 these are left together many days before the compound is laid on 

 the furrows, and ploughed into the soil before the wheat is sown. 



* The stronger affinity of lime for carbonic than for phosphoric acid may cause a 

 decomposition and conversion of carbonate of lime, or of marl, or marlstone, or chalk, 

 into phospliate of lime by a substitution of phosphoric for carbonic acid; and a similar 

 effect might follow if slightly baked or sun-dried clay or marl, or powdered chalk, be 

 submerged in sewage. The result being (as Dr. Lyon Playfair has suggested) a kind 

 of exchange and pseudomorphic conversion of carbonate to phosphate of lime. It is 

 probable that by a similar process the phosphorites were formed in all the strata of the 

 crag. But the formation of these strata occupied longer periods of time than the 

 chemists have at their disposal, and the great desideratum to which I would call 

 attention, is the discovery of a cheap and easy method of accelerating this process. I 

 repeat what I have before stated, that the addition of carbonic acid to sewage, and of 

 protoxide of iron and salt, and moderate heat, may induce conditions, approaching to 

 those under which analogous compounds were formed from putrescent animal and 

 vegetable matter in ancient deposits, both under salt and fresh water, throughout all 

 ^- '' fical time. 



