CHEMICAL FERTILIZERS 



the riddles it is dusted over with a finely powdered rock 

 phosphate. The object of this riddling and dusting is to 



obtain the material in a fine, 

 dry condition, suitable for 

 sowing. 



The advantage of dusting 

 a very small quantity, perhaps 

 only | %, of rock phosphate 

 is partly chemical, but very 

 largely a purely physical 

 one. In Fig. 8 is shown the 

 manner in which large damp 

 particles are prevented from 

 uniting by the interposition 

 of small dry particles equal to 

 one-half of a per cent, of the 

 total mass. This action can 

 be seen very well from the 

 way in which dusty globules 

 of mercury do not coalesce. 

 As the particles do not roll 

 on one another without limit 

 seeking to coalesce, it is only 

 necessary to reduce the chance 

 of coalescing. Hence a small 

 percentage will serve the pur- 

 pose. On the other hand, the 

 dust grains are not evenly dis- 

 tributed, and therefore many 

 are wasted. Hence more dust 

 is needed. The two errors tend 

 to balance one another. Moreover, many of the particles of 

 dust may be nearer one-thousandth of an inch in diameter. 

 Hence, in practice, small proportions of dust are effective, if 

 skilfully used. There is, however, great difficulty in anything 

 approaching even distribution. Very minute particles of a 

 substance not acted on by the liquid phase of superphosphate 

 would be almost equally effective, but are not used in practice. 



FIG. 8. Section of dusted 

 particles. 



SS are two particles of sticky super- 

 phosphate, very highly mag- 

 nified. 



DD are the dust particles which 

 adhere to the sticky surface, 

 but prevent the sticky super- 

 phosphate particles from co- 

 alescing. The diagram is 

 drawn to a scale of about % 

 of dust particles, assuming 

 them to be evenly distributed. 



