234 SOILS AND MANURES 



the slag. It does not react with nitrate of soda, but on 

 all grounds it is much better that it should not be mixed 

 with that or any other substance before it is applied to 

 the soil. 



Mechanical Condition of Basic Slag. The manurial 

 efficiency of basic slag depends very largely upon the 

 fineness. Samples which contain a comparatively small 

 proportion of phosphates will often produce a better 

 result than those which are richer in that respect, but 

 not so finely ground. The fineness cannot be judged 

 with any degree of accuracy by inspection ; it must be 

 tested with a sieve. Not less than 80 per cent, should 

 pass through a piece of wire cloth having 100 wires to 

 the inch, i.e., 10,000 holes to the square inch. 1 



Adulteration of Basic Slag. Basic slag is very rarely 

 adulterated with chalk or other non-phosphatic ingre- 

 dients. If the price be fixed as it generally is in pro- 

 portion of the amount of phosphate it contains, such 

 admixture would be a source of loss, not of gain, to the 

 seller. It has, however, occasionally been adulterated to 

 a considerable extent by admixture of finely ground 

 native phosphates. The presence of these substances is 

 easily detected by microscopic examination. Tricalcic 

 phosphate introduced into the converter along with the 

 raw slag previously to the operation by which Thomas' 

 phosphate is produced, cannot be detected in this way. 

 It is doubtful whether phosphates so introduced would 

 be regarded as adulterants. If not converted into basic 

 phosphates, they would remain comparatively insoluble 

 and of lower agricultural value. Purchasers of basic 

 slag would therefore be well advised to obtain a guar- 

 antee of the solubility of the phosphates. Many firms 

 now give a guarantee that 80 per cent, pf the total phos- 



1 The standard sieve for this purpose is that known as 100 E of 

 Amandus Kahl, which is of slightly larger mesh. 



