224 SOILS AND MANUBES 



cent, of soluble phosphates and 4 per cent, of potash. 

 It is convenient for certain purposes, especially when only 

 small quantities are required. Equivalent quantities ' of 

 superphosphate and sulphate of potash applied separately, 

 give equally good results, and if they can be obtained 

 at cheaper rates, are more economical. 



The quality of a sample of superphosphate cannot 

 be judged by the colour, smell or other outward pro- 

 perties. All that can be done by simple inspection is to 

 determine whether it is in good sowable condition. 

 Beyond this, the sole criterion of quality is the proportion 

 of soluble phosphates it contains, and that can only be 

 determined by chemical analysis. 



BASIC SLAG.. 



Basic slag, basic cinder, Thomas' phosphate meal or 

 powder, for it is known by all these names, is a substance 

 of entirely different origin and character. It is a bye- 

 product from steel works, and has been known only for a 

 comparatively short time. The process by which it is 

 produced is of a highly technical character, but the main 

 outlines are easily followed. 



In the well-known Bessemer process for the manu- 

 facture of steel, the crude product from the blast fur- 

 naces, called " pig iron," is melted, and air is blown 

 through the molten mass in order to oxidise the carbon. 

 The operation is performed in a specially constructed 

 apparatus known as the " converter " a pear-shaped iron 

 vessel lined with infusible fireclay. As originally devised, 

 the process could not be applied to samples that con- 

 tained much phosphorus because the oxide of that ele- 

 ment, being non- volatile, does not escape like that of 

 carbon, and its presence affects the properties of the steel 

 in such a way as to make it useless for many purposes 



