28G THOMAS-SLAG OR BASIC CINDER. 



to refer to iu\estigation.s niatle by cliemists to discover what is 

 the nature and composition of the substance. 



It was found by Hilgenstock that the phosphate of lime con- 

 tained in the slag differed from ordinary phosphate of lime, such 

 as is found in natural phosphates and in bone ash. Ordinary 

 phosphate of lime is known as tribasic phosphate of lime or 

 tricalcic phosphate, in which three equivalents of lime are 

 imited to one equivalent of phosphoric acid, and whose com- 

 position is thus expressed — 



CaO^ 56^ 



CaO VPoO,= 56 Vl42, 

 CaOj ' ' 56j 



viz., 168 parts by weight of lime, united to 142 parts by weight 

 of phosphoric acid ; but the slag phosphate was found to have 

 four equivalents of lime united with one of phosphoric acid. Its 

 composition woidd therefore be expressed thus — 



OaO 

 CaO 

 CaO 

 CaO J 



56- 

 PA= II 1^142, 

 56. 



viz., 224 parts of lime to 142 parts of phosphoric acid. It is 

 therefore called tetrabasie phosphate of lime. 



This is an entirely new substance, and is one of a number of 

 new substances that have been found in the slag, but which 

 might not have been discovered had the steel companies not 

 adopted the plan of pouring the slag into boxes instead of pour- 

 ing it on the ground, as was the practice at first. When the 

 slag is allowed to cool in the boxes there is a hollow part in the 

 centre, in which these new bodies are found crystallised in well- 

 marked forms. The tetrabasie phosphate is found in well- 

 defined flat square crystals. It is a very interesting substance, 

 from a chemist's point of view, but there is one characteristic it 

 possesses that is especially interesting to agriculturists. It is a 

 phosphate which may be said to be supersaturated with lime, 

 and as a result of this supersaturation the lime and phosphoric 

 acid are in a somewhat feeble state of combination, so that it is 

 capable of being decomposed by the carbonic acid of the soil, 

 and by the acids contained in the roots of plants. Therefore, 

 when it is applied as a manui'e to crops, they are able to extract 

 the phosphoric acid from it. 



The amount of phosphoric acid contained in the slag varies 

 a good deal, for it depends upon the amount of phosphorus con- 

 tained in the pig-iron. 



The following shows the composition of two slags, — No. 1, 

 made from iron containing If per cent, of phosphorus, and No. 

 2, from a sample of iron containing 2^ per cent, of phosphorus.* 



* Stead ,111(1 Readsilale, Journal of Iron and Steel Institute, 1887. 



