184 



NATURE 



[April 8, 1920 



would be no falling off in total yield, in spite of the 

 diminished area. 



Mr. Bainbridge gave an account of the experiments 

 by Dr. Stead and Mr. Jackson on the solubility of 

 basic slag in citric and carbonic acids. The reaeon 

 why fluorspar makes the phosphoric acid in slag 

 insoluble is that a reaction occurs between fluoride 

 and phosphate, producing an artificial apatite, which, 

 as regards insolubility, resembles natural fluorapatite. 

 Even the most soluble phosphatic slags undergo this 

 change and become insoluble on melting with fluor- 

 spar. Carbonic acid, after long-continued attack, 

 generallv dissolves out more phosphoric acid than a 

 single attack by the standard citric acid. 



Mr. G. Scott Robertson gave details of the field tests 

 made to compare the effect of various types of open- 

 hearth basic slags on g^rassland. These experiments 

 were made in Essex on London clay, Boulder clay, and I 

 chalk. They show that all the phosphatic slags are 

 effective fertilisers; but there are important differences | 

 in the agricultural effects, which are not connected j 

 with solubility according to the citric acid test ; indeed, i 

 this test affords no indication of the fertilising value I 

 of open-hearth slags. Details of the botanical exam- 

 ination of the plots showed the striking effect of the , 

 basic slags in reducing the amount of bare space and 

 in increasing the amount of clover. ! 



Mr. Daniel Sillars made an important contribution ^ 

 from the metallurgical side, discussing the formation { 

 of basic slag in the manufacture of steel. The phos- j 

 phide of iron, FejP, in which state of combination \ 

 phosphorus exists in molten iron, is oxidised by 

 reactions of the type — 



5Fe30, + 2P=i5FeO + P*05 ! 



5Fe30, + 8P = i5Fe + 4P,03. 



The P2O5 formed may combine with FeO to form ' 



Fe3(P04)2, which, however, is unstable in the pres- , 



ence of a large excess of iron, and a reaction such as j 

 Fe3(PO.,)3+iiFe = 8FeO + 2Fe3P results, and it is in 



consequence of this reaction that the acid process of \ 



steel-making is unable to remove phosphorus. In the | 



basic process the presence of lime affords an oppor- i 



tunity to the phosphoric acid to fcrm a stable body by ' 



the reaction — ! 



Fe3(PO,)2 + 4CaO = Ca.P.O, + 3FeO. 



The calcium phosphate formed is only feebly \ 

 attacked and decomposed by the metallic iron, but 

 manganese and carbon attack it more vigorously and 

 cause the phosphoric acid to be reduced and the metal 

 to be re-phosphorised. These reactions are, of course, 

 proceeding concurrently, and it is necessary to main- 

 tain a certain concentration of ferrous oxide in the 

 slag to minimise, so far as possible, the tendency to 

 re-phosphorisation. Re-phosphorisation is probably 

 due to the reaction between ferrous phosphate and 

 lime being slightly reversible, whereby a small con- 

 centration of ferrous phosphate is always present, 

 which is reduced by the carbon unless a source of 

 oxygen is supplied by ferrous oxide in the slag. 



In ordinary practice the open-hearth process is 

 carried out by allowing the slag formed by the oxida- 

 tion of the silicon, phosphorus, and manganese to flow 

 over shutes made in the fore-plates into slag-pots 

 under the furnaces, and no attempt is usually made 

 to remove more slag than that which flows out natur- 

 ally when the level of the slag in the furnace is higher 

 than the level of the fore-plate. The slag left behind 

 is carried on, and forms part of the finishing- slag, 

 which latter is therefore much greater in volume, and 

 therefore lower in phosphoric acid, than the slag 



NO. 2632, VOL. 105] 



removed in the middle of the process. By this method 

 of operation the time spent in tapping the furnace for 

 separation of the slag and for the formation of a new 

 slag is saved, but the slag is inferior both in richness 

 and in citrate solubility if that still forms a criterion 

 of excellence to the agriculturist. 



Mr. Ridsdale took part in the discussion, and 

 exhibited specimens of slags examined in the classic 

 investigations by Stead and Ridsdale ; and Mr. W. S. 

 Jones contributed a paper on the improvement of 

 low-grade basic slags. 



As a result of the discussion it was decided to ask 

 the Ministry of Agriculture to form a Committee 

 which should study possible practical steps to effect 

 improvement in quality and in quantity of the phos- 

 phatic slags. 



Verification of Screw Gauges for Munitions 

 of War. 



THE Bulletin de la SociiU d' Encouragement pour 

 I'lndustrie Nationale (November-December, 1919, 

 No. 6) contains an article by M. Cellerier, of the ( 

 Conservatoire des Arts et Metiers, on the verification 

 of screw gauges, with particular reference to the 

 methods advocated by Mr. Bingham Powell, who was 

 engaged in the United States during the war as 

 Inspector of Gauges and Standards for the British 

 Ministry of Munitions. These methods related chiefly 

 to the measurement of the full, effective, and core 

 diameters ; the verification of pitch was neglected 

 until quite a late period of the war, owing to the 

 lack of instruments possessing the requisite precision 

 and rapidity. 



Extreme accuracy is of the highest importance in 

 measurements of pitch, as any error in the pitch 

 makes it necessary for the maximum limit of effec- 

 tive diameter to be reduced by double the amount 

 of this error if the gauge is to be accepted as correct. 

 Where the permissible deviations are very small, 

 an error in pitch of a few ten-thousandths of an inch 

 may thus completely annihilate the tolerance on effec- 

 tive diameter. Inaccuracies of pitch are often 

 regarded as essentially progressive ; but this is not 

 always the case, as deformations due to hardening 

 may introduce variable errors of quite appreciable 

 mas^nitude. The method frequently adopted of 

 verifving the pitch by measurements made on a 

 length comprising a number of threads is accordingly 

 much less trustworthv than the practice, long in 

 vogue in France, of testing separately a number of 

 consecutive threads. 



For the latter orocess measuring machines of the 

 pattern used at the National Phvsical Laboratory are 

 narticularlv suitable, but at the time when the demand 

 for extreme accuracv in screw gau£/es for war-work 

 first became pressing it was impossible to obtain one 

 of these machines in America without considerable 

 delav, and accordinglv Mr. Powell found it necessarv 

 to devise an instrument on the spot. He dispensed 

 with the optical contrivance which forms an essential 

 feature of the laboratory machine, and substituted for 

 the spherical contacts a lever terminating in a small 

 sphere which rests freely in the screw and can be 

 guided convenientlv in the axial plane from one thread 

 to another. The lever consists of a very light needle, 

 arranged in such a way that the apparatus can also 

 be used for testing- internal screws or nuts by means 

 of appropriate casts taken bv an ing^ious and 

 delicate method, but only a small^ segment of the 

 internal thread can be obtained in this way for testing 

 purposes. 



