608 SUMMARY OF CURRENT RESEARCHES RELATING TO 



mercury arc in vacuum lamp gives excellent results. The form used 

 by the author is shown in fig. 132. The upper vacuum space is very 

 large, so as to increase the radiating surface, and therefore decrease the 

 heating of the lamp. The central tube is covered with a refractory 

 coating which greatly lengthens the life of the lamp. With a projection 

 eye-piece an exposure of from one to two minutes is required. This 

 lamp should preferably be worked by a continuous current. The differ- 

 ence of electro-motive force between the two poles varies from 15 to 25 

 volts as the lamp gets heated. Three amperes is the best current 

 intensity for normal work. 



Fig. 133, plate IX., is an interesting example of abnormal structure 

 in grey cast iron, etched with tincture of iodine. It was photographed 

 by the above apparatus, and the magnification is 660 diameters. 



Iron and Phosphorus.* — J. E. Stead, after an extensive examination 

 of samples of phosphorisel iron, finds that they may be conveniently 

 divided into 4 classes, according to the percentage amount of phosphorus. 

 (1) From to 1-70 p.c. (2) From 1-70 to 10-2 p.c. (3) From 10-2 

 to 15-58 p.c. (1) From 15-58 to 21-68 p.c. Alloys containing much 

 above 21 p.c. of phosphorus have not been investigated, as they are of 

 little metallurgical value. The selected photomicrographs will give an 

 idea of the micro-structure. 



Fig. 134, plate IX., is the type of all the metals in the first class. 

 The crystalline grains are polygonal, and are solid solutions of Fe 3 P 

 in iron. The grains, under like conditions of heating and cooling, 

 increase in size with the increase of phosphorus ( X 50). 



Fig. 135 contained about 8 p.c. of phosphorus. The broad light 

 parts are crystallites of the metal containing about 1 - 7 p.c. phosphorus. 

 The white mottled ground mass is the eutectic containing 10*2 p.c. 

 phosphorus ( x 250). 



Fig. 136, plate X., contained 10*2 p.c. phosphorus, and is the 

 eutectic of phosphorus and iron. It has only one critical point at about 

 950° C. ( X 350). 



Fig. 137 contained 11-07 p.c. phosphorus, and shows sections of rhom- 

 bic or oblique idiomorphic crystals of Fe 3 P imbedded in a ground mass 

 of the eutectic ( X 60). 



Mr. Stead's second paper is wholly based upon micro-structure, and 

 is illustrated by a series of important photomicrographs. He summarises 

 his results under eleven heads, which are briefly : — 



(1) That on melting saturated solid solutions of iron phosphide in 

 iron with carbon, the latter causes a separation of the jmosphide near 

 to the point of solidification, which appears in the solid metal as a 

 eutectic in irregular-shaped areas. A residuum always remains in solid 

 solution. 



(2) That the residuum appears to be retained in the pearlite. 



(3) That a portion of the iron phosphide in steels containing less 

 than 0-10 p.c. of phosphorus is thrown out of solution by carbon when 

 it exceeds 0*9 p.c, and the phosphide so separated is liable to form a 

 brittle cell-structure enveloping the grain, yielding a more or less 

 fragile mass. 



* Metallographist, 1901, pp. 89-111, 199-28(3 (27 figs.). 



