194 Allen, Crenshaw, Johnston, and Lai' sen — 



it contain any disulphide of iron because this is insoluble in 

 hydrochloric acid, while only sulphur remains after pyrrhotite 

 has been boiled with this reagent. Judging from the proper- 

 ties ascribed to Fe 2 S 3 , he concluded that this also could not be 

 present. Habermehl was thus forced to conclude, like Lind- 

 strom, though on the basis of further evidence, that pyrrhotite 

 was variable in composition, and it may here be said that no 

 evidence of later date has ever disproved this conclusion. At 

 that time a homogeneous solid of variable composition was an 

 anomaly. To-day such substances are quite generally recog- 

 nized under the category of solid solutions. 



Hypothesis of solid solution. — We proposed to test the 

 hypothesis of solid solution by preparing a series of synthetic 

 pyrrhotites and measuring some property of them. Pyrrhotite 

 was prepared by Berzelius, Kammelsberg and others, generally 

 by heating pyrite. One can also begin with marcasite, which, 

 as we have seen, is first changed to pyrite between 450° and 

 600°, or with sulphur and iron. We have tried all three 

 methods, though most of the work has been done with pyrite 

 from Elba. This very pure mineral, an analysis of which was 

 given on p. 177, Part I, was kept in a vacuum desiccator, 

 from which portions were taken from time to time as required. 



Apparatus. — The apparatus used in the synthesis of pyrrho- 

 tite appears in fig. 6. The crucible, C, containing the pyrite 

 is of unglazed porcelain, 48 mm highx37 mm outside diameter at 

 the top and 22 rmn at the bottom. It has a doubly perforated 

 graphite cover, E, through the central orifice of which passed 

 the glazed Marquardt jacket, A, which shields the thermo- 

 element. Through the second orifice passes a similar tube, B, 

 open at the lower end, which is traversed by a current of 

 hydrogen sulphide.* 



The crucible is inclosed in a large porcelain tube, D, 40— 45 mm 

 inside and 50 mm outside diameter, and 50 cm in length. In some 

 of the experiments the crucible was supported in the hot zone 

 by a strong graphite rod, G, which was fastened to the cover, 

 E, and clamped outside the large tube, D, while the cover itself 

 was fastened to the crucible by three small pegs. By means of 

 this device, the crucible could be quickly lowered at any time 

 to the bottom of D and thus rapidly cooled. In other experi- 

 ments, a much shorter porcelain tube was substituted for D, 

 in which instances the crucible was supported by a fire-clay 

 pedestal which rested on the bottom of the tube. The upper 



* Since the ferrous sulphide from which hydrogen sulphide is generated, 

 contains free iron, the gas was passed through boiling sulphur to remove 

 hydrogen, before reaching the furnace. 



