62 DISCOVERY REPORTS 



Museum collection. He identified the calcium oxalate crystals with the much smaller 

 "envelope" crystals found in human urine. " Sometimes these crystals are opaque and 

 the octahedron is remarkably flattened : the calculus then looks as if studded with pearl- 

 spar." His careful study shows that the calcium oxalate may be intergrown with uric 

 acid, sodium urate, ammonium urate, magnesium ammonium phosphate, calcium phos- 

 phate or calcium carbonate. Golding Bird does not give, however, the dimensions of the 

 renal crystals of calcium oxalate. The crystals of both the calculi I have examined are 

 roughly square in outline, measuring 2-3 mm. across, but the pyramidal faces are curved 

 and the thickness of the crystals varies considerably owing to subparallel growth and 

 possibly twinning. Only small transparent wedge-shaped fragments suitable for optical 

 and X-ray work could be detached from the edges. These give a positive uniaxial picture 

 and yield approximate refractive indices w 1-523, e 1-544 (Becke method). The broken 

 fragments exhibit no well-marked cleavage directions; their hardness is about 4, 

 Mohs' scale. Light reflections from the pyramid faces show that their curvature is due 

 to the presence of a large number of vicinal faces between (loi) and (001). A few crystal 

 fragments give values cr varying from 30° i' to 30° 56', but natural faces are too imper- 

 fect for refractive index measurements by the prism method and the fragments are too 

 small to be ground and polished. Gypsum needles crystallized from a solution of a 

 crystal fragment in sulphuric acid, and the residual liquid also decolorized a drop of 

 potassium permanganate solution. These preliminary measurements and chemical tests 

 therefore suggested the identity of the deep-sea crystals and the crystals from the renal 

 calculi. 



X-ray rotation photographs of some crystal fragments from both the renal calculi were 

 then taken about the supposed [100] axis, i.e. about an edge of a square plate. All the 

 spots of each photograph correspond exactly in position and intensity with those of 

 a similar photograph of a deep-sea crystal. These photographs constitute the most 

 reliable test of the identity of the two compounds since they do not depend upon the 

 perfection of crystal form but only upon the atomic arrangement within a crystal. A 

 chemical analysis of carefully selected fragments from the renal calculi should there- 

 fore reveal the chemical composition of the deep-sea crystals. 



The specific gravities of crystal fragments from both calculi were separately de- 

 termined by balancing in mixtures of bromoform and benzene. Values varying from 

 1-98 to 2-00 were obtained, but no systematic difference could be detected between the 

 specific gravities of the white and pale-brown crystals. The first chemical analysis on 

 white crystals, sp. gr. 1-99, from C. 90, gave CaO 37-3 per cent (by ignition), C2O3 46-1 

 per cent (by titration with potassium permanganate solution), HgO 16-7 per cent (loss of 

 weight at 270° C), total 100- 1 per cent, which corresponds to a chemical formula 

 CaC204 . 1 IHaO and was the first indication that the usually accepted trihydrate formula 

 might be incorrect. The formula relating the specific gravity J, atomic contents nM and 

 unit-cell dimensions of a tetragonal compound is «M x i-648/«^c= d. Since X-ray 

 rotation photographs of the deep-sea crystals and renal calculi crystals are identical the 

 values a 12-40, c 7-37 A. may be inserted in the above formula. The value of M corre- 

 sponding to CaCgO^.iiHaO is 155, the observed specific gravity is 1-99, so that 



