Correspondence—Edward Greenly. 379 
and present the appearance of crystals rich in faces. Mr. Barker has 
found that they do not lie in zones or obey the laws of distribution of 
ordinary crystal faces, and cannot therefore be regarded as the faces of 
a single crystal. There is, however, no evidence, from etching by 
acid, that the bead is an aggregate of crystals. The nature of these 
remarkable faces is difficult to understand. A bead of platinum 
presenting the same peculiarities was measured by the late Professor 
Miller. — Chlormanganokalite, by Dr. H. J. Johnston- Layis and 
Mr. L. J. Spencer. A preliminary account of this new Vesuvian 
mineral was given by Dr. Johnston-Lavis in Watwre on May 31, 1906. 
A new analysis of the mineral gives the formula MnCl,, 4 KCl. 
The crystals are rhombohedral, with a rhombohedral angle of 
57° 36’; they are optically uniaxial with very weak positive 
birefringence; the refractive index is 1°59 and the specific gravity 
2°31.—Mr. L. J. Spencer exhibited a suite of beautifully crystallized 
minerals, presented to the British Museum by Mr. Percy C. Tarbutt, 
from the Rhodesia Broken Hill mines in North-Western Rhodesia. 
In driving a tunnel through one of the kopjes, which consist mainly 
of cerussite and hemimorphite, a cavern containing flint implements 
and bones of recent mammals was encountered, and a cavity in the 
bone-breccia on the floor of this cave was encrusted with magnificent 
groups of hopeite crystals (the rare hydrous zinc phosphate discovered 
by Sir David Brewster in 1823). In the vicinity of the cave, crystals 
of another hydrous zinc phosphate were found in association with 
descloizite (hydrous vanadate of lead and zinc). The crystals of this 
new species, for which the name tarbuttite is proposed, are anorthic ; 
they possess a perfect cleavage in one direction, through which 
emerges obliquely the acute negative bisectrix of the optic axes. 
Cavities in the ordinary ore are lined with large twinned crystals of 
water-clear cerussite, which are encrusted with small crystals of 
hemimorphite.—A group of quartz-crystals from British Guiana was 
exhibited by Mr. Anderson and a fine crystal of apatite by Mr. Gordon. 
CORRESPON DEHN CE. 
CHEMICAL REACTIONS BETWEEN SOLIDS. 
Srr,—The attention of geologists should be drawn, I think, to some 
recent experiments by Mr. E. P. Perman, briefly described in Nature 
for June 20th last, in the abstracts of the Proceedings of the Royal 
Society ; and of which further details will no doubt soon be available. 
The author finds that reactions take place between many pairs of salts 
in the solid state, but he also finds that careful drying prevents such 
reactions. From this he concludes that the reactions are carried on by 
means of thin films of water, acting as ionizing solvents. Many salts 
react on merely shaking their powders together; but all the changes 
are greatly aided and accelerated by heat and by pressure. 
The importance of these experiments for geologists will be obvious 
enough, and needs no insisting upon, particularly in connection with 
metamorphic problems. Indeed, geologists might claim a sort of 
priority, for Mr. Perman’s ‘“‘ Heat, Pressure, and Water’ have been 
allies of ours for many years past. Not that this detracts from the 
