24 T. STERRY HUNT ON THE GENETIC 
be formed by aqueous action. Chrysolite also, is produced by igneous fusion, but its 
presence in crystalline limestone in the form of forsterite, and in massive magnetite as 
hortonolite, shows that, like the related and similarly associated chondrodite, it may be 
formed in the presence of water. ! 
§ 41. The evidences of the intervention of water in eruptive rocks have, since the 
time of Scrope, been too often pointed out to need repetition here. Its elements may even 
be retained in fused compounds at the temperature of ignition, under the ordinary atmos- 
pheric pressure, as seen not only in the hydrate and the acid sulphate of potassium, but 
in certain vitreous borates of sodium and potassium, long -since described by Laurent, 
which at a red heat and in tranquil fusion hold an amount of hydrogen equal to 1.2 and 
1.3 hundredths of water, and are, under these conditions, slowly decomposed by metallic 
iron, with abundant disengagement of hydrogen gas, which burns with a green flame 
from the presence of combined boron.* That, under greater pressure, water may be held 
by other compounds, such as silicates, is undoubted. Hydrous glasses like pitchstone and 
perlite are examples of these, and differ from obsidian in containing three or four hun- 
dredths of water. 
§ 42. The late researches of Tilden and Shenstone on “The Solubility of Salts in 
Water at High Temperatures” throw much light on the geological relations of water. 
While the solvent power of this liquid rapidly increases, when under pressure, at tempe- 
ratures above 100° C.. they have shown that “the increase of solubility follows the order 
of the fusing-point of the solid.” Thus, of potassium-iodid, which melts at 634°, 100 parts 
of water at 180° dissolve 327 parts, while of barium-chlorate, melting at 400°, 100 parts of 
water at 180° dissolve 526 parts. Of potassium-nitrate, melting at 339°, 100 parts of water 
at 120° dissolve 495 parts, or nearly five times its weight; while of silver-nitrate, whose 
fusing-point is 217°, 100 parts of water at 125° dissolve 1622.5 parts, and at 133° 1941.4 
parts, or nearly twenty times its own weight. Of certain substances it can be said that 
they are infinitely soluble at certain temperatures. This is true of the decahydrated 
sodium-sulphate, which melts at 34°, and nearly true for benzoic acid. This substance, 
which melts at 120°, requires for its solution 600 parts of water at 0° and 25 parts at 100°; 
but when heated in a sealed tube to a few degrees above its fusing-point it is miscible 
with water in all proportions. These heated solutions, in the case at least of barium- 
chlorate and potassium-nitrate, are described as notably viscous, a condition which perhaps 
indicates that they are colloidal. * 
§ 43. From these results it is easy to conceive what might be expected at elevated 
temperatures with materials as insoluble, at ordinary temperatures, as quartz or the 
natural silicates. A few hundredths of water at several hundred degrees Centigrade would 
probably convert these into a viscid fluid, from which, as from an anhydrous magma, by 
rest or by partial cooling, definite compounds might successively crystallize,—the mixture 
becoming, to use the simile of Poulett Scrope in speaking of lavas, like a syrup holding 
grains of sugar. From such mixtures partially cooled, or from a heterogeneous plutonic 

1 Trans. Roy. Soc. Canada, Vol. ii. Sec. iii. p. 61. 
2 The potassium-borate in question, apart from combined water, contained boric oxyd 58.6, potash 16.3, 
giving the oxygen-ratio 72:5; and the sodium-borate had the same atomic ratios. Aug. Laurent, Compte Rendu 
des Travaux de Chimie, 1850, pp. 86-42. 
3 Philos. Trans., 1884, Part i, pp. 23-36. 
