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In Professor Shand's reply to my criticism of his original paper on the application of the principle of saturation with respect to silica in the classification of igneous rocks, some points arise which make it clear that he has misunderstood me in several particulars. In the beginning of my paper, it was explicitly stated that what was to be considered was (1) whether the principle, as enunciated by Professor Shand, could be extended to minerals other than the silica ones (quartz, tridymite, etc.), the felspars and the felspathoids, and (2) whether the criterion whereby the rock minerals were divided into the two groups of saturated and unsaturated was sufficiently exact. The criterion used was that of “the observed facts of distribution”, and any mineral which was found coexisting with quartz or some other form of silica in igneous rocks was said to be saturated, while those which did not occur along with quartz were said to be unsaturated. Correspondingly, rocks which consisted entirely of minerals of the former type were classed as saturated, those consisting of mixtures of the two types as part-saturated, and those consisting solely of the second type as unsaturated. I endeavoured to show that the rock-forming minerals cannot be satisfactorily divided into two classes in this empirical fashion, without any consideration of the cooling-histories of the individual rocks in which the minerals may be found.

In a preliminary note presented to the Society in June, 1887, the results of twenty-one experiments on the composition of water by volume were given in detail. The ratio deduced from these experi­ments was less than two volumes to one of oxygen. This result was unexpected, because of the greater compressibility of oxygen than of hydrogen, but as every one of the experiments pointed to this result, the evidence for it seemed conclusive.

In a preliminary note, “ On the Composition of Water by Volume,” presented to the Society in June, 1887, I remarked that up to that time we had no direct measurements of this important constant, except those of Gay -Lussac and H umboldt. The value they deduced from their experiments corresponded so far with the number I then gave as the most probable one, in that it required rather less than two volumes of hydrogen to one volume of oxygen. The results of my experiments, in spite of this coincidence, astonished me, for it was to be expected that as oxygen is more compressible than hydrogen (to say nothing of its coefficient of expansion), it would contain more molecules per unit volume than hydrogen, and, therefore, necessarily require more than two volumes of hydrogen for one volume of oxygen. In a short note, published in “ Nature,” of March 8th, 1888, I announced that I had detected a most important, and hitherto unsuspected, source of error which had led to the consumption of oxygen, and this naturally accounted for the relatively small number for the hydrogen. This source of error lay in the use of a lubricant of a combustible nature, not because of its volatility, but because of its tending to form a greasy layer on the surface of the eudiometer, and which was, to a certain extent, burnt with each explosion in the eudiometer.