Page:Proceedings of the Royal Society of London Vol 69.djvu/127

Rh trations required for simple molecular combinations only of nitric acid (HN0 3 ) with water. Some of these combinations have been alluded to by previous writers on this and kindred substances, while others have either not been examined or have formed the subject of a diversity of opinion.

In the case of the densities and contractions the best denned points of alteration correspond to the composition of the hydrates HN0 3 with 14, 7, 4, 3, 1*5, and 1 molecular proportions of water respectively, while in the case of the refractive indices, apart from consideration of density, the most marked points correspond to the 14, 7, and 1-5 hydrates. In the case of the contractions and electric conductivities, but to a less degree in that of the refractiA r e indices, there is remarkable discontinuity at concentrations 95 to 100 per cent., which can possibly be explained rather by some decomposition than by some combination.

Further, the more detailed experiments on the contractions show that these points of discontinuity, though to some degree real, yet to another degree are ideal, in that an apparently abrupt change of events is resolved into either a gradual transition or a transition stage, which differs from either the preceding or successive phase. In the present state of knowledge it appears difficult to interpret the course of events between each phase, as there are the possibilities of at least two cases involving two entities, namely, (i) combined acid and water -with free water, and (ii) combined acid and water with free acid.

Investigations into a possible case not readily provided by nature or art, of two liquids which rapidly mix but slowly combine each with each, might solve many of the problems of solution towards which the present memoir may serve as a slight contribution.

Comparatively little attention has been directed to the subject of the development of the stem. This research concerns itself chiefly with the development of the cauline fibro-vascular skeleton, since this appears to be most interesting from the phylogenetic and morpho- logical standpoints. A study of numerous examples drawn from the main groups of the Pteridophyta and Gymnospernis has led to the conclusion that the polystelic type of Van Tieghem does not originate, .as he states, by the repeated bifurcation of the epicotyledonary central