I have thrown together, in this chapter, the ores of several members of a family of the elements, which may be called the family of the magnetics;* and I have also, from its frequent chemical association with iron, placed titanium along with them. No attempt was made by me to analyse the ordinarily-employed iron ores, but only such as, from their apparent purity, or from the excellence of the specimens, appeared to be of mineralogical interest.

This research was undertaken with a view to ascertaining if metallic chlorides have any tendency to combine with a further quantity of chlorine. The well-known fact that potassium iodide unites with iodine to form a tri-iodide suggested that potassium chloride might, under suitable conditions, form a similar compound with chlorine. The method of investigation which first presented itself was to expose crystals of various chlorides to the action of dry chlorine gas at low temperatures, when any chemical action taking place would be expected to show itself in changes in the appearance of the crystals. A few experiments were made with no apparent result, and the method finally adopted was to determine the quantity of chlorine absorbed by solutions of the salts. It was thought that in this way quantitative results could be obtained which would reveal any tendency towards the formation of perchlorides. If a body having the formula KCl3 could be shown to exist, it would strengthen the position of those chemists who contend that the generally accepted ideas regarding atomicity need modifying. The discovery of a compound having the formula KCl2 would be a still stronger argument. At an early stage, however, the research resolved itself into an investigation of the solubility of chlorine gas in solutions of metallic chlorides in water, the question of the existence of perchlorides becoming a side issue. The influence of salts in solution on the solubility of gases in water is a subject of considerable interest, as throwing light on the nature of solution in general. Of late, certain chemists are inclining to adopt the old view that solution should be included under the general head of Chemical Action.

§1. As a preliminary to the establishment of the law in question, it is necessary to state and exemplify another law to which I have elsewhere directed attention, viz.,

THE LAW OF COMPLEMENTARIES.

To every general theorem which takes the form of an identical relation between a number of the minors of a determinant or between the determinant itself and a number of its minors, there corresponds another theorem derivable from the former by merely substituting for every minor its cofactor in the determinant, and then multiplying any term by such a power of the determinant as will make the terms of the same degree.

The following remarks on the dragon's blood tree of Socotra are intended to serve as introduction to Dr Dobbie's and Mr Henderson's paper on the red resin obtained from the tree, and to furnish a technical description of the plant which has not hitherto appeared. In my account of the Botany of Socotra, which will shortly be published by the Society, further remarks upon this interesting tree will be found, along with a figure.

§1. It is well known that by a simple transformation of a determinant we may cause a zero to take the place of any one of the elements. The theorems of Hermite and Horner, for example, for depressing the order of a determinant may each of them be viewed as the result of repeated transformations of this kind, the operation being continued until all the elements of a row or column except one are replaced by zeros.

“If the n roots of the equation

“are all real, then all the roots of the equations

“and

“are also real, and one of the n—1 roots of the latter lies between each adjacent “two roots of the former.”

Two specimens of the worm which forms the subject of this memoir were brought to Professor Moseley, in 1872, by a coolie trained for the purposes of collecting by Dr. Thwaites, F.R.S., the distinguished curator of the Peradeniya Gardens at Kandy; each was found at the bottom of a deep burrow, in company with a single egg-case, in the neighbourhood of that town. Professor Moseley entrusted me with them for study and description, and I have to thank him for much valuable assistance during the course of my work, which was carried on in the Oxford Natural History Museum.

I am indebted to the kindness of Professor Ramsay, Director-General, and of Professor Geikie, Director of the Scottish Branch of the Geological Survey of Great Britain, for the privilege of examining and describing a remarkable collection of fossil fish-remains from the Lower Carboniferous rocks (Calciferous Sandstone Series) of Eskdale and Liddesdale. Most of the specimens were collected by Mr Arthur Macconochie, one of the collectors attached to the Scottish Geological Survey; and Mr Walter Park of Brooklyn Cottage, Langholm, has also willingly co-operated in the search, so far as the district of Eskdale is concerned. I have myself also had the pleasure of twice visiting Eskdale, along with Mr Macconochie and Mr B. N. Peach, and on these occasions I obtained a few specimens for my own collection.

The resin to which the following paper refers was obtained by Professor Bayley Balfour from a species of Dracæna which he discovered in Socotra during his visit to that island in the year 1880, and to which he has given the name of Dracæna Cinnabari. In outward appearance and in general physical properties the resin differs from the varieties known commercially under the name of dragon's blood. With the view of ascertaining if these differences depend upon difference of chemical composition, we undertook, at Professor Balfour's request, to make an examination of the resin.

By the permission of Professor A. C. Ramsay, LL.D., F.R.S., Director-General of the Geological Survey of Great Britain and Ireland, and Professor Geikie, LL.D., F.R.S., Director of the Geological Survey of Scotland, I have been permitted to describe several new Crustaceans which have come under my notice in my capacity of Acting Palæontologist to the Scotch Survey. They are from the cement-stone group of the Calciferous Sandstone series of the Scottish border, and, with a very few exceptions, were got from one locality on the river Esk, about four miles south of Langholm, in Dumfriesshire, and were almost all collected by A. Macconochie, Fossil Collector to the Geological Survey of Scotland. They belong to two orders, viz., Phyllopoda and Decapoda.

I have again been privileged by Professor A. Geikie, Director-General of the Geological Survey of Great Britain and Ireland, in being allowed to bring before this Society the results of a further study of the Crustacea and Arachnida obtained by the Geological Survey from the Lower Carboniferous rocks of the Scottish Border. Since my former papers on those subjects were read, a considerable amount of fresh material has come to light, which has allowed of many species, which were formerly represented by fragmentary specimens, to be more fully described, and several altogether new to science to be added to our former list.

Through the kindness of Dr. A. Geikie, Director-General of the Geological Survey of Great Britain, I have had the pleasure of examining the fossil plants collected by the Scottish Geological Survey from the Calciferous Sandstones of Eskdale and Liddesdale.

Of all the various spectra which the progress of experimental science has enabled man to observe in the present day, none are so rich, varied, and important, as those of gases. And no wonder! for it is only when matter has been reduced to the gaseous condition, that it is able to specialise itself and write its character with much of its history in any otherwise smooth, undefined, continuous spectrum; while, if in former times, men would have found it an impossibility to drive many of the more refractory substances into the state of incandescent vapour, what is there now anywhere on the surface of this earth which, in small quantity, can resist the action of a powerful and condensed induction spark of electricity; and what application of that spark is so neat, elegant, convenient, and economical, as when it is employed in conjunction with so-called gas-vacuum tubes.