hypotheses to the most crucial and varied practical tests, and conclusively proved their truth, or determined the limits of error involved by them. He had the power of arranging almost intuitively simple experiments for qualitatively testing the value of an idea, and his mathematical knowledge and power of close and accurate reasoning enabled him to work out the quantitative results of a difficult problem with great facility. His experimental tank at Torquay, with all the delicate and interesting contrivances in connection with it for measuring and recording the behaviour of models in rolling or their resistance to motion through the water is a marvel of philosophical arrangement and practical skill. Mr. Froude's published papers include but a small portion, we believe, of his work. It would be a worthy tribute to his memory, and a great boon to science and to the shipping interests of the country if the result of his researches could be published in a complete form, and thus made readily accessible.

Mr. Froude had not much encouragement during_the early days of his investigations upon these subjects. The first to appreciate their value were the late Prof. Rankine and Mr. Crossland, one of the constructors of the Navy. Mr. Crossland was one of the first to see that Mr. Froude, in his first paper on the Rolling of Ships, read before the Institution of Naval Architects in 1861, had indicated the true laws of rolling motion, and in the following year he contributed an original paper upon the same subject. Mr. Reed was the first to apply the principles enunciated by Mr. Froude to the construction of ships; and did so with great ability and success. Canon Moseley, Dr. Woolley, and others did not see, however, for a considerable time, that Mr. Froude had made a great stride in advance of previous knowledge, and had really discovered the means that had long been wanted of arriving at a due comprehension of the dynamical laws which govern a ship's behaviour at sea. Mr. Froude's lucid and painstaking explanation of his theory and replies to the objection of Dr. Woolley and others produced in due time their full effect, and in the course of a very few years all who were capable of understanding the arguments upon which the theory was based were thoroughly convinced that Mr. Froude's method and its results were sound, and were such as could alone lead to improvement in this branch of science.

Mr. Froude's scientific reputation and the value of his work now rest upon a solid foundation. His discoveries have revolutionised whole theories of hydrodynamics, and have stood the test of practical application. He has received various honorary distinctions, such as the degree of LL.D. from the University of Glasgow, and the Royal Medal of the Royal Society; but his greatest distinction, and that with which his name will always be associated, is that, in an age when science is fashionable and many of its professors look more to the show than the substance, Mr. Froude devoted his energies to a long and unwearied search after truth in a department of science that few knew anything about, and that could have no interest for the many, and he looked only to success for his reward. Happily, in this sense he was bountifully rewarded, and has left, both in the subject-matter of his researches and the example he set in pursuing them, a legacy to those who follow after which should stimulate them to work with all their might, with the one object of endeavouring also to attain unto truth and to be worthy of being admitted within the veil of the temple of nature.

KARL KOCH

TH 'HERE are very few even among professed botanists, who avail themselves to any thing like the extent they might do of the teachings of a garden. And yet for the study of the life-history of plants and for the due estimation of their precise degree of relationship one to the

other a garden offers in some ways—in many ways—unrivalled opportunities.

Karl Koch, whose death we lately recorded, was one of the few who had a right appreciation of the resources of a garden and who knew how to turn them to account. His tall, attenuated form and keen eye were to be observed at most of the International Botanical and Horticultural Congresses which have been held in various continental cities and in London in 1866. Everywhere, by horticulturists as by botanists, his claims to high rank among his fellows and his title to respect and even affection for his personal qualities were acknowledged, so that it became a pain to those who saw him recently to notice his gradually failing powers and to see how the old spirit was curbed and checked by impaired physical health.

Karl Koch was born in Weimar in June, 1809. In that little capital he came in contact, as a youth, with Goethe, and it was partly owing to his influence and advice that Koch made his visits to the Caucasus and various parts of Asia Minor. Shortly after he had completed his studies in medicine and natural history at Jena and at Würzburg he set out on his travels, his special objects being the investigation of the vegetation and an inquiry into the original sources of our cultivated fruittrees. After two years' research he suffered so severely from the effects of sunstroke on Mount Ararat that he was obliged to return to Jena, but in 1843 he set out a second time for the East. Of his first journey an account was published in 1842, under the title of "Travels through Russia," of his second, in 1845, under that of "Wanderings in the East." A general account of his travels may be found in the Linnea for 1848, in which publication also may be found catalogues and descriptive lists of the plants collected by him, together with remarks on the geographical distribution of plants in the Caucasus, &c. On his return from this second expedition he became assistant-director of the Botanic Garden at Berlin, secretary of the Prussian Horticultural Society, and, a few years later, Professor of Botany in the University.

His position at Berlin gave him exceptional facilities for studying cultivated plants, and, accordingly, much of his botanical work consisted of monographs of Arads, Bromeliads, Agaves, and other plants, necessarily imperfectly preserved in herbaria. Many such monographs are scattered through the Wochenschrift of the Berlin Horticultural Society, and which was for many years edited by him. As a pomologist also he held no mean position, but the most interesting and valuable part of his labours, so far as this branch is concerned, are those relating to the origin of cultivated fruit trees, a subject intimately connected with the history and migrations of our own race.

His magnum opus, however, is his "Dendrology”—a scientific description of the trees and shrubs cultivated in the forests and gardens of central Europe, a work for which his travels had well prepared him. For the purpose of compiling this volume Koch visited almost every country in Europe. All the great nurseries of the Continent and of our country were also inspected by him with the object of study or of securing specimens.

Despite small defects of method Koch's descriptions are excellent and characteristic, so much so, that it is a great pity that his work has not been translated into English. The technical details of his subject are enlivened by short biographical notices of the botanists and horticulturists whose names are the most prominently associated with the department of botany, of which his work treats. The reader of these interesting notes to an otherwise necessarily dry technical book will have no difficulty in understanding the estimation in which Koch's popular lectures on trees and on fruit trees in particular were held by the Berlin public.

In private life Koch was beloved for his uprightness, loyalty, and warm-hearted devotion to his friends.

THE ELECTRIC DISCHARGE WITH THE CHLORIDE OF SILVER BATTERY

MESSRS. De La Rue and Müller, in the second part of the researches which they have carried on during three and a half years, have contributed facts of the highest value towards the solution of the problem presented by the beautiful phenomenon of stratification produced by electric discharges in vacuum tubes. The following are some of the more important results of these experiments as described by the authors.'

These phenomena, first noticed by M. Abria in 1843, were independently re-observed by Mr. (now Sir William) Grove in 1852, and have since engaged the attention of many physicists. The late Mr. Gassiott, working at first with an induction coil, but more recently on the same lines as the voltaic batteries of high potential, published results of great interest; while, on the other hand, Mr. W. Spottiswoode is still pursuing with great acumen and originality a similar investigation, both with the induction coil and the Holtz machine, with which he has recently used condensers of great capacity.

Throughout our labours we have felt so strongly the necessity of obtaining numerical results as data for the foundation of a theory, that we have not hesitated to risk much in this cause. By the fusion of terminals, or the sudden discharge of the condenser, we have lost a vast number of very beautiful tubes; but gradually by the adoption of various devices, and by the employment of instruments specially constructed and insulated to suit the high potentials we deal with, we have succeeded in overcoming the various impediments, so that we can now readily obtain values for the physical quantities that enter into consideration in our experiments.

There is a serious trouble connected with the study of the discharge in rarefied gases, for, after a very short time, the tubes completely and permanently change, so as no longer to present the splendid stratifications witnessed on a first trial. We believe these changes occur much more rapidly with the battery in consequence of the greater amount of current, than with the induction coil; but the fact appears to have been well known to Dr. Geissler, of Bonn, who, on the occasion of a visit to our laboratory, brought with him some tubes through which no current had previously passed (virgin tubes, as he called them), which presented most beautiful phenomena lost for ever after too brief a period.

Tube 123 (cyanogen), for example, when first connected

After spending much time in experiments with tubes prepared for us by Dr. Geissler, Messrs. Alvergniat Frères, of Paris, and Mr. Hicks, of Hatton Garden, with the vexation of finding that we could not often enough repeat our experiments, we ultimately came to the conclusion to have others made, but not exhausted, and to perform ourselves the charging and exhaustion. The tubes we usually employ have a glass stop-cock fitted. to them at each end; they are 32 inches long, and from 175 to 2 inches in diameter; the terminals are of aluminium, and about 29 inches apart, one being a ring, the other a wire bent at a right angle, so as to point in the direction of the axis of the tube (see No. 144, Fig. 3), for we have found that the phenomena vary according as the ring or wire is made positive.

144

These we exhaust and fill with any gas we may wish to experiment with, and gradually exhaust again, noting the phenomena presented at different pressures, with different potentials, and with different amounts of current. We re-fill and exhaust the tube again and again, and mostly obtain, under the same conditions, as nearly as possible the same phenomena, of which we are careful to make sketches and, if possible, to obtain photographic records.

In some cases we make use of tubes provided with a

1 See Phil. Trans., vol. clxix., PP. 55-121.

Mr. Gassiott made several batteries of different kinds in the course of his experiments; on the occasion of a visit to his laboratory, January 26, 1875, the current of his Leclanché battery was measured by us with a volta

meter.

The current of 1000 new cells was found to be 007464 W; that of the whole 3000 cells, 1000 of which had been a long time in use, o'04718 W. Taking the Leclanché as 148 volt the internal resistance of the new battery must have been 1983 ohms per cell; that of the whole 3000, 31 87 ohms per cell. The striking distance of the whole 3000 between a conical point and a disk 0125 inch diameter was only o 125 inch; whence the inference is that the insulation was, at that time, imperfect.

calibrated chamber between two stop-cocks, as a-b, No. 145, Fig. 4,the chamber in this particular case having th of the capacity of the tube, this tube has also an absorption chamber communicating through a cock and intended to contain spongy palladium. After a tube has been exhausted so as to produce a particular phase, and in the course of the experiment the exhaustion has been carried beyond that degree which permits of the reproduction of that phase, one or more charges of gas may be successively admitted into the tube by filling the calibrated chamber with gas at any particular pressure, and then opening the stop-cock communicating with the tube; the lost phase is thus reproduced.

The apparatus which we have found it advantageous to adopt for the exhaustion of our tubes is shown in Fig. 5 ; it comprises three means of exhaustion which are successively employed as the vacuum becomes more perfect. The first is an Alvergniat high-pressure water trompe in connection with the high-pressure water-main of the

West Middlesex Water Company, the head of water being 106 feet; it produces a vacuum to within half-an-inch (047 in. 12 millims.) of the height of the barometer. The pipe leading to it is so marked in the drawing; it is

attached, through a cock, to a four-way-junction-piece F, provided with three more cocks, communicating:-one to one end of the tube T, one to the last drying bottle of the gas generator G G', and one to a mercurial gauge. The

a ს

other end of the vacuum tube T communicates by means of a Y-piece to both, an Alvergniat mercurial pump, on the right of the figure, and a Sprengel pump, on the left. After the trompe has done its work, the Alvergniat is used for rapid exhaustion, and then shut off by means of the glass cock C, leaving the exhaustion to be completed by the Sprengel; we have thus obtained, by the pumps alone, in tubes 32 inches long and 2 inches in diameter, vacua of only o'002 millimetre pressure, equal to 2.6 millionths of an atmo

145

sphere-a vacuum so perfect that the current of 8040 cells would not pass. The apparatus is in connection with a McLeod gauge, by means of which pressures to 0'00005 mm. can be determined. Besides this gauge, the Sprengel and Alvergniat pumps have their own gauges, which read to a millimetre. M is a rotating mirror consisting of a four-sided prism mounted on a horizontal axis and provided with a multiplying wheel; on each face of the prism is fastened a piece of looking-glass. The reflection of the tube in the mirror enables one to examine

whether an apparently nebulous discharge is simply nebulous or consists really of strata, also whether and in what direction there is a flow of s rata which may appear quite steady to the eye. The observations are facilitated by covering the tube with a half cylinder of cardboard having a slit in the direction of its axis about inch wide. R is a radiometer attached to the Sprengel; d, d, a drying tube containing sticks of potash used when gas is introduced from a reservoir through the Alvergniat.

The resistance of vacuum tubes does not depend solely

or mainly on the distance between the terminal, but it does greatly on their diameter.

In order to test how much of this depends on the length of any constriction, we had made two tubes, 154 and 155, Fig. 6, of nearly the same length (16 inches), and internal diameter 18ths of an inch, the residual gas in each case being carbonic acid, CO. From results obtained with these tubes where the constricture varied in length in the ratio of 125 to 1, it became evident that the main effect is due to the simple constricture of the tube.

The diagram Fig. 7, shows the arrangement by which,