5. On a Glass Digester in which to Heat Substances under Pressure. By Dr E. A. Letts.

The objections to the use of sealed tubes are known to every practical chemist, and are a serious drawback to their employment. The chief of these are the time expended in the manufacture of the tubes, the amount of skill in glass-blowing required, the danger experienced in opening them, and above all, the fact that only a small quantity of material can be heated at one operation. Moreover, the same tube can seldom be used for more than three or four experiments, as each time it is sealed up its neck must be drawn out, and its length thus considerably decreased. These disadvantages were especially felt by me whilst preparing bromacetic acid, which was required in considerable quantities, and where as many as half a dozen tubes of bromine and acetic acid had to be heated before 100 grammes of the acid could be obtained. To obviate these objections I have had an apparatus constructed, which consists of a cylinder of glass, the walls of which are about half an inch thick. Its length is fifteen inches, its external diameter three inches, and its capacity about 600 cubic centimetres. At one end it is drawn out to a tube, whose aperture is only about one-sixth of an inch in diameter, though its walls are as thick as the rest of the apparatus. Originally this tube was provided with a stopcock, but at Professor Brown's suggestion, I have substituted a glass plate, which is ground fiat, and accurately adapted to the top of the tube.

In order to keep the glass plate pressing on the tube the whole apparatus is placed in a frame, consisting of three brass wires arranged symmetrically around the cylinder, and attached by means of nuts, below, to a brass ring, and above, to a brass plate, through which latter a screw passes, which, when turned, presses on a brass plate placed on the glass cap.

As any experiments with such an apparatus would be attended

with danger, were it necessary to be in its neighbourhood, it occurred to me that an automatic arrangement might be employed to give notice that the temperature had been reached to which it was intended to subject the digester.

For this purpose I employed a thermometer with a somewhat wide tube and large bulb. A platinum wire is sealed into the bulb, and touches the mercury, whilst a brass wire passes down the tube, and is held in position by a binding screw. The two wires are connected with an electric bell, the brass wire being so adjusted, that when a particular temperature is reached the mercury touches its end, and thus completes the circuit, and causes the bell to ring.

In order to test the digester, about 200 grammes of a mixture of two-parts bromine and three of acetic acid was placed in it, and after fixing it in its frame, the whole apparatus was immersed in an oil bath and heated to 150° C., the temperature at which reaction in this case takes place. The experiment was made in a cellar, and the bell placed in a room some distance off. The gas to heat the oil bath was led by a tube from another cellar, so that it could be regulated without going near the digester. In about an hour and a half the bell rung, and thereupon the gas was shut off; and on examining the digester next day, it was found that the reaction had taken place, and that only twelve grammes of product had been lost-a very inconsiderable quantity.

As the action of bromine on acetic acid is very sudden, and accompanied by the disengagement of a large volume of hydrobromic acid, the apparatus may be considered to have undergone a very severe test, and that its efficacy for all ordinary purposes is established.

Should the digester come into general use, it will certainly save chemists much time and labour.

The following Gentlemen were elected Fellows of the Society:

Rev. FRANCIS LE GRIX WHITE, M.A.

JAMES DUNCAN, Esq., Benmore.

Rev. NORMAN MACLEOD.

J. S. FLEMING, Esq.

JAMES DOUGLAS H. DICKSON, M.A. Glasg., B.A. Camb.

Monday, 20th March 1876.

SIR WILLIAM THOMSON, President, in the Chair.

The following Communications were read:

1. On the Connection between Cohesion, Elasticity, Dilatation, and Temperature. By Professor George Forbes.

(Abstract.)

At various times there have arisen supporters of one or other of two extreme hypothesis concerning the nature of what we define as force. These are the hypothesis of "action at a distance" and of "no action at a distance."

According to the latter hypothesis, the centre of gravity of no hody, however large or however small, can be moved from a position of rest, nor can its motion be altered in direction or amount, except by direct collision with another portion of matter.

Starting from this supposition as a basis of argument, and without assuming anything further as to the manner in which the different physical forces are caused by collisions, it is possible to arrive at some very general theorems; and from these theorems conclusions may be drawn as the nature and connection of some of the physical forces, which are necessarily true if the hypothesis of no action at a distance be true.

The principal result of these theorems is the following:-Let a rod be chosen of any substance whose cohesion and elasticity do not vary enormously with the temperature. Let a be its expansion, in terms of its length, when the temperature is raised 1°. Let ẞ be the compression of the same rod, in terms of its length, when a unit weight is supported at its summit. Let c be the number of these units which, when suspended by the rod, suffice to break it by sudden rupture. Let be the absolute temperature at which all these experiments are made. Then the theory leads us to the conclusion that

Only a few experiments have been made by which we can test

this law. But the following values are the most accurate, and tend to prove the truth of the law. The apparent discordance in the case of iron is in part due to the variations in the qualities of that metal in different specimens.

In calculating this table, the values of c from the experiments of M. Wersheim are used; those of a from the experiments of Mr Mathiessen (except iron); those of ẞ from the experiments quoted by Prof. Balfour Stewart in his Text-Book; and the assumed temperature is 18° C., or 283° absolute temperature.

2. Notice of the Completion of the Works designed by Sir Charles A. Hartley, F.R.S.E., for the Improvement of the Danube. By David Stevenson, Esq., V.P.R.S.E.

In 1868 I presented to the Society, on behalf of Sir Charles A. Hartley, a memoir published by the European Commission of the Danube, on the improvement of that river, and at the same time gave a notice of the works designed by Sir Charles Hartley for effecting that important object. These works have now been completed, and Sir Charles Hartley has again asked me to present to the Society a second memoir published by the Commission, which brings the history of the works constructed under their charge down to the time of their completion in 1873.

In supplement of the notice formerly communicated, which referred to a work in progress, it may not be uninteresting, now that the work is completed, to state briefly what has been effected by this most important and successful example of hydraulic engineering.

The engineering problem to be solved by the European Commission was the removal of the bar which obstructed the Sulina mouth of the Danube, which, in 1856, had a varying depth of channel never exceeding 11 feet. The design of Sir Charles Hartley-the engineer to the Commission-consisted in piers so constructed as to confine the current of the river in its passage into the Black Sea. At the date of my last notice the north pier had been extended to the length of 4640 feet, and the south pier to 3000 feet, and a maximum depth of 171⁄2 feet instead of 11 feet had been obtained. I, however, suggested in that notice, that as the Danube must continue to bring down an enormous mass of detritus, so in course of time the works which had proved so successful must be extended; and it appears that this has been found necessary, as the south breakwater, completed in 1871, has been extended to 3457 feet in length, and even with this additional length it is, I think, not improbable that in the course of time still farther extension may be required, for the Sulina mouth of the Danube will still discharge the same amount of water, bearing with it the same amount of alluvial matter, estimated in high floods at about 70,000 tons in twenty-four hours, the deposit of which at the extremity of the piers will still have a tendency, though in deeper water, to form a bar.

The works have, however, proved most successful, and reflect the highest credit on Sir Charles Hartley, by whom they were designed and executed, and the following is a summary of the results that have been obtained.

The total length of piers executed is 8789 feet, at a cost of L.185,352, being L.21 per lineal foot, in an average depth of 14 feet at low water. The navigable depth of the channel over the bar has been increased from 11 feet in 1856, to 20 feet in 1873. In 1853, 2490 vessels, of 339,457 aggregate tonnage, left the port; in 1869 there were 2881 vessels, with a tonnage of 676,960. Thus, while the number of vessels increased only at the rate of 16 per cent., the tonnage, due to the greater draught, had been increased at the rate of 50 per cent., a good practical proof of the value of the improvements. The number of shipwrecks at the mouth of the Danube has also been greatly diminished.

VOL. IX.

U