Monday, 3d April 1876.

SIR WILLIAM THOMSON, President, in the Chair.

The following Communications were read:

1. Chapters on the Mineralogy of Scotland. By Professor Heddle. Chapter I. -On the Rhombohedral Carbonates. Communicated by Professor Tait.

Professor Heddle read a paper on the "Rhombohedral Carbonates occurring in Scotland," the first of a series of Chapters intended to embrace the analytical results of an investigation of all unknown or insufficiently determined Scottish species.

In this paper many analyses of the carbonates were submitted; and the pseudomorphic changes taking place in these were referred to in a special manner.

2. On Thermo-Dynamic Motivity. By Sir W. Thomson.

3. On the Vortex Theory of Gases, of the Condensation of Gases on Solids, and of the Continuity between the Gaseous and Liquid State of Matter. By Sir W.

Thomson.

4. On two new Laboratory Apparatus. By William Dittmar.

The object of this communication is to submit to the notice of the Society two little inventions of mine, which, whatever may be the degree of originality which they can claim, will, I venture to hope, prove useful additions to the catalogue of chemical-laboratory appliances. The one is a new form of the precision balance, which pretends to execute exact weighings with a hitherto unattained degree of rapidity; the other is a contrivance for maintaining a constant pressure in a supply of gas, and thus making it possible, with comparative facility, to keep, say an air-bath, for any length of time, at a constant temperature.

The new balance differs from the instrument in its customary

form only in two points, of which the more important is a modification of the centre of gravity "bob" arrangement, which enables one, at a moment's notice, to shift the centre of gravity of the instrument from a certain definite position, I., to a certain other (higher) position, II., matters being arranged so that in passing from I. to II., the sensibility, i.e. the deviation, corresponding to an overweight of, say 1 milligramme, is increased in an exactly pre-determined ratio, such as of 1: 10, for instance. For this purpose the "bob" is made very light, so that the distance through which it has to travel in order to effect the desired change of sensibility is not too small, and, instead of to a screw as usual, is fixed by mere friction to a vertical triangular steel rod forming part of the needle. The other new feature in the balance is, that the rider-principle, besides being discounted in a slightly different manner from the customary one, is extended to the determination of differences of weight up to 100 (instead of 10) milligrammes.

ΔΟ

10

Bob

10 A

Fig. 1.

The arrangement adopted is represented in the accompanying sketch, for the interpretation of which it is only necessary to say that C (10) and (10) O are equal to C (0), and (0), (10), respectively, and that both O(10) and 0, (10), are each divided into 10 equal parts, the former by notches filed into the beam, the latter by marks; and to add, that there are two riders, one weighing p centigrammes for the left arm, and another weighing p milligrammes

for the right arm, the balance being adjusted so that, when both riders are at their zero-points, it is in equilibrium, and p being chosen so, that, supposing the large rider to be shifted to the n mark, and the small one to the m mark, this virtually amounts to the addition of 10n+m milligrammes to the charge in the right pan.

There is no need of my explaining how the balance is meant to be used; I will rather avail myself of this opportunity for drawing the attention of readers interested in the subject to a few inferences from the theory of the balance, which, obvious as they are, have hitherto not been sufficiently appreciated by either the authors of our physical handbooks or by practical balance-makers.

I. Given a balance in which everything is constant except the distances of the centre of gravity of the empty instrument from the axis of rotation, and it is easily shown that (for a constant charge) the deviation a of the needle for a given over-weight ▲, and

α

consequently the "sensibility" a = is the greater the less s. This, of course, is duly stated by all authors; but what is always forgotten to be pointed out are two things, viz.-1st, That the "sensibility" has nothing to do with the inherent precision of the instrument; and 2dly, That supposing the sensibility to be increased, all the other good qualities of the balance get less; we diminish the rate of vibration (this rate being proportional to

); we diminish the range of differences of weight determinable by the method of vibration; we diminish the relative constancy (in opposition to variations in the charge) of the sensibility and the time of vibration. Considering now,

II. The case of a balance to be constructed, the arm-length 7 and weight w of the empty beam also become variables, related to each other, according to some equation like w const. 1, and (assuming each of the pans to bear a certain medium charge p) we have

i.e., by diminishing we can increase the sensibility without diminishing the rate of vibration (or vice versa); but the other

disadvantages mentioned under I. must be taken into the bargain, and, besides, the inherent precision of the balance gets less.* To pass to an example: What we gain by substituting a 7-inch for a 14-inch beam is that, for the most convenient t, the sensibility becomes 2 to 4 times greater; but this advantage is secured without expense in good qualities by placing before the graduated limb a lens magnifying the excursions of the needle into 2 to 4 times their natural size. This is the theory of the "short beams," which have lately come so much into fashion.

To come back to my own balance, I must not forget to thank Messrs Becker Sons of Rotterdam for the readiness with which they have, at their own risk, tried to realise my ideas in an actual instrument, which, by the way, is now being exhibited at South Kensington. To increase the usefulness of the instrument, I have caused Messrs Becker to add to it a glass plunger, which is adjusted so that it displaces exactly 10 grammes of water at 15°, and which consequently enables one with great rapidity to determine the specific gravities of liquids by the method of immersion.

To pass now to the new gas-governor, its most essential part consists of a mercury-manometer (fig. 2), of which one limb, A, is

about 20 mms. wide, and stands vertical; while the other, C, is of the width of a thermometer tube, and is placed horizontally.

For fuller explanations, see my article "Balance" in the "Encyclopædia Britannica."

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The empty part of the wider limb communicates, through F, with the gas-supply, through G with the gas-lamp serving to heat the air-bath; and the quantity of mercury is adjusted so that, when the gas is at the lowest pressure which, in the course of the experiment, it is likely to assume, the mercurial index in C occupies a certain convenient position a.

The manometer is connected with a constant battery (the circuit of which includes an electro-magnetic arrangement for opening or shutting the gas-tap), in such a manner that, as soon as, through an increase of pressure in E, the index in C travels ever so little towards the right of a, the current is closed, and the gas cut off.

The following Gentlemen were elected Fellows of the Society:

JOHN MACMILLAN, M.A.

JOHN GIBSON CAZENOVE, D.D.

The following Gentlemen were elected Honorary Fellows of the Society:

1. On an Improved Form of Galvanic Battery. By J. Cook, Esq. Communicated by Professor Tait.

I wish to direct attention to a simple improvement on battery cells, whereby porous cells are dispensed with, and the inconveniences of gravity batteries avoided.