fumes of nitrous acid are evolved. The residue is a strongly acid syrup, which becomes finally crystalline. It is repeatedly dissolved in water and evaporated on the water-bath in order to expel the nitric acid. Thus purified, the new acid is dissolved in boiling water, neutralized with pure carbonate of barium and separated from sulphate of barium. On cooling, the barium-salt of the new acid crystallizes. It is soluble in boiling water, less so in cold water, and almost insoluble in alcohol, by means of which it may be precipitated from its solution in water.

On examining the appearance and deportment of this salt, Dr. Hofmann, in whose laboratory I performed these experiments, at once recognized the identity of this compound with the barium-salt of disulphetholic acid which he and Mr. Buckton have lately discovered.

This view was fully confirmed by the analysis which I made. The composition of the barium-salt, dried at 160°, is represented by the formula

C. (H, Bag) S, O12.

The potassium salt of this acid is readily soluble in water; it crystallizes easily, and is likewise precipitated by alcohol from its solution Dried at 100° C., it contains

in water.

At 160° it suffers no decomposition; when exposed to a higher temperature, however, it blackens and intumesces, empyreumatic substances being evolved.

It is obvious that this bibasic acid stands in the same relation to ethylene as the monobasic ethylsulphurous acid to ethyl.

The origin of the two acids is perfectly analogous, the latter acid, according to Mr. Muspratt, being obtainable also by the action of nitric acid upon sulphocyanide of ethyl.

Sulphocyanide of ethyl, C, H, Cy S2, produces ethylsulphurous acid, C, H, H, S2 0%.

4

2

Sulphocyanide of ethylene, C, H, Cy, S, produces ethylensulphurous acid, C4H4, 2H, 2S2 O6.

This reaction appears to throw some light upon the constitution of polybasic compounds. The compounds of monatomic molecules of the hydrogen-group with elements or compound radicals of the oxygen-group, are all remarkable for the simplicity of their construc

2

tion. The union of biatomic radicals of the hydrogen-group with molecules of the oxygen-group gives rise to combinations of a far more complicated character. Whilst one molecule of water, H, O21 most conveniently may be considered as the type of many compounds of the former class, the corresponding compounds of biatomic radicals frequently correspond to a double molecule of water, 2H, O..

Sulphocyanide of potassium may be viewed as water, in which the oxygen is replaced by sulphur, one of the hydrogen molecules by cyanogen, the other by potassium.

4

KJ

In the production of sulphocyanide of ethylene two equivalents of chlorine in chloride of ethylene (C HCl) have to be eliminated by two equivalents of potassium. Thus the very reaction of the two factors, chloride of ethylene (C, H, Cl2), and two equivalents of sulphocyanide of potassium 2(K Cy S2), joins 4 equivs. of sulphur and 2 equivs. of cyanogen with one molecule of ethylene. This reaction may be expressed by the following equation, which will illustrate at once my view in regard of the constitution of this substance :

The acid produced by the action of nitric acid upon sulphocyanide of ethylene obviously belongs to the same type. In this compound, which in the conception of this view may be called ethylene-sulphurous acid, the cyanogen is replaced by hydrogen, whilst the sulphur has been oxidized into the compound radical S, O6, which in sulphurous acid we assume united with hydrogen.

Since we find that the hydrogen-molecules in polybasic acids are replaceable by two or more molecules of different metals or radicals,— witness tartrate of potassium and sodium, oxalovinate of potassium,— the idea naturally suggests itself that the biatomic alcohol-forming radicals may be capable of uniting two molecules of different elements

or compounds of the oxygen-group. It is probable, for instance, that the ethionic acid, discovered by M. Magnus, may be such a compound, namely ethylene-sulphuro-sulphurous acid.

The following Table contains some of the known ethylene and succinyl compounds compared with the corresponding derivatives of the ethyl and propionyl series.

Propionyl-series.

Succinyl-series.

II]

}

Propionic acid.. C, (C,II.)}O, Succinic acid.. C(C,II, ̧)0 ̧

VII. "Description of an Instrument for registering Changes of Temperature." By BALFOUR STEWART, Esq. Communicated by J. P. GASSIOT, Esq., F.R.S., Chairman of the Kew Observatory Committee of the British Association. Received June 12, 1856.

It lately occurred to the author that advantage might be taken of the capillary action of mercury to construct an instrument similar to a thermometer, but in which the mercury should expand from heat only in one tube, and contract from cold only in another. Accordingly a bulb was blown between two thermometric tubes of differentlysized bores, in such a manner that the tubes lay in one straight line, with the bulb between them. The bulb was then filled with mercury, and the tubes were hermetically sealed at both ends, having been first carefully deprived of air. When the instrument thus constructed was laid in a horizontal position, or nearly so, its action was precisely what the author had hoped; the mercury contracting from cold only in the narrow bore, and expanding from heat only in the wide one,—even when viewed by a microscope of considerable magnifying power.

It was suggested by Mr. Welsh, Director of the Kew Observatory, that such an instrument might be used for measuring fluctuations of atmospheric temperature; and the following use afterwards occurred to the author. Were it required to exactly estimate the radiating

effect of a source of heat, it might perhaps be done by placing this instrument near the source, alternately exposing it to the influence of the calorific rays proceeding from the source, and intercepting these by means of a screen. Owing to the peculiar action of the mercury alluded to, the effect of the rays would be multiplied by the number of times the screen was interposed, provided it were always retained long enough to permit the mercury to cool down. The comparison of an instrument thus acted upon with another similar instrument near it, screened entirely from the source of heat, might furnish us with a means of exactly estimating the heating effect of the source.

The author desires to express his obligation to Mr. Welsh, who, besides finding a use for the instrument, suggested the selection of tubes which appears to answer best, and whose experience was of great assistance in arranging details. He is also indebted to the Kew Committee of the British Association, who kindly examined the instrument, and authorized the construction of several by way of trial. Mr. Casella undertook the operative part in their construction, and his glass-blower, Mr. J. E. Griffin, took pains to discover some of the circumstances that interfere with the proper action of the instrument, and constructed those that have proved successful.

Without attempting to explain all the peculiarities of this action, it would seem that the mercury is kept in the narrow bore, and prevented from retreating into the bulb, by friction; but, when a moving force is supplied by means of a change of temperature, the motion of the mercury takes place in that direction in which it is least opposed, or most aided, by its capillary action.

As the result obtained is due to the difference between two forces, neither of which is very great, the construction of such an instrument requires care; and the author will now state what appear to be the chief points which demand attention, as derived from his own experience, and that of those who have interested themselves in the construction of the tubes; although this experience is necessarily very limited.

1st. The tubes should be quite clean and free from moisture.

2nd. They should be in one straight line, and should expand symmetrically into the bulb.

3rd. It seems the best arrangement, to have the narrow tube of flat bore, not too flat; its greatest width being about equal to the