Page:Proceedings of the Royal Society of London Vol 4.djvu/150

Rh specimen of lace which Mr. Talbot exhibited to the Society, and which was made five years ago, was preserved in this manner. But his usual method of fixing is different from this, and somewhat sim- pler, or at least, requiring less nicety. It consists in immersing the picture in a strong solution of common salt, and then wiping off the superfluous moisture and drying it. It is sufficiently singular that the same substance which is so useful in giving sensibility to the paper, should also be capable, under other circumstances, of destroy- ing it, but such is nevertheless the fact. Now, if the picture which has been thus washed and dried, is placed in the sun, the white parts colour themselves of a pale lilac tint ; after which they become in- sensible. Numerous experiments have shown the author, that the depth of this lilac tint varies according to the quantity of salt used relatively to the quantity of silver. But by properly adjusting these, the images may, if desired, be retained of an absolute whiteness. He mentions also, that those preserved by iodine are always of a very pale primrose yellow, which has the extraordinary and very remark- able property of turning to a full gaudy yellow whenever it is ex- posed to the heat of a fire, and recovering its former colour again when it is cold.

A paper was also read, entitled, " A Description of a Hydro- pneumatic Baroscope." By J. T. Cooper, Esq., Lecturer on Che- mistry.

The liability of the ordinary mercurial barometer to derangement and to fracture, led the author to the construction of an instrument for measuring atmospheric pressure that should be exempt from these objections. It consists of a float, formed by a brass tube, having the shape of the frustum of an inverted cone, nine inches long, two inches in diameter above, and one inch below, and its content being about fourteen cubic inches. From the centre of the upper and wider end, which is closed, a brass wire proceeds, sur- mounted by a cup, for the purpose of holding such weights as are necessary for bringing the float, when immersed in water, to the same constant level. The lower and smaller end of the tube is closed by a brass plug, sufiiciently heavy to sink the instrument to the proper depth, and maintain its position, and having a small perfora- tion in its centre. This float is inclosed in a case, containing the water in which it is to be immersed, and which is to be raised to a constant given temperature by a spirit lamp burning beneath it. The float being first filled with water, a given portion of this water is poured out into a measure of known capacity, and is consequently replaced by an equal volume of air, the dilatations or contractions of which will, when the temperature is constant, be dependent only on the external pressure of the atmosphere 5 and the latter will, there- fore, be indicated by the weights requisite to be placed in the cup of the float, in order to maintain it at the same level in the fluid, on the principle of the hydrometer. The author gives a minute description of all the parts of the apparatus, of the method of using it, and of the adjustments and calculations required for determining by its means the difference of level of two stations.