126 



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. 



