8. With respect to the use of dry quicklime as a desic cative, Saussure seems to suspect that this substance pro duces a degree of dryness less perfect than the fixed alka lies in a caustic state ; and that, at any rate, the slowness with which it attracts moisture, renders it less fit for the purpose. Strongly concentrated sulphuric acid, or paper soaked in muriate of lime, and then well dried, absorb moisture more rapidly than either : the former, in particu lar, when inclosed under a receiver with a given volume of air, causes the index of a hygrometer to advance as many degrees towards extreme dryness, in a few hours, as dry quicklime would do in as many days. As the attrac tion of sulphuric acid for moisture varies, however, with its degree of concentration, it is proper to employ it always of the same specific gravity, otherwise the point of ex treme dryness will not be the same in all hygrometers. An uniformity of scale would thus be obtained, whether the point of extreme dryness were absolute, or merely re lative.
9. For examining the hygroscopic state of small quanti ties of air, Dc Luc gives the instrument a form, which renders it more commodious for being introduced under a receiver. Fig. 3. represents a front view of the instru ment constructed for that purpose, of its actual size. a a a a is a frame of brass, which is connected with a similar Irame behind. The dotted line cbbbd represents the slip of whale bone fixed at c to an adjusting screw, passing over the ponies b b b b, and joined at d to a slender metallic plate of annealed silver. This metallic plate d of move a over a pulley e, and is joined at f to the moveable part g of the vernier h h h h. From the top of the moveable vernier i. proceeds another slender metallic plate of the same pliant material, the opposite extremity of which is fixed to the pulley k. This pulley is supported on the same axis with a smaller pulley 1. which is connected, by means of a slen der metallic plate, with the upper extremity of the bent lever 7n, n, the shorter arm of which n is pressed by a spring. It is easy to see from the figure of the instrument, that when the whale-bone is dilated by absorbing moisture, the vernier will ascend by the action of the spring upon the lever ; and vice versa.
10. The instrument may be fitted up in the form of a watch, by fixing the whale-bone to the circumference of a wheel, and distending it gently by a weak spring. The contraction and enlargement of the whale-bone might then be indicated by a hand fixed to the arbor of another wheel, and moving along the circular graduation of a dial-plate. Other contrivances of a similar kind will readily suggest themselves to the ingenious artist.
1 I. Hygrometers have also been constructed of sub stances, which seem to suffer a dilatation in all directions by moisture. On this principle, De Luc constructed some time ago a hygrometer of ivory, by giving a portion of that substance intended to be affected by moisture the form of a thin slender tube, and then inserting into it, at one ex tremity, a capillary tube of glass, about 14 inches long, and inch in diameter. The ivory is recommended to be taken from an elephant's tooth of considerable size, a few inches from the top, and as near the surface as possible, in order that it may be of an uniform grain in different in struments. This hygrometer is represented in Fig. 4. Plate CCCXXI. where b g b represents the ivory tube open at b b, and shut at g. This tube, which is 2 inches 8 lines in length, and 2i lines in diameter, internally, is bored in the direction of its fibres, and reduced by turning it on a lath, till its thickness is about of a line, except at the two extremities, where it is left somewhat thicker, to give it greater strength. The piece a a d d, which is
made of brass, connects the ivory with the glass tube, by means of gum mastich, or any other adhesive substance. The part of the ivory tube b c b c, is protected from the moisture of the air by a brass verrel, which prevents it from splitting. Before being fitted up, the ivory tube is usu ally moisted on the outside, which may be done very con veniently, and to a proper degree, by surrounding it with wet cambric, and allowing it to remain till the moisture per meates through the ivory.
12. The mercury is introduced by first inserting a horse hair into the bore of the glass tube, sufficiently long to pass completely through it, and reach about an inch be yond its lower extremity, into the ivory tube. A slip of paper, four or five inches long, is then wrapt round the up per part of the glass tube, and tied tightly to it ; leaving about three inches of the paper projecting beyond it, so as to form a kind of funnel for pouring the mercury into the tube. A quantity of mercury being then poured into the paper tube, is afterwards gradually made to descend into the ivory tube, partly by gentle agitation, and partly by means of the horse hair, which is moved continually up wards and downwards, till the whole of the air is extricat ed, and the instrument filled with the proper quantity of mercury. This being done, the range of the scale is ad justed in the following manner : The instrument is im mersed in a vessel of water, kept at the freezing point, by little bits of ice floating in it, where it is allowed to remain until the ivory has attained its greatest possible dilatation, which is known by the mercury in the tube becoming sta tionary. The lowest point which it reaches is then mark ed as the zero of the scale. One fixed point being thus obtained, the other divisions are determined by previously ascertaining the relation subsisting between the internal capacity of the ivory tube and that of the glass tube, and graduating the instrument accordingly. For this purpose, De Luc employed a glass tube which had formerly been used as a thermometer, the divisions of which he and the bulb of which • was intentionally broken, in order that the quantity of mercury contained in it might be accu rately weighed. The weight of the mercury in the ivory tube being also determined, it was easy to form a new ther mometrical scale adapted to the glass tube and the ivory bulb. For let M be the weight of the mercury in the ori ginal thermometer, D the length of a degree upon its scale, in the weight of the mercury in the ivory tube, and d the length of a degree upon its scale, we have evidently, The scale affixed to the glass tube was accordingly divided into as many divisions as it admitted, each of which was made to equal d. By this mode of forming the scale, it is unnecessary to observe, that the instrument would be in reality a thermometer, provided the ivory were not affect ed by moisture ; but the ivory being dilated and contract ed by moisture, will give rise to corresponding deviations from the mere effect of temperature, and thus, by means of an attached thermometer, the effect due to dryness or moisture may be accurately separated from the rising and sinking of the mercury in the glass tube, by means of heat and ccld. The excess of the hygrometrical degrees above the degrees indicated in similar circumstances by an ordi nary thermometer, is to be considered as the sole effect of dryness in contracting the ivory ; while a difference of a contrary kind is to be ascribed to an opposite cause.