DIFINITION OF TERMS. 



157 



pressure of &11 parts of the surface in the 

 vessel is af;;ain restored. 



Although what has been said respect- 



Iing the surfaces of fluids being at the 

 same level (in a system generally con- 

 nected by pipes,) while at rest, and the 

 fluid matter at the same density ; yet 

 when the equilibrium to the system is 

 disti5rbed by any extraneous force an un- 

 dulatory motion will ensue, in which the 

 different parts of the surface will be some- 

 times above, sometimes below, the gene- 

 ral level, and the undulation will con- 

 I tinually diminish until at length friction, 

 and other causes will restore the equili- 

 brium — thus if a tube have one end stop- 

 ped with a plug to which a thread or wire 

 is attached, and the end thus stopped be 

 plunged to the depth of one or two feet 

 .in a vertical direction below the surface 

 of water, and the plug be then suddenly 

 withdrawn, the water will rise in the 

 tube above the surface of the water into 

 which the tube is plunged, and this in 

 proportion to the depth of the lower end 

 of the tube. 



Note. This appears to be the principle 

 upon which Montgojfier's Hydraulic Ram 

 acts to raise water from a lower to a hicrh- 

 er level. The water issuing from this ma- 

 chine, at or near the lower part of it, at a 

 certain velocity closes the orifice, by car- 

 rying up the valve; the current is, by 

 these means diverted, and acts upon the 

 valve opening into the air vessel, which 

 it opeiis, and a portion of water and air 

 rush into the vessel until the pressure of 

 the air in the vessel acting upon the water 

 restores the equilibrium; the valve in the 

 air vessel then shuts; and the first men- 

 tioned valve at the same time opens, by 

 descending in the water. The waterthen 

 runs out of the orifice as at first; and again 

 closes it by shutting the valve. The valve 

 in the air vessel is again opened, and other 

 portions of water and air pass into the air 

 vessel and so on; the valves opening and 

 closing alternately, while the compress- 

 ed air in the vessel by its elastic force 

 causes the water to ascend in the discharge 

 p^pe in a constant stream during each pul- 

 sation, as the valves, alternately open and 

 close, 



. Hydrosulphurets'xn chemistry. Sulphur- 

 eted or sulphurated hydrogen gas, possess 



es the properties of an acid. It is absorb- 

 ed by water, in considerable quantities, 

 and the solution reddens vegetable blues; 

 it combines also with alkalies and earths; 

 and with several metallic oxides. The 

 combination's which sulphureted hydro- 

 gen forms with bases, have been called 

 hydrosulphurets. Sulphuretted hydrogen 

 combines with alkalies, and earths, and 

 forms with them compounds which may 

 be distinguished by the following proper- 

 ties. 



1. They are all soluble in water, and 

 the solution is colorless. 



2. When the solution is exposed to the 

 air, it becomes green or greenish-yellow, 

 and deposits sulphur on the sides of the 

 vessel in the state of a fine black crust. 



3. After long exposure to the air, the 

 solution becomes limpid and colorless; and 

 on examination is found to contain only 

 the sulphate of the base of the original 

 hydrosulphuret. 



4. The solution of the hydrosulphurets 

 precipitates all metallic solutions; iron and 

 lead, black; antimony, orange; arsenic, 

 yellow. 



Hygrometer. A machine, or instru- 

 ment whereby to measure the degrees ol 

 dryness, or moisture of the atmosphere. 



There are several ways of making these 

 instruments, founded upon the circum- 

 stance of certain substances possessing the 

 properties of imbibing and parting with 

 moisture. Thus, a piece of sponge sus- 

 pended to one end of a lever, and the 

 other end of the lever formed into a hand 

 or index to move by the side of a segment 

 of a circle marked off into regular divi- 

 sions or degrees, and a small cord attached 

 to the lever on the index side of the ful- 

 crum with small grains of shot attaclied to 

 it at regular distances asunder, so that 

 when the sponge imbibes the greatest de- 

 gree of moisture, the cord and attached 

 shot is entirely suspended in space, but 

 when the sponge loses a portion of its 

 moisture it rises and the cord descends 

 and a portion of it rests on a small plat- 

 form or floor and in proportion to the de- 

 gree of dryness of the sponge. The in- 

 dex at the same time pointing to the num- 

 ber on the circle opposite, denoting the 

 degree of moisture or weight of the 

 sponge. Or a horizoiital axis may be 



