HYPHOMI 



ilYI'liOPHOniA. 



7M 



To the inferior extremity of the item, from which the hollow tail a 

 bout uiie inch ilictont, a permanent pear-shaped weight is attached ; 

 so that \\hrn the instrument U placed on a fluid, the other extremity 

 may float perpendicularly to the surface. There are aUo ten weights 

 uf different magnitudes, nine of which are circular, and applicable by 

 means of a slit to the lower branch of the item. These are marked 1 0, 

 80, SO, 40, 50, 60, 70, 80, and 90 respectively, and by their successive 

 application the instrument may be sunk so as to obtain the complete 

 range of specific gravity, from that of pure alcohol to that of distilled 

 water. The other weight u of the form of a parallelepiped, and may 

 be fixed when necessary to the upper branch of the stem. The upper 

 branch of the stem is divided into ten equal parts or degrees, each of 

 which u subdivided into two parts. The whole is adjusted at thu 

 temperature of 60 Kahr., and tables are computed whereby the neces- 

 sary corrections may be determined for all variations above or below 

 that point. In order to determine the strength of spirit by means of 

 the hydrometer, a portion is placed in a tall glass cylinder, and the 

 temperature observed. One or more of the circular weights is then 

 attached to the lower stem of the instrument, so that the lower extre- 

 mity of the scale may sink beneath the surface of the fluid, and when 

 the whole has become stationary the number upon the scale in con- 

 tact with the surface of the fluid is observed. This number added 

 to the number marked upon the circular weight employed will give a 

 third number, adjacent to which, in the tables above mentioned, and 

 under the head of the proper temperature, will be found the per 

 centage of strength required. 



Sykf* Hydrometer. 



The most convenient method of obtaining the specific gravities of 

 fluids U by means of what chemists call a */ "It, or a 



tkotaand yrai* bottle. This is a bottle of a globular form, with a 

 ground-glass stopper, ra adjusted as to contain exactly 1000 grains of 

 dwtilled water, at the temperature of 00 Fabr., and accompanied by a 

 weight, which is an exact counterpoise for the bottle when thus filled. 

 In order to determine the specific gravity of a fluid by this means, it is 

 simply necessary to fill the bottle with that fluid at the temperature of 

 60*, and place it in one of the scale-pans of a delicate balance, the 

 counterpoise being placed in the other scale-pan ; then the number of 

 grains which it will be found necessary to add to one of the scale-pans, in 

 order to produce equilibrium, will be the difference between the specific 

 gravity of the fluid and that of water taken at 1000. For example, 

 the bottle filled with the sulphuric acid of commerce will require about 

 845 grains to be added to the counterpoise to produce equilibrium, 

 whence we say that the specific gravity of such a sample of sulphuric 

 acid U 1-846. Whereas if the bottle were filled with spirit of winr, 

 we should have to add weights to the scale-pan containing the bottle, 

 and in the can* of what is called proof ipint, 80 grains would have to 

 be so added, in which case the specific gravity would be 0-920. 

 [ALCOHOLOHFTRT ; SPECIFIC GRAVITY.] 



Various forms of hydrometer have been contrived under different 

 name*, one or two of which we may mention, such as the Arttonuter 

 (from Apofi, " light" or " thin," and nirpor, " a measure "). This in 

 employed for the purjmse of ascertaining the specific gravities of llniil -. 

 generally such as are less dense than water, but it may be used to 

 determine those of any fluids, and also of solid bodies. Its best form 



is probably that which was devised by Nicholson. The instrument 

 which bears the name of tint rx|>erimcnter consist* of a hollow 

 cylinder, or sphere, of copper, from each extremity of which proceeds a 

 stem ; that which, when the instrument is in a vertical position, is upper- 

 most, is a wire of hardened steel about l-40th of an inch in diameter, 

 carrying at its top a small cup, in which weights may be placed : to 

 the lower stem, which is short, is attached a stirrup carrying a cup in 

 which may be placed a solid body whose specific gravity it may be 

 required to determine. The instrument is so adjutrd. that if a weight 

 equal to 1000 grains be placed in the upper vessel, the hole will sink 

 in distilled water at the temperature of GO" Kahr., till a mark made on 

 the stem is on a level with the fluid surface. When the specific gravity 

 of a fluid, whether lighter or heavier than water, is to be determined, 

 the instrument is plunged into the fluid, and weights are placed in the 

 upper vessel till the mark on the stem is at the surface ; then, if c- in 

 grains be the weight of the instrument when unloaded, and v' the 

 weight placed in the vessel as just said : also if be the specific gravity 

 of distilled water, and *' that of the fluid in which the instrument is 

 placed, we have, by hydrostatics, 



K + 1000 : ic + tc* : : i : '; 



whence ' may be found. 



To obtain the specific gravity of a solid which does not imbibe water : 

 after placing the instrument in distilled water, let the body be laid in 

 the upper vessel, and weights, in grains, added till the mark on the 

 stem is at the level of the surface ; these weights being subtracted from 

 1000 grains, leave the weight of the body in air; then placing the 

 body in the lower vessel, let other weights be added in the upper one 

 till the mark on the stem is again at the surface of the water ; then* 

 additional weights will express the loss which the body sustains by 

 being immersed in water. Consequently if ic" be the weight of the 

 body in air, '" the loss just mentioned, the specific gravity of the 

 water, and <" that of the body, we have by hydrostatics, 



"' : ic" : : : " ; 



whence i" may be found. 



If the substance whose specific gravity is required be, like wood, 

 capable of imbibing water during the experiment, it should be left in 

 the lower vessel, while under water, till the instrument is stationary, 

 when the additional weights in the upper vessel will express the weight 

 of the displaced water, together with that which in imbibed ; then, 

 having dried the surface, let the substance be placed in the upper 

 vessel and weighed in air as at first : the difference between this weight 

 and that which was found before the substance was put in the water, is 

 the weight of the water imbibed ; and this must of course be subtracted 

 from the observed weight of the substance in water, previously to 

 substituting the values of the terms in the lost proportion. 



For fluids of greater specific gravity than water, such as the concen- 

 trated acids, an instrument of the like kind which was invented by 

 Guyton de Morveau, and is designated " gravimeter," may be used. 

 (Gregory's ' Mechanics,' vol. ii.) Instruments of this kind, however, 

 are seldom resorted to, the best plan being to moke use of the hydro- 

 static balance, as will be more particularly described under Sri-iiir 

 GRAVITY. 



HYDRONITROFERROCYANIC ACID. [CYANOGEN.] 



HYDRON1TROPRUSSIC ACID. [CYANOOKX.] 



HYDROPERSULPHOCYANIC ACID. [CTAXWIES.] 



HYDROPHO'BIA (from Stop, water, and <fxf/3os, fear) is the disease 

 occasioned by inoculation with tic saliva of a rabid animal, and is so 

 called from the violent and suffocating spasms of the throat which 

 occur whep the patient attempts to drink, or when, in the latter stages, 

 the mere idea of drinking arises in his mind. The disease is never 

 produced in man by any other cause than the saliva of a rabid anim.il : 

 those cases which have been said to arise spontaneously have not pre- 

 sented all the true characters of the affection, and have in general I ecu 

 only severe cases of hysterical or other convulsions, in many of which 

 the imagination and the fear of real hydrophobia had much influence. 

 Whether it is ever spontaneously generated in animals is less certain, 

 because its origin in them is lees easily traceable ; but the fact that it is 

 possible to ward off the disease from pocks of dogs, in which every fresh 

 comer is submitted to a kind of quarantine, and the many instances 

 now known of isolated situations in which, although dogs are very 

 numerous, no case of hydrophobia has occurred for many years, tend 

 to prove that in the dog also it arises only in consequence of the bite 

 of some other rabid .inini.il. It is probable that all animals are subject 

 to hydrophobia, for all that we have an opportunity of observing, that 

 is, all our domestic species, are; but it has not n].| in <1 th.it any, 

 except the wolf, fox, cat, and dog, arc capable of communicating it to 

 each other, or to other species. There is no evidence whatever to 

 prove that the disease can be communicated from one human being to 

 another ; men affected by it are not disposed to bite, and it is doubtful 

 whether, if they did bite, the saliva would have any effect, for the 

 experiments made with it upon animals are as yet contradictory and 

 inconclusive. 



The disease may be communicated to man either by the saliva being 

 carried into a wound mode by the tooth of a rabid animal, or by its 

 being placed on the surface of a previous wound, as where dotfi have 



