78.1 



HYDROTIC ACID. 



HYGROMETRY. 



791 



Whtn hydrothorax results from inflammation of the pleura, it gene" 

 rally exists on one side only of the chest. When it is a consequence of 

 obstacle to the circulation through the heart, or of organic disease of 

 the kidney, it is generally double, although the effusions into the two 

 pleural cavities may not take place quite simultaneously. In the latter 

 case, also, hydrothorax is found in connection with general dropsy. At 

 first there is oedema of the lower extremities ; this oedema gradually 

 extends to the integuments of the trunk, to the arms, and even to the 

 face ; and, subsequently, serous fluid is effjised into the cavities of the 

 pleura, giving rise to hydrothorax, and into those of the pericardium 

 and peritoneum. 



A collection of fluid in the cavity of the pleura may be detected by 

 physical signs : a dulness on percussion, and, when the effusion is 

 moderate, a diminution of the respiratory murmur, and the presence of 

 rcgophpny, on the affected side. If the effusion be so considerable as 

 entirely to prevent the expansion of the lung, there is a total absence 

 of aegophony or of any respiratory murmur on that side, whatever be 

 the force with which inspiration is made, while on the opposite side 

 the 'respiratory murmur is unusually audible. 



When one side only is affected, the patient generally lies on that 

 Bide ; when the effusion is double, he lies on his back ; or, which is 

 the case when the effusion is considerable, he can breathe in the erect 

 position only. The difficulty of breathing is generally greater in pro- 

 portion to the rapidity with which the fluid is effused. [PLEUBITIS.] 



HYDROTIC ACID. [SuooBic ACID.] 



HYDRURETS. [HYDBOUEN.] 



HYDRUS (constellation), the Water-make, commonly called the 

 Southern Snake, a constellation of Lacaille. It is situated between 

 the south pole and the bright star in Eridanug (Achernar). 



Character. 



u 



a 

 S 



No. in Catalogue 

 of Lacaille. 



595 



605 

 747 



No. in Catalogue 



of British 



Association. 



603 

 623 

 756 



Magnitude. 

 4 

 3 

 4 



HYGIEIA (*iyia), the goddess of health, called also Salus by the 

 Romans, was the daughter of .-Esculapiue, with whom she was fre- 

 quently associated in worship, both by the Greeks and Romans. She 

 was worshipped aa the goddess of both mental and bodily health, and 

 in some of her attributes resembled Athena, one of whose titles was 

 Athena Hygieia. Representations of Hygieia, both alone and with her 

 father, are very numerous in ancient art. She appears always ag a 

 virgin of beautiful form, and pleasing expression, and is clad in a 

 long full robe. Most frequently, she is giving drink to a serpent from 

 a patera which she holds in her left hand. The annexed cut is from a 

 terra-cotta bas-relief in the British Museum. 



Hygieia from the British Museum. 



UYGRO'METER (typos, moist, and niTpov, measure). In physical 

 experiment* it sometimes becomes necessary to ascertain the quantity 

 of aqueous vapour contained in the atmosphere or other aeriform fluid 

 under examination. To attain this object several instruments called 

 hygrometers have been invented, and are known by the names of their 

 authors, a De Luc's, De Saussure'n, ftc. These for the most part rest 

 upon one common principle, the diminution of bulk which takes 

 place in organic tissues consequent upon the abstraction of moisture. 

 Thus De Luc employed a thin slip of whalebone, the contractions of 

 which indicated the variations of moisture ; and Do Saussure had re- 

 course to a human hair, by means of which he constructed a far moro 

 delicate instrument ; but this was exceedingly liable to derangement, 

 and unless prepared with extraordinary care was uncertain. All of 

 these, however, were nearly superseded by the hygrometer invented 

 by the late Mr. Daniel), professor of chemistry at King's College, 



London, first described by that gentleman in 1819, in the 8th volume 

 of the ' Quarterly Journal of Science ; ' subsequently in his ' Meteoro- 

 logical Essays,' and in his posthumous work on ' Meteorology.' It 

 consists of two thiu glass balls one inch and a quarter in diameter, 

 connected by a glass tube about seven inches in leugth. The tube is 

 bent in two places at right angles so as to form three arms of unequal 

 length, the longest of which contains a small thermometer, whose bulb 

 descends into the lower of the two glass balls. This ball, after being 

 filled about two-thirds with ether, is placed over a spirit-lamp until 

 the vapour of the ether has expelled the contained air through a capil- 

 lary tube which is left open for the purpose, and afterwards hermeti- 

 cally sealed. The other ball is then covered with a piece of muslin, 

 and the instrument thus adjusted is placed upon a stand, to which is 

 attached a small thermometer indicating the temperature of the ex- 

 ternal air. When about to be used a small portion of ether is poured 

 upon the muslin, which, by evaporating, lowers the temperature of the 

 glass ball, and thereby occasions a rapid condensation of the ethereal 

 vapour contained within the instrument. The condensation of the 

 vapour within the tube produces a continuous evaporation from the 

 surface of the ether in the lower ball, by which the temperature of the 

 included ether is continually reduced until a deposit of moisture from 

 the surrounding atmosphere is observed to take place upon the exterior 

 of the glass. At this instant the inner thermometer, which always 

 indicates the temperature of the ether, is observed, and thua the dew 

 point, or that at which the precipitation of atmospheric moisture takes 

 place, is determined with considerable accuracy. Having ascertained 

 the dew point, and likewise the temperature of the external air, the 

 actual quantity of moisture contained in a cubic foot of air will readily 

 be found from the formula, 



* 

 Weight in grains 



- 

 448 + 1 



p, 



where t denotes the temperature of the external air, and p the elas- 

 ticity of aqueous vapour at the temperature indicated by the interior 

 thermometer. The value of p for every degree of the thermometer is 

 given in Mr. Dalton's tables of the expansive force of steam. (' Man- 

 chester Memoirs,' v. 659.) 



But this instrument, so excellent in theory, is very costly on account 

 of its great consumption of ether, and is scarcely useable in hot 

 climates, owing to the difficulty of preserving that liquid. It has also 

 been objected to by competent authority, because an experiment has 

 necessarily to be made to obtain an observation. At the Royal Obser- 

 vatory at Greenwich, however, it appears to have been employed for 

 the determination of the dew-point from the commencement of the 

 meteorological observations in the year 1840 until the end of 1847, 

 since which, to the present time, that element has been exclusively 

 inferred in those observations from the simultaneous reading of two 

 thermometers, the bulb of the one being dry and the other wet, agree- 

 ably to the method which has now almost universally superseded 

 every other, and which is explained in the next article. 



Under the head DEW, some experiments of Dr. Wells and of Mr. 

 Glaisher have been noticed, having an important bearing upon the 

 use of many inorganic aa well as organic substances for hygrometrical 

 purposes, as well as upon the necessity of screening hygrometrical 

 apparatus from the effects of its own nocturnal radiation. 



HYGROMETRY is that part of natural philosophy which relates to 

 the determination of the humidity of bodies, particularly of the atmos- 

 phere : it comprehends also the theory of the instruments noticed in 

 the preceding article, which have been invented for the purpose of 

 ascertaining the quantity of water contained in a given volume of air. 



The experiments of Dr. Dalton have proved that the water received 

 from the earth is not dissolved in the atmosphere, and that it exists 

 thre in the state of vapour, and forming, statically, a distinct 

 atmosphere from that of the air, though coexisting with it in the same 

 space. That philosopher discovered also, that the quantity of vapour 

 contained in a portion of the atmosphere depends greatly upon the 

 temperature of the latter, and that it is very variable even when the 

 temperature is constant. He ascertained moreover that when a 

 quantity of aqueous vapour at a given temperature is diffused through 

 any space, it will support the game external pressure, whether previously 

 that space had been void or occupied by air. On these principles are 

 founded the methods which have been used for determining the 

 absolute quantity of moisture in a given volume of air by means of the 

 hygrometer : the requisite data being the elasticities of aqueous vapour 

 at different temperatures, and the corresponding indications of the 

 instrument. 



The tension or elasticity of watery vapour corresponding to every 

 degree of Fahrenheit's thermometer, from zero to the point of boiling 

 water (measured by the height in inches of the column of mercury 

 which the vapour will support when the density of ; the atmosphere is 

 represented by 30 inches), has been determined by Drs. Dalton and 

 Ure, who for this purpose introduced a small quantity of water into 

 the vacuum of a barometer, and observed how much, at different tem- 

 peratures, the vapour arising from it depressed the column of mercury ; 

 and tables of such tensions are published in the fifth volume of the 

 Manchester Memoirs,' and the ' Philosophical Transactions' for 1818. 



Previously to stating the manner of determining the relation between 

 the indications of an hygrometer and the state of aqueous vapour with 



