Richter’s hydrometer resembles, for the most part, Gay-Lussac’s.
Syke’s hydrometer is that adopted by the Revenue authorities in England for ascertaining the strength of spirits, and has been already fully noticed.
Tralles’s hydrometer resembles Gay-Lussac’s (nearly).
Twaddell’s hydrometer is much used in the bleaching establishments of Scotland, and in some part of England. According to this scale, 0 is equal to 1000 or the sp. gr. of distilled water, and each degree is equal to ·005; so that, by multiplying this number by the number of degrees marked on the scale, and adding 1·, the real specific gravity is obtained.
Obs. Hydrometers, unless manufactured with great care and skill, merely afford approximate results; but which are nevertheless sufficiently correct for all ordinary purposes. They also require several ounces of liquid to float them, and hence cannot be used for very small quantities. Those of Fahrenheit, Nicholson, and Sykes are the most accurate, both in principle and application. They are all employed with a tall glass cylinder termed a sample, test, or hydrometer glass, in the way already noticed; but the thermometer for ascertaining the temperature must be covered with a glass case, or arranged with a folding scale to allow of its immersion in corrosive liquids.
Alcoholometers, Elaiometers, Saccharometers, Urinometers, &c., are simply hydrometers so weighted and graduated as to adapt them for testing spirits, syrups, urine, &c. See Alcoholometry, Alonholmetry, Areometer, Specific Gravity, &c.
HYDROM′ETRY. Syn. Areometry. The art of determining the specific gravity of liquids, and hence their strength and commercial value. The instruments used are noticed above; their action depends upon the fact that a floating body displaces a bulk, equal to itself in weight, of the fluid in which it floats, and consequently that a body of a given weight sinks deeper in a lighter than in a heavier fluid. In hydrometric determinations the temperature of the samples must be carefully attended to, for fluids expand as their temperature is increased. The hydrometers used in England are generally adjusted to the standard temperature of 60° Fahr., and when ‘Hydrometer Tables,’ giving the corrections for the variations of the thermometer, are not accessible, the fluids to be examined should be brought to this standard temperature by applying heat directly to the vessel, when the temperature is below the standard, or by surrounding the vessel, with cold water, when it is above the standard. The principal applications of hydrometry are described in different parts of this work. See Acetimetry, Alcoholometry, Chlorometry, Specific Gravity, &c.
HYDROP′ATHY. Syn. Water cure; Hydropathia, L. A mode of curing diseases by the copious use of pure cold water, both internally and externally, together with dry sweating, and the due regulation of diet, exercise, and clothing. This “treatment of diseases undoubtedly includes powerful therapeutic agents, which, in the hands of the educated and honourable practitioner, might be most beneficially resorted to as remedial agents.” (Pereira.)
HYDROPHO′BIA. Syn. Canine madness; Rabies canina, L. A disease which is generally considered as the result of a morbid poison being introduced into the system by the bite of a rabid animal. A clear case of idiopathic or spontaneous hydrophobia has never yet been known to occur in the human subject.
The common symptoms of hydrophobia are great excitability and horror at the sight of water, or the attempt to drink, fever, vomiting, excessive thirst, spitting of viscid saliva, difficult respiration, irregular pulse, convulsions, syncope, delirium, and death.