above and below this mark on the basis that the amount of the stem between any two scale divisions should equal of the volume of the part immersed in water. With an instrument graduated in this manner the specific gravity is obtained by dividing the reading into 100. In practice, it is more convenient to use a hydrometer whose scale is graduated to read specific gravity direct, but in this event the scale is ir regular, and great care must be exercised in the construction and testing of the instrument. The accuracy of a hydrometer is greatly increased by making its stem as slender as possible, and consequently increasing the space between the divisions, so it is customary to construct a number of instruments, each having a limited range, and designed for liquids of different densities. The hydrometer when used for a special purpose or substance, frequently undergoes modifications. The alcoholmeter, for example, is so graduated as to give at once the percentage of pure alcohol in a mixture of alcohol and water. The urinometer, lactometer, sacharimeter, and other instruments are all hydrometers used for special purposes. Hydrometers with arbitrary scales also play an important part in scientific work, though the present tendency is toward the exclusive use of specific gravity values. In the Baumé scale, which is encountered frequently in chemical processes, a second fixed point on the hydrometer stem is determined in addition to that given by water. This is obtained by dissolving one part by weight of common salt in nine parts of water, and then the space between these two points is divided into ten equal parts, which are called degrees. The water-point is marked 10 on the scale, and the division is carried beyond for 40 degrees. For liquids heavier than water the second fixed point is determined by immersion in a solution of 15 parts of salt in 85 parts of water, and the space between it and the water-point, which is marked 0, is divided into 15 equal divisions. The scales of Cartier and Beck are also used, though less frequently than that of Baumé. The following tables from Kohlrausch, Leitfaden der praktischen Physik (Leipzig, 1900), afford a comparison of these scales with the true values of specific gravity:

LIQUIDS LIGHTER THAN WATER

which the instrument is made, or to use a thermometer, and then apply suitable corrections. For this purpose many of the finer hydrometers contain within their stems a thermometer tube whose bulb is placed in the lower portion of the instrument.

In the United States Internal Revenue Service the hydrometers furnished to the inspectors are so graduated as to indicate the number of parts by volume of proof spirit equivalent to the volume of the liquor at the standard temperature, which is 60° Fahrenheit. They are constructed so as to read 100 for proof spirit, and 200 for absolute alcohol. Proof spirit in the United States is defined by law to be that mixture of alcohol and water which contains one-half of its volume of alcohol, the alcohol when at a temperature of 60° Fahrenheit being of specific gravity .7939 referred to water at its maximum density. Proof spirit has at 60° Fahrenheit a specific gravity of .93353, 100 parts by volume of the 50 consisting of

same

parts of absolute alcohol, and 53-71 parts of water.

A hydrometer of a different type from those described above is the weight hydrometer, where the subvolume merged

remains

constant, but as the specific gravity of the liquids changes, the weight of the instrument must be varied in order to immerse it to a given point. The Nicholson hydrometer is representative of this class, and consists of a brass tube with conical ends, which floats upright and carries above a thin stem the carrying pan in which may be placed a substance whose specific gravity is

SET OF UNITED STATES

NAL REVENUE OFFICE.

to be found. When used to determine the specific STANDARD HYDROMETERS gravity of a liquid the AS USED BY THE INTERweight of the apparatus is first ascertained, and then it is placed in water and weights added until a marked point on the stem is at the surface of the water. If the instrument is placed in a liquid of greater specific gravity, then additional weights must be placed on the pan in order to sink the stem to the mark, while if the liquid is less dense the number of the weights must be diminished. The weight of the instrument increased by the amount of the weights added when the instrument was placed in water, divided by the weight of the instrument and the weights added when the instrument was in the liquid under test, will give the specific gravity. This hydrometer can also be used to determine the specific gravity of a solid, in which case the latter is first placed in the upper pan, while the instrument is in water, and the number of weights which must be removed is noted. The substance is then placed in the lower pan, and the amount of weight which must be removed to restore it to its former position ascertained. The difference in the two amounts

represents the weight of the water displaced by the substance, and if divided into the sum of the weights removed when the body was placed in the upper pan, will give the specific gravity. The hydrometer of Fahrenheit is based on a similar principle, but is made of glass instead of metal, and has a bulb filled with mercury at its lower end instead of a weighted cup. It can only be used for liquids. These instruments are not as reliable as the ordinary hy. drometers, and are not as widely used. In the most accurate determinations of specific gravity a chemical balance is employed, and equal volumes of the liquid actually weighed, or if the substance is a solid it is weighed both in the air and in distilled water, suitable corrections being applied for temperature, and other disturbing influences. The hydrometer is without doubt one of the earliest pieces of physical apparatus, its invention being generally ascribed to Archimedes, to whom is due the principle on which it is based. The instrument is mentioned by Priscian, who died about A.D. 500, and it is also described by Synesius of Ptolemais in a letter to Hypatia of Alexandria, under the name of hydroscopium, as follows: "It is a cylindrical tube the size of a reed or pipe, a line drawn HYDROMETER. along it lengthwise which intersected by others, and these point out the weight of water. At the end of the tube is a cone the base of which is joined to that of the tube so that both have but one base. This part of the instrument is called baryllion. If it be placed in water it remains in perpendicular direction so that one can readily discover the weight

NICHOLSON

is

of the fluid." The date of this letter can be approximately fixed by the fact that Hypatia was murdered in A.D. 415. The use of the hydrometer

was known to the Saracens of the tenth and

eleventh centuries, and one of their writers, AlKhâzinî, attributes its invention to a Greek philosopher named Poppius, a contemporary of Theodosius the Great. In this connection this same writer refers to the fundamental discovery of Archimedes.

HY DRONI'TRITES. ACID.

See HYDRONITROUS

HY'DRONI'TROUS ACID, HN. An acid gaseous compound of hydrogen and nitrogen first obtained by Curtius in 1890. An easy method of preparing this acid was subsequently worked out by Angeli, and consists in adding hydrazine hy drate (see HYDRAZINE) to a concentrated solution of silver nitrate, the result being a precipitate of silver, hydronitrite, AgN, from which free hydronitrous acid may be obtained by decomposing with sulphuric acid. The salts of hydronitrous acid, or hydronitrites, are violently explosive compounds, and their preparation should not be undertaken by inexperienced persons.

HYDROP'ATHY (from Gk. űdop, hydōr, wateráloç, pathos, disease). A synonym for hydrotherapy, the method of treating diseases by external and internal use of water. See HYDROTHERAPY.

HY'DROPHANE (from Gk. dwp, hydōr, water + pavós, phanos, clear, from paive,

phainein, to appear). A translucent variety of opal that is either whitish or light-colored, and becomes more transparent or translucent in water.

HYDROPH'IDÆ. See SEA-SNAKE.
HY'DROPHILIDÆ

(Neo-Lat. nom. pl., from Gk. dop, hydōr, water + pios, philos, loving). A family of scavenger beetles, most of which swim in the water or crawl on the submerged parts of plants in quiet pools. They are elliptical black beetles with club-shaped antennæ. Because of the fact that they carry a film of air on the ventral surface of the body, this part has a silvery appearance. The larvæ are carnivorous, but the adult beetles live chiefly on decaying vegetation. There are one hundred and fifty species of this family in the United States. One of the most common species is Hydrophilus triangularis. A few forms live in moist earth or dung, and subsist upon maggots which inhabit such places.

HYDROPH’ILOUS. A term which has been applied particularly to plants which are pollinated through the agency of water. It has also been applied by some authors to hydrophytic plants, but should be entirely discarded there, both because of its previous use in another sense and because it is an undesirable word in all genses. The term water-pollinated is preferable in the first sense. See POLLINATION.

HY'DROPHO'BIA (Lat., from Gk. idpopoßia, hydrophobia, from idpopóßos, hydrophobos, dreading water, from dop, hydōr, water + φόβος, phobos, fear), or RABIES, also called LYSSA. An

acute infectious disease of warm-blooded animals

produced in man by the implantation of a specific virus through the bite of an animal sick with the disease. This disease has been known since the earliest historical times. Democritus

of Abdera gave an account of it in the fifth century B.C. Aristotle in the fourth century B.C. described it. Celsus, B.C. 21, detailed some of its

symptoms. Xenophon, Ovid, Vergil, Horace, Plutarch, all referred to rabies in their works. Boer

haave (q.v.) and Van Swieten described the disease intelligently. John Hunter (q.v.), Magendie (q.v.), Marochetti, Morgagni, Trousseau, and Dupuy all wrote of it. Youatt (1850), the celebrated veterinarian, considered rabies at length in his books, and relied on early cauterization with nitrate of silver to obviate infection. Yet after being bitten seven times with impunity, he is reported to have died of rabies at the last. Virchow and Von Ziemssen propagated the disease by the hypodermic injection of saliva of a rabid animal. After many others had added to our knowledge of hydrophobia, Pasteur (q.v.), in 1882, began to treat the disease with scientific accuracy.

The geographic distribution of the disease is wide. It has been found in Greenland, where it was epidemic in 1860; in Constantinople, where it prevailed in 1839; in Egypt, in London, in Hamburg, in Saxony, in Bavaria, and in many parts of the United States, the prevalence of the disease in Washington, D. C., in 1899-1900 raising grave apprehension. In Russia and Siberia very many instances of rabid wolves have been recorded. The disease is found most frequently in the dog, the wolf, the cat, and the cow. The skunk, also, seems to be peculiarly liable to hydrophobia, and the disease in this animal has

been considered distinct and worthy its own name of rabies mephitica. A careful consideration of all the known facts, however, leads to the conclusion that the disease is one to which all the lower animals, and man also, are liable; that it is protean in its manifestations, chiefly because of the thousand and one concomitant circumstances and elements; and that the collected statistics are mostly so tainted, from manifold sources of error, that they need to be scanned and sifted with the utmost discretion and care. It is, however, safer to treat every skunk-bite as though certainly by a rabid animal, since in most cases the facts cannot be known. (The curious will find the question most interestingly discussed in a candid manner in Coues's Fur Bearing Animals.) The nature of the infecting virus is as yet unknown. It is probably found in the saliva. In man there is a variable incubation from three weeks to six months, the period depending largely on the site of the infection. Wounds about the neck and face are especially unfavorable in prognosis; next are those of the hands. Punctured wounds, because of the difficulty of cauterizing them, are most dangerous. Three stages are described. In the premonitory stage there is pain, numbness, or irritation about the bite. Irritability, nervousness, and depression are common. In the stage of excitement there is great hyperesthesia. Almost any slight stimulus will cause intense reflex excitability and convulsions. The muscles of deglutition are extremely rigid, or even paralyzed, and any attempt at swallowing may cause spasms of the throat and general convulsions; these may even follow the thought of swallowing. The name hydrophobia is erroneous, and based upon false inferences from these facts. The sufferer from rabies does not fear waterhe intensely desires it, as he is devoured with thirst, but he cannot swallow it, nor even "go through the motions" of swallowing. There is also a secretion in the throat and mouth of a thick, viscid mucus, with thickened saliva, and the effort to get rid of this with muscles refusing to act, causes the bark-like cough and hawk so often described as 'barking like a dog.' The patient does not bark, and it is doubtful if he bites. The temperature rises, and the patient may become maniacal. This stage may last from one to four days, and then the paralytic stage supervenes. In this stage the spasms stop, unconsciousness supervenes for the first time, the action of the heart ceases, and the patient dies. The pathological changes found after death from hydrophobia are not characteristic. The most constant are found in the nervous system, especially in the region of the medulla oblongata and pons. The changes consist in a varying degree of inflammation, marked by small roundcell infiltration of the blood-vessel walls, exudation into the pericellular lymph-spaces, small hemorrhages, and sometimes thrombosis of the small blood-vessels. More recently extensive degenerative changes in the nerve-cells have been described. Lesions have also been noted in the sympathetic, consisting in degeneration of the nerve-cells and increase in the thickness of their endothelial capsules. In addition to the changes in the nervous system, there is usually congestion of the mucous membrane of the gastro-intestinal tract and of the pharynx, larynx, and bronchi. Despite the fact that innumerable attempts to discover the cause of the disease have been made

without success, it still seems probable that hydrophobia is due to a micro-organism. This organism is not apparently widely distributed throughout the body, but confined mainly to the saliva and the central nervous system. An emulsion made of the medulla of a rabid animal, injected into dogs, cats, rabbits, guinea-pigs, etc., produces symptoms characteristic of the disease, although in rodents there is little or none of the stage of excitement.

Although we have no knowledge as to the specific germ of the disease, hydrophobia furnishes our most remarkable example of the success of artificial immunization by means of protective inoculation. To the French student Pasteur (q.v.) is due the credit not only of the discovery of the preventive treatment of hydrophobia, but of demonstrating through it a principle in therapeutics which is of constantly widening applica tion. Pasteur found that he could induce the disease in rabbits by inoculations with portions of the spinal cords of rabid animals, and that the spinal cords of these rabbits possessed a high degree of virulence. Drying in air reduced the virulence in direct proportion to the length of the drying. It was found that, while inoculation of man or animals from the fresh rabbit-cords was invariably fatal, if the man or animal was first inoculated from one of the cords the virulence of which had been greatly reduced by the drying, and then from cords of gradually increasing virulence, he could become so accus tomed to the virus that injection of the fresh cord would no longer be fatal, and such a series of inoculations made sufficiently soon after the bite of a rabid animal was found to prevent entirely the development of the hydrophobia. Advantage is taken of the long period usually elapsing between the bite and the onset of the disease to practice these preventive inoculations, and the result has been a marked decrease in the mortality from the bites of rabid animals. In view of the uniformly fatal results of hydrophobia and the success of the Pasteur treatment, the importance of determining at the earliest possible moment whether the animal by whom a person has been bitten had rabies can be readily appreciated. The animal should not be killed, for as rabies is invariably fatal to canines, the recovery of a sick animal definitely disproves rabies. On the contrary, the animal should be carefully watched, and if it dies, should be sent to a laboratory where examination can be made and the question of rabies definitely settled by inoculation experiments on animals. The treatment should be cauterization in every case, less than one hour after the bite, by means of the actual cautery, strong acid, or acid nitrate of mercury. Wounds must be opened by the surgeon, and even amputation may be necessary. Washing and syringing wounds with water at 130° F. is desirable. Sucking the wound to draw out the poison has been practiced, and may be safely done if there are no breaks of the membrane of the lips or mouth. Administration of morphine or alcohol does harm. Immunizing by the Pasteur method should be practiced in all cases. By this method the patient is inoculated with attenuated virus by the injection hypodermically of emulsion made from the brain of a rabid animal, repeated in stronger and stronger concentration during twenty-one days. The results of the Pasteur method are now indisputable,