H Y D H Y D 433 head, gives a remarkably aged expression to the child. There is generally defective development in other respects, the body being ill nourished, the bones thin, the hair scanty and fine, and the teeth carious or absent. As illustrating the extent to which this disease may proceed, it may be mentioned that the average circum ference of the adult head is about 22 inches, while in the child it is of course considerably less. In chronic hydro- cephalus the head of an infant three months old has been known to measure 29 inches ; and in the well-known case of the man Cardinal, who died in Guy s Hospital, the head measured 33 inches. In the museum of the faculty of medicine in Paris there is a hydrocephalic skull measur ing 39 inches. In aggravated cases the head cannot be supported by the neck, and the patient has to keep in the recumbent posture. The expansibility of the skull pre vents destructive pressure on the brain, yet this organ is materially affected by the presence of the fluid. The cerebral ventricles are widely distended, and the convolu tions flattened, while occasionally the fluid escapes into the cavity of the cranium, which it fills, pressing down the brain to the base of the skull. As a consequence of such changes, the functions of the brain are interfered with, and in general the mental condition of the patient is impaired to a greater or less extent. The child is dull and listless, irritable, and sometimes imbecile. The special senses become affected as the disease advances, especially vision, and sight is often lost, as is also hearing. Towards the close paralysis is apt to occur. Hydrocephalic children rarely live long, generally dying from the malady in a few years, or succumbing to some of the disorders of childhood, which they are little able to resist. Nevertheless there have been many instances of persons with this disease reaching maturity, and even living to old age. It must also be borne in mind that there are grades of this affection, and that children may present many of the symptoms of it in a comparatively slight degree, and yet recover, the head ceasing to expand, and becoming firmly ossified. Various methods of treatment have been employed in this disease, but the results are seldom satisfactory. Com pression of the head by bandages, and the administration of mercury with the view of promoting absorption of the fluid, are now little resorted to. Tapping the fluid from time to time through one of the spaces between the bones, drawing off a little, and thereafter employing gentle pressure, has been tried, but seldom with permanent benefit. On the whole, the plan of treatment which aims at maintaining the patient s nutrition by appropriate food and tonics, is at once the most rational and successful, provided it be resorted to in time to admit of the arrest of the progress of the symptoms. (j. o. A.) HYDROGEN (from v8wp, water, and yeiWco, to generate) is a chemical element which in the free state occurs in volcanic gases, but exists for the most part in combination with oxygen as water. As its elemental characteristics and the chief compounds of hydrogen have already been described in the article CHEMISTRY, vol. v. pp. 478, 483, 491, 499, 544, &c., it is here only necessary to refer to tiro liquefaction of gaseous hydrogen. 1 At the close of 1877 and in the beginning of 1878, not only hydrogen, but all the so-called permanent gases, were reduced to the liquid state, an achievement the more remarkable as it was the result of the simultaneous but entirely independent labours of two distinguished physi cists, M. Cailletet of ChjlJillon-sur-Seine and M. Pictet of Geneva. The experiments of the former, who clearly demonstrated the possibility of liquefying acetylene, car- bonic oxide, hydrogen, methane, nitric oxide, nitrogen, The statement in the article CHEMISTRY, vol. v. p. 479, that hydrogen has never been liquefied, was true at the date of publication. and oxygen, are described in detail in the Annales tie Chimie et de F/ii/siyue, ser. 5, vol. xv. p. 132 ; those of M. Pictet, who operated only upon oxygen and hydrogen, are detailed in the same journal, ser. 5, vol. xiii. p. 145. The instrumental means employed by them were very dif ferent, as will be evident from the following description. M. Cailletet s apparatus is represented in the annexed sketch (fig. 1). The gas under experiment is contained in a stout glass tube of narrow bore of the form shown in fig. "L FIG. 1. Cailletet s Apparatus for Liquefaction of Gases. To fill this tube with gas, both ends being open, a globule of mer cury is first introduced at the lower curved extremity ; the tube is then placed in a nearly horizontal position, the curved extremity is connected with the holder containing the gas, or with the apparatus in which the gas is being evolved, by means of caoutchouc tubing, and a current of the pure dry gas is passed through the tube until the air is entirely ex pelled ; this being effected, the point opposite to the curved extremity is sealed in the blowpipe flame ; the tube is then brought into a vertical position, so that the globule of mercury closes the lower ex tremity, the caoutchouc tube is withdrawn, and the tube A A thus filled is screwed into its place in the cylinder 15. The lower part of the cylinder contains mercury, the upper part water. The pressure is exerted by forcing a plunger into a massive steel cylinder filled with water by means of the screw and wheel seen on the left in fig. 1, the water being forced from this cylinder through the fine coiled tube into the upper part of the cylinder in which the glass tube is fixed ; by rapidly turn ing the wheel seen on the right of the figure, a valve may be opened, allowing of the escape of the water, and thus the pres sure on the gas may be sud- denly withdrawn. The pressure to which the gas is submitted is measured by means of eitlijcr of the manometers seen on the right. The gas may be cooled by surrounding the tube A with liquid sulphur dioxide, car bon dioxide, or nitrous oxide, and the deposition of ice on the out side of the cylinder containing the refrigerating liquid is prevented by covering it with a glass cylinder or bell jar, under which is placed some desiccating material. By strongly compressing a gas in this apparatus, and then suddenly relieving it from pressure, an enor mous reduction in its temperature is effected, owing to the sudden expansion of the gas, and it is under these circumstances that lique faction takes place in cases where pressure alone is ineffective. On submitting hydrogen to a pressure of nearly 300 atmospheres, and then suddenly withdrawing the pressure, M. Cailletet observed the formation of a fine mist in the interior of the tube ; the experiments of Andrews and his own previous observations bad shown that this result afforded incontestable proof of the presence of liquid, if not of solid, particles.
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