HYD 



HYD 



travasation of the lymph, which, when 

 collected withinside of the bones of the 

 cranium, the hydrocephalus is then term- 

 ed internal ; as it is external, when retain- 

 ed between the common integuments and 

 the cranium. See MEDICINE. 



HYDROCHAR1S in botany, a genus of 

 the Dioecia Enneandria class and order. 

 Natural order of Palmae. Hydrocharides, 

 Jussieu. Essential character: male, spathe 

 two-leaved ; calyx, trifid ; corolla, three 

 petalled ; filaments the three inner style 

 bearing: female, calyx trifid: corolla three 

 petalled ; style six ; capsule six-celled, 

 many seeded, inferior. There is but one 

 species, with many varieties, viz. H. 

 morsus ranse. frog bit. 



HYDROCOTYLE, in botany, marshpen- 

 nywort, a genus of the Pentandria Digynia 

 class and order. Natural order of Um- 

 bellatae. Essential character : umbel sim- 

 ple, with a four-leaved involucre ; petals 

 entire ; seeds semi-orbiculate, compress- 

 ed. There are fifteen species. 



HYDRODYNAMICS treat of the pow- 

 ers, forces, and velocities, of fluids in mo- 

 tion. Having entered fully into the de- 

 tail of all relating thereto, while treating 

 of FLUIDS, HYDRAULICS, HYDROSTA- 

 TICS, MILLS, and WATER Wheels, we 

 forbear from that repetition, which would 

 trespass on the space allotted to other 

 articles, referring the reader to those 

 heads for what appertains thereto. 



HYDROGEN. It had been long known 

 to the chemists, that a vapour or air is 

 disengaged in some processes, which 

 kindled on the approach of an ignited 

 body. Van Helmont gave this the ncme 

 of gas igneum, and it seems to have at- 

 tracted the attention of Boyle, Mayow, 

 and Hales. The chemists knew, that such 

 a vapour or air was commonly disengaged 

 during the solution of certain metals in 

 muriatic or dilute sulphuric acid, that it 

 burnt at the mouth of the phial, and if 

 mixed with atmospheric air, exploded 

 when kindled by a match. 



Mr. Cavendish, however, first examined 

 its properties fully, showed that it is per- 

 manently elastic, not absorbed by water, 

 .and that it is much lighter than atmos- 

 pheric air. (Philos. Trans, vol. Ivi p. 141). 

 This sub stance form ing water when com- 

 bined with oxygen, and being therefore 

 the radical of that compound, the name 

 hydrogen was given to it at the formation 

 of the new nomenclature. 



It is always obtained from the decom- 

 position of water, as it cannot, from other 



substances, in which it exists, be easily 

 disengaged in perfect purity. Some sub- 

 stance is made to act on water, which 

 exerts an attraction to the oxygen, with- 

 out combining with the hydrogen, when, 

 of course, the hydrogen is disengaged, 

 and passes into the elastic form. 



At the common temperature of the 

 globe, this decomposition cannot be ef- 

 fected with rapidity .by any single affinity. 

 Iron, moistened with water, decomposes 

 it very slowly, and evolves hydrogen ; but 

 at the temperature of ignition, the de- 

 composition is more rapid. If a coil of 

 iron wire, or a quantity of iron filings, be 

 put into an iron or coated glass, or 

 earthen tube, which is placed across a 

 small furnace, and surrounded with burn- 

 ing fuel, so as to be brought to a red heat, 

 on distilling water from a retort connect- 

 ed with it, the vapour, in passing over the 

 surface of the ignited iron, is decomposed, 

 the iron attracts its oxygen, and hydro- 

 gen gas issues from the extremity of the 

 tube. 



This process is a troublesome one, and 

 by the agency of an acid, water is decom- 

 posed as rapidly by iron or zinc, at a na- 

 tural temperature. Zinc affords the hy- 

 drogen in the greatest purity. One part 

 of it, in small pieces, is put into a retort, 

 or a bottle with a bent tube adapted to 

 it ; two parts of sulphuric acid, previously 

 diluted with five times its weight of water, 

 are poured upon it, an effervescence is 

 immediately excited, hydrogen gas es- 

 capes, and is to be collected in jars filled 

 with water, and placed on the shelf of the 

 pneumatic trough. Its disengagement 

 continues until the zinc is dissolved. Iron 

 may be employed in place of zinc, but 

 containing generally a little carbon, which 

 is dissolved by the hydrogen, it affords a 

 gas less pure. Muriatic acid serves the 

 same purpose as sulphuric acid, but must 

 be diluted with only twice or three times 

 its weight of water. 



In the experiment, the hydrogen gas is 

 derived entirely from the decomposition 

 of the water, the oxygen of which is at- 

 tracted by the metal. Thut the acid suf- 

 fers no decomposition is proved, by the 

 liquor, at the end of the experiment, be- 

 ing capable of saturating as much of an 

 alkali as the quantity of acid employed 

 would have done in a pure state. The j ' 

 agency of the acid was formerly explain- 

 ed, on the absurd doctrine of disposing 1 

 affinity, that it had no attraction to the 

 pure metal, but to the oxide ef the meted? 



