HYDRA

hydra, in astronomy, a constellation of the southern hemisphere, mentioned by eudoxus (4th century b.c.) and aratus (3rd century b.c.), and catalogued by ptolemy (27 stars), tycho brahe (19) and hevelius (31). interesting objects are: the nebula _h. iv. 27 hydrae_, a planetary nebula, gaseous and whose light is about equal to an 8th magnitude star; [epsilon] _hydrae_, a beautiful triple star, composed of two yellow stars of the 4th and 6th magnitudes, and a blue star of the 7th magnitude; _r. hydrae_, a long period (425 days) variable, the range in magnitude being from 4 to 9.7; and _u. hydrae_, an irregularly variable, the range in magnitude being 4.5 to 6. hydracrylic acid (ethylene lactic acid), ch2oh·ch2·co2h. an organic oxyacid prepared by acting with silver oxide and water on [beta]-iodopropionic acid, or from ethylene by the addition of hypochlorous acid, the addition product being then treated with potassium cyanide and hydrolysed by an acid. it may also be prepared by oxidizing the trimethylene glycol obtained by the action of hydrobromic acid on allylbromide. it is a syrupy liquid, which on distillation is resolved into water and the unsaturated acrylic acid, ch2:ch·co2h. chromic and nitric acids oxidize it to oxalic acid and carbon dioxide. hydracrylic aldehyde, ch2oh·ch2·cho, was obtained in 1904 by j. u. nef (_ann._ 335, p. 219) as a colourless oil by heating acrolein with water. dilute alkalis convert it into crotonaldehyde, ch3·ch:ch·cho.