This oxide may likewise be obtained in the moist way, and then it is soluble in dilute acids : it is precipitated in the state of hydrate, by adding ammonia to the green solution of chloride of uranium ; the precipitate is of a reddish-brown colour, which by ebullition becomes black and dense, probably because it is dehydrated. It may also be procured by putting fragments of marble into the green solution of chloride of uranium.
Black Oxide of Uranium (2U0, U,0,) is obtained by Calcining the nitrate at a high temperature. It is not decomposable by heat ; when added to acids they do not directly combine with it, but a mixture of salts of the protoside and peroxide is formed.
Olive Oxide of Uranium (UO, any of the preceding oxides are submitted at a low red heat to the action of oxygen, the olive-coloured oxide is formed. It has a velvety appearance, and when strongly heated it loses oxygen and is converted into the black oxide, and when acted upon by acids there is formed a mixture of yellow and green salts, in which the salts of the peroxide exist in the larger proportion, and this is an advantageous process for preparing them.
Peroxide of Uranium, or Oxide of the Yellow Salts oxide, which is of all the most important, is obtained with difficulty in a separate state : when nitrate of uranium is decomposed with a gentle heat, an orange-coloured subsalt remains, which by the applica tion of a stronger heat becomes olive and then black oxide ; when an alkali is poured into a saline solution of this oxide, the yellow pre cipitate formed retains alkali in combinatiou; even uranate of ammonia resists the prolonged action of boiling water and also of a vacuum; by heat the ammonia and water are not expelled till peroxide itself under goes decomposition.
Chlorine and Uranium.—The protochloride (UCI) is obtained by passing a current of dry chlorine gas over an intimate mixture of equal parts of any oxide of uranium and charcoal submitted in a glass tube to a high temperature. The chloride of uranium formed appears in the state of a red vapour, and condenses in the cool part of the tube in very regular octohedrorus of a metallic lustre, and of a black.or green colour according to their size.
Chloride of uranium is volatile, and attracts water so strongly that it very soon becomes fluid by exposure to the air, the moisture of which also decomposes it.
Suhchloride of Uranium compound is obtained by passing a current of dry hydrogen gas over the chloride of uranium moderately heated in a glass tube. The residue of this operation is of a deep brown colour, iu fine filaments which are but slightly volatile at the temperature at which it is formed : it is very soluble in water ; the solution is purple at first, but in a few seconds it becomes green ; it gives out hydrogen gas, and at the same time deposits a red powder, which is very probably oxide of uranium, yielded in consequence of the transformation of this substance into chloride of uranium.
Sulphide of Cranium of a black colour may be obtained by adding the alkaline sulphides to solutions of uranium, or by passing the vapour of sulphide of carbon over the oxide at a high temperature.
We shall now briefly notice some of the oxyealts of uranium.
Sulphate of Protoxicle of Uranium.—This salt is obtained by adding sulphuric acid to the protochloride of uranium, and heating the mix ture, by which hydrochloric acid is expelled, and sulphate of uranium remains; by dissolving the residue in water, and evaporating the solution, green prismatic crystals of the sulphate are formed.
It frequently happens that the crystals possess a silky lustre, are greenish, and but slightly soluble in water ; in this case they contain excess of base. This salt yielded by analysis :— Oxalate of Protoxide of Uranium—This salt is of a greenish-white colour, and very slightly soluble in water either cold or hot. It may be prepared by mixing solutions of oxalic acid and chloride of ura. nium ; the precipitate formed is to be repeatedly washed with boiling water, in order to dissolve the yellow oxalate of the peroxide, which is more soluble, and which is first precipitated. The protoxalate of uranium, after being dried, may be exposed to the air without under going any perceptible change.
Nitrate of Peroxide of Uranium. salt is easily obtained in fine regular crystals. It is of a yellowish colour, effiorescea in vacuo, and loses half its water of crystallisation.
Uranium forms a considerable number of double salts, which we have not thought it requisite to describe.
Peroxide of uranium is employed in colouring glass, to which it imparts a fine lemon yellow.