CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 281 



but always the anhydride, Cr0 3 . The corresponding hydrate, Cr0 4 H 2 , 

 or any other hydrate, is not even known. Nevertheless, it must be 

 admitted that chromic acid is bibasic, because it forms salts isomorphous 

 or perfectly analogous with the salts formed by sulphuric acid, which is 

 the best example of a bibasic acid. A clear proof of the bibasicity o 

 Cr0 3 is seen in the fact that the anhydride and salts give (when heated 

 with sodium chloride and sulphuric acid) a volatile chloranhydride, 

 Cr0 2 Cl 2 , containing two atoms of chlorine as a bibasic acid should. 5 



5 Berzelius observed, and Rose carefully investigated, this remarkable reaction, 

 which occurs between chromic acid and sodium chloride in the presence of sulphuric 

 acid. If 10 parts of common salt be mixed with 12 parts of potassium dichromate, fused, 

 cooled, and broken up into lumps, and placed in a retort with 20 parts of fuming sul- 

 phuric acid, it gives rise to a violent reaction, accompanied by the formation of brown 

 fumes of chromic chloranhydride, or chromyl chloride, CrO 2 Cl 2 , according to the re- 

 action : Cr0 3 + 2NaCl -H H 2 S0 4 = Na 2 S0 4 + H 2 O -h CrO 2 Cl 2 . The addition of an excess of 

 sulphuric acid is necessary in order to retain the water. The same substance is always 

 formed when a metallic chloride is heated with chromic acid, or any of its salts, in the 

 presence of sulphuric acid. The formation of this volatile substance is easily observed 

 from the brown colour which is proper to its vapour. On condensing the vapour in a 

 dry receiver a liquid is obtained having a sp. gr. of T9, boiling at 118, and giving a 

 vapour whose density, compared with hydrogen, is 78, which corresponds with the above 

 formula. hromyl chloride is decomposed by heat into chromic oxide, oxygen, and 

 chlorine: 2CrOoCl 2 = Cr 3 O 3 + 2Cl 2 + 0; so that it is able to act simultaneously as a 

 powerful oxidising and chlorinating agent, which is taken advantage of in the investiga- 

 tion of many, and especially of organic, substances. When reated with water, this 

 substance first falls to the bottom, and is then decomposed into hydrochloric and chromic 

 acids, like all chloranhydrides : CrO 2 Cl 2 + H 2 O = CrO 5 + 2HC1. When brought into con- 

 tact with inflammable substances it sets fire to them ; it acts thus, for instance, on 

 phosphorus, sulphur, oil of turpentine, ammonia, hydrogen, and other substances. It 

 attracts moisture from the atmosphere with gredt energy, and must therefore be kept in 

 closed vessels. It dissolves iodine and chlorine, and even forms a solid compound with 

 the latter, which depends upon the faculty of chromium to form its higher oxide, 

 Cr 2 7 . The close analogy in the physical properties of the chloranhydrides, CrO 2 Cl 2 and 

 SO 2 C1 2 , is very remarkable, although sulphurous anhydride is a gas, and the corresponding 

 oxide, CrO 2 , is a non- volatile solid. It may be imagined, therefore, that chromium di- 

 oxide (which will be mentioned in the following note) presents a polymerised modification 

 of the substance having, the composition CrOj ; in fact, this is obvious from the method 

 of its formation. 



If three parts of potassium dichromate be mixed with four parts of strong hydrochloric" 

 acid and a small quantity of water, and gently wanned, it all passes into solution, 

 and no chlorine is evolved ; on cooling, the liquid deposits red prismatic crystals, known 

 as Peligot's salt, very stable in air. Thfs has the composition KCl,CrO 3 , and is formed 

 according to" the equation K 2 Cr 2 O 7 -f 2HCl = 2KCl,CrO 5 + H 2 O. It is evident that this 

 is the first chloranhydride of chromic acid, HCrO 3 Cl, in which the hydrogen is re- 

 placed by potassium. It is decomposed by water, and on evaporation the solution yields 

 potassium dichromate and hydrochloric acid. This is a fresh instance of the reversible 

 reactions so frequently encountered. With sulphuric acid Peligot's salt forms chromyl 

 chloride. The latter circumstance, and tHe fact that Geuther produced Peligot's salt 

 from potassium chromate and chromyl chloride, give reason for thinking that it is a 

 compound of these two substances . 2KCl,CrO 3 =K 2 CrO 4 + Cr0 2 Cl 2 . It is also sometimes 

 regarded as potassium dichromate in which one atom of oxygen is replaced by chlorine 

 that is, K 2 Cr 2 6 Ci 2 , corresponding with K 2 Cr 2 O 7 . When heated it parts with all its 

 chlorine, and on further heating gives chromic oxide. 



