284 



On Chemical Tests. 



[Oct. 



lxxxiv. Introduce the remaining air, or a portion of it, into a small 

 bent tube, containing a bit of phosphorus; heat it so as to kindle the 

 phosphorus, and note the diminution of bulk when cold. It is pro- 

 portional to the oxygen present, and, if equal to one-fifth of the whole 

 bulk, the gas may be regarded as atmospheric air. 



lxxxv. If sulphuretted hydrogen be present it may be separated by 

 alcoholic solution of iodine, which absorbs it, and scarcely takes up 

 more than its own volume of carbonic acid gas. Chlorine, added to a 

 mixture of sulphuretted hydrogen and carbonic acid, will also produce 

 the absorption of the former, if a little water be present; but it cannot 

 be conveniently used over mercury. 



lxxxvi. During the ebullition it not unfrequently happens that a 

 precipitation ensues, indicating that the substances thrown down were 

 dissolved by carbonic acid ; and in that case they should fee separated 

 upon a filter, (a) after which the remaining water may be evaporated to 

 dryness in a glyzed porcelain basin ; the dry residue must be transferred 

 to a silver, capsule and perfectly desiccated at a temperature not exceed- 

 ing 500° (&). 



lxxxvii. The precipitate (a) may consist of carbonate of lime, of 

 carbonate of magnesia, or of oxide of iron ; or it m-.iy be a mixture of 

 the three. Dissolve it in dilute muriatic acid, and add oxalic acid, which 

 throws down oxalate of lime ; separate this by filtration, and saturate 

 the filtrated portion with carbonate of ammonia, which precipitates the 

 peroxide of iron, and having removed this, evaporate the residuary 

 mixture, and expose the dry salt to a red heat in a small platinum cap- 

 sule ; the magnesia, if any were present, will remain; if not, there will 

 be no residue; for the oxalic acid and muriate of ammonia will be 

 destroyed and volatilized. 



lxxxviii. When carbonic acid holds iron in solution, the metal is in 

 the state of protoxide, and if air be excluded, it requires long boiling 

 to decompose it; for the same reason, if the water be exposed under 

 the exhausted receiver of the air-pump, it does not readily become 

 brown, as is the case when it is exposed to air; a drop or two of nitric 

 acid facilitates the deposition of the red oxide. 



lxxxix. The dry residue (b) is to be digested in six or eight parts of 

 boiling anhydrous alcohol, which will take up muriate of magnesia, 

 and in some rare cases (where no sulphates are present) muriate of 

 lime. Filter off the alcoholic solution, and wash the residue (c) with a 

 little fresh alcohol, which add to the former, and evaporate to dryness, 

 the dry mass (d) exposed for some time to a heat of 500,° is general- 

 ly t pure muriate of magnesia: if it contain muriate of lime, the latter 

 earth may be separated by solution of oxalic acid, in the state of oxalate 

 of lime. 



