8 Prof. R. Threlfall on the Electrical 



can be completely deprived of the latter by allowing it to 

 bubble once through a flask containing the blue liquid. To 

 test this I placed two flasks in series, both containing a few 

 hundred centimetres of the solution, and fitted with tubes 

 allowing air to be sucked through both, one after the other. 

 Though I allowed the air to pass much more briskly than if 

 I had been actually using the process (say three times faster 

 than one ordinarily allows the oxygen to pass in an organic 

 analysis), I did not observe any change in the second flask 

 till the first had long been quite opaque and had become more 

 than warm, and then suddenly the contents of the second 

 flask became attacked. In manipulating the chromous- 

 chloride solution, it is useful to have a flask of the blue liquid 

 permanently at hand to purify the carbonic acid or nitrogen 

 which must be used in the subsequent operations. In many 

 cases it does not matter whether the gas has a trace of hy- 

 drogen in it or not : but for my purpose I was not willing to 

 run any risk of having hydrogen about, and consequently I 

 never allowed the purifying flask to contain zinc. 



Having satisfied myself as to the reliability of the chromons 

 chloride as an absorber of oxygen, I undertook a number of 

 experiments with the object of discovering, 1st, the most 

 suitable form in which to use it ; 2nd, whether in the process 

 of absorption any other gas was given off, or any vapour 

 which could not be safely absorbed. In these experiments 

 large vessels containing copper and ammonia (arranged in a 

 manner to be presently explained), and capable of furnishing 

 a continuous steam of nitrogen mixed with oxygen in pro- 

 portions which could be varied at will, were used throughout. 

 The ammonia was removed from the nitrogen by strong sul- 

 phuric acid that had been heated to boiling and cooled in a 

 vacuous desiccator. With regard to the best way of using the 

 chromous chloride, I began with the impression that tubes 

 filled with the dry salt would be the most convenient, espe- 

 cially as Recoura shows that small crystals of dry chromous 

 chloride have an affinity for oxygen which surpasses that of 

 the solution. I therefore prepared large quantities of the 

 blue solution, and obtained the crystals from it by the method 

 recommended by Eecoura, which is briefly as follows : — The 

 blue liquid is mixed with a saturated solution of sodium 

 acetate, which precipitates chromous acetate in the form of a 

 dark red powder. This powder is then dissolved in dilute 

 hydrochloric acid, and the chromous chloride precipitated 

 from the solution by passing in gaseous hydrochloric acid so 

 as to gradually increase the strength of the hydrochloric-acid 

 solution 



