262 



Dr. F. W. Pavy on 



[Jan. 16, 



liquid ; and this, from the facility with which it absorbs oxygen, 

 quickly assumes a blue colour under exposure to air from the recon- 

 version of the cuprous into the cupric oxide. 



If ammonia be added to the ordinary Fehlmg's solution, a liquid is 

 obtained which is rendered colourless by boiling with a sufficiency of 

 sugar to effect the complete reduction of the cupric oxide present to 

 the state of suboxide. As the saccharine product is dropped in the 

 blue colour gradually fades, without any occurrence of precipitation 

 to interfere with the perception of the precise moment when the point 

 of complete decoloration is attained. The ammonia exerts no in- 

 terference with the process of reduction, but simply dissolves the 

 reduced oxide, leading, when complete decoloration is effected,' to the 

 production of a perfectly colourless, limpid liquid. 



Enough ammonia must be present to secure that the suboxide is 

 held in solution, and precaution must be taken that whilst the analysis 

 is being performed the reduced oxide does not become reconverted 

 into the oxide by exposure to the air. To obviate this the operation 

 should be conducted in a flask instead of an open capsule. 



The appliance that naturally suggests itself as most suitable for 

 employment is a flask of about 80 cub. centims. capacity, with a cork 

 inserted into the neck, through which a delivery tube from a Mohr's 

 burette, graduated in tenths of a cub. centim., passes for dropping in 

 the product to be examined. Through the cork, also, there must be 

 an exit tube for the escape of air and steam from the flask. Should 

 it be desired to avoid the impregnation of the surrounding atmosphere 

 with ammonia, the exit tube may be connected by vulcanized tubing 

 with a U-shaped tube containing fragments of pumice stone moistened 

 with water or a weak acid. The burette being fixed in the stand, the 

 flask is allowed to hang suspended, so that there may be nothing to 

 obstruct the full view of its contents. The heat is applied by means 

 of the flame of a spirit lamp, and the best position for watching the 

 disappearance of colour is by the light reflected from a white back- 

 ground specially provided for the purpose. It is convenient to have 

 another burette, graduated in cub. centims., and of 100 cub. centims. 

 capacity, fixed in the stand for holding and delivering the ammoniated 

 copper solution. Messrs. Griffin, of Garrick Street, have constructed 

 an arrangement adapted to meet the requirements. 



I at first took it for granted that in the action occurring the same 

 relation existed between the amount of oxide of copper reduced and 

 that of sugar oxidised, as under the employment of the copper test in 

 the ordinary way, viz., that 5 atoms of oxide of copper were reduced 

 by 1 atom of sugar, and the liquid I first employed was prepared 

 by adding to 100 cub. centims. of Fehling's solution 300 cub, centims. 

 of strong solution of ammonia (sp. gr. '880) and 600 cub. centims. 

 of distilled water. The liquid thus made contained one-tenth of 



