1879.] 



Volumetric Estimation of Sugar. 



261 



obviate the difficulty here presented, I suggested, in a communication 

 published in the "Proceedings of the Royal Society " for June, 1877, 

 that the precipitated suboxide should be collected and dissolved, and 

 the copper subsequently thrown down by the agency of galvanic 

 action upon a platinum cylinder, as is now frequently done in the 

 assaying of copper ores. The process has been found, as shown by 

 the closeness observable in the results of counterpart analyses, to 

 admit of the greatest precision, and I have turned it to extensive 

 account in some recent physiological investigations I have conducted. 

 In its application to such a purpose, it may be held that time and 

 labour should be considered as of no moment, but it frequently 

 happens that a more ready process of estimation is needed than the 

 gravimetric supplies, and on this account a volumetric method, free 

 from the objection I have pointed out as belonging to the ordinary 

 plan, constitutes a desideratum. 



A few years back Bernard introduced, for physiological purposes, a 

 modification of the ordinary volumetric process, which is attended 

 with reduction and the non-precipitation of the reduced oxide. The 

 process involves the employment of a large quantity of caustic potash, 

 and the presence in the product to be tested of extraneous organic 

 matter. Under these circumstances it happens that the reduced sub- 

 oxide is held in solution instead of being allowed to fall, and thus 

 decoloration without precipitation occurs and enables the point of 

 disappearance of the colour of the test to be ascertained with pre- 

 cision. Bernard, in his remarks upon the test, simply made mention 

 of the fact that under these conditions, reduction without precipita- 

 tion took place, but Dr. d'Arsonval,* his Preparateur at the College of 

 France, refers the effect to the solvent influence of the extraneous 

 organic matter in presence of the alkali. 



Whilst engaged upon an inquiry into the merits of this test, the 

 conclusion suggested itself to me that the agency preventing the 

 deposition of the suboxide was the development of -ammonia. With 

 an absolutely pure solution of sugar, such as may be obtained by 

 inverting the ordinary crystallized cane sugar (refined loaf sugar) no 

 amount of potash will hinder the instantaneous precipitation of the 

 suboxide. With commercial grape sugar, however, and in a still 

 more marked manner with honey, interference with precipitation is 

 temporarily exerted, and this, I am led to conclude, is due to the 

 action of the potash in producing ammonia from the small quantity 

 of nitrogenous organic matter incidentally present. 



With this before me, the idea presented itself of resorting to the 

 direct employment of ammonia for attaining the same result. It is 

 well known to chemists that ammonia is a powerful solvent of the 

 suboxide of copper, leading to the production of a perfectly colourless 

 * " Gazette Hebdomadaire de Medecine et de Chirurgie," Sept. 14, 1877, p. 454. 



