506 



SCIENCE 



[N. S. Vol. XLV. No. 1169 



lieved that these latter represent but question- 

 able evidence for the presence of any true re- 

 ducing substances, or if conceded to be posi- 

 tive evidence for such, it is held that the re- 

 ducing substances can not be present in great 

 amounts. 



An investigation of this series of changes 

 from a colloid-chemical point of view reveals 

 the fact that these color changes are coinci- 

 dent, not with differences in the nature of the 

 reduction products, but only with diiferences 

 in the size attained by the particles of reduced 

 substance. If the copper oxide particles are 

 brought down in very fine (subcolloid or col- 

 loid) form, the greenish discolorations are 

 produced; as the copper oxide particles grow 

 in size they become yellow, then orange, and 

 when very coarse they are red. The series 

 shows, in other words, what has been observed 

 by different colloid-chemical workers : that one 

 and the same material may, in the colloid 

 state, show different colors, the color order fol- 

 lowing WoKgang Ostwald's color law, accord- 

 ing to which the most highly dispersed par- 

 ticles of a given substance are likely to be yel- 

 low, turning, as the size of the particles in- 

 creases, to orange or red, and finally to violet, 

 blue or black. 



As to which of these possible colors will be 

 obtained from a Fehling's solution undergoing 

 reduction depends upon the conditions sur- 

 rounding the reduction, the greenish discolora- 

 tions being obtained whenever the conditions 

 are such as will keep the cuprous oxide, as pro- 

 duced, in its finely divided state; while the red 

 will result when opposite conditions obtain. 



Three factors are chiefly concerned in the 

 process : 



1. Contrary to the generally accepted no- 

 tion, the presence of too much reducing sub- 

 stance (as too much dextrose) is more likely to 

 yield a greenish result than the presence of too 

 little. This is because with much reducing 

 substance the reduction starts from many 

 points at once, but with exhaustion of the 

 available copper salt the process comes to a 

 halt before the cuprous oxide particles have 

 attained any great size. 



2. Irrespective of the amount of reducing 



substance present, the greenish discoloration 

 will be obtained whenever materials are pres- 

 ent in the reaction mixture which tend to sta- 

 bilize the cuprous oxide in its finely divided 

 form. Such materials are of the group of the 

 lyophilic (hydrophilic) colloids and whenever 

 present, either because added experimentally 

 to reaction mixtures prepared in the laboratory 

 or brought in with the mixtures being tested 

 for reducing bodies (like diabetic urine) they 

 incline to stabilize the cuprous oxide when this 

 is still in a finely divided state. 



3. With certain reducing substances (like 

 dextrose) such " protective " hydrophilic col- 

 loids may be produced in the course of the 

 reactions incident to the Fehling's test itseK. 

 In the action of the alkali of Fehling's solu- 

 tion upon dextrose, for example, there are pro- 

 duced, from a chemical point of view, not only 

 the various degradation products which are 

 responsible for the reduction of the copper 

 salt, but, from a colloid point of view, many 

 of these are colloid in nature and so tend to 

 inhibit a precipitation of the cuprous oxide in 

 coarse form. 



Consideration of these various facts not only 

 renders intelligible many of the excellent em- 

 piric instructions which different chemists 

 have long found useful when Fehling's test for 

 the qualitative or quantitative determination 

 of various reducing bodies is employed, but 

 they indicate also what schemes should be fol- 

 lowed if it is desired to get the copper oxide 

 precipitated in its coarse red form. 



To allow adequate time for the growth of the 

 cuprous oxide particles, it is better to make 

 reductions at low temperatures than at higher 

 ones. A Fehling's test carried out at room tem- 

 perature by mixing the Fehling's solution with 

 the suspected material and setting this aside 

 for twenty-four hours is therefore more likely 

 to yield a red precipitate than if the test is 

 made by boiling the two together in the ordi- 

 nary way. 



Care should also be taken not to use exces- 

 sive quantities of the material containing the 

 reducing bodies. This not only avoids the 

 possibility of using more reducing substance 

 than there is available copper salt that may be 



