ON THE JUICE OF THE SUGAR-CANE. 425 
which is personal in some measure to me, and which for this reason I 
take the liberty of remarking on before concluding the present chapter. 
This reagent is formed by the oxide of copper in solution in a concen- 
trated solution of caustic soda ; and in its preparation some little care 
must be taken, without which the dissolution of the metallic oxide is 
not produced. A cubic demi-centimetre of a weak solution of sulphate 
of copper is introduced into a glass, upon which is rapidly poured about 
twenty-five cubic centimetres of soda, at the same time stirring the 
liquid rapidly in the glass with a small rod ; after decanting it, in order 
to separate a small quantity of the precipitated oxide of copper, a liquid 
of a beautiful deep blue is obtained which must be kept from the light. 
The contact of ammoniacal vapour must be carefully avoided, for any 
traces of this alkali would completely neutralise the effect produced 
by the solution. When required for experiment, about the proportion of 
one-third is added to juice which is thought to contain interverted sugar ; 
the mixture is then left to itself, and at the end of two or three minutes 
it changes from deep blue to violet red if there be the most feeble trace 
of glucose. No other substance existing in cane-juice has the property 
of producing such a decided change of colour, so that this reagent on 
account of its sensitiveness, and the great ease with which it may be 
employed, is very convenient when we wish simply to recognise the 
presence of interverted sugar in the liquid extract of any plant ; but on 
account of its instability, occasioned by the slow separation of the 
metallic oxide, it cannot be used as a means of testing, like the liquid of 
Frominherz.* 
III. On other Organic Matters besides Sugar. 
A considerable number of organic substances more or less defined 
have been pointed out as existing in cane-juice ; but as all of them are 
far from having the same importance during the operation of extracting 
the sugar from the cane, I shall at present confine myself to the exami- 
nation of those only which have a positive influence on the qualities of 
the juice, reserving the others for some future occasion when they will be 
made the object of a special study. The basic acetate of lead mixed 
with the juice generates an abundant precipitate into which is drawn 
the greatest portion of the vegetable matters besides sugar ; this preci- 
pitate, as can easily be ascertained, contains also as salts of lead some of 
those acids found in the juice in combination with its alkaline bases. If 
* This reagent as I showed several years ago, has the property of changing to 
violet under the influence of the albumen of egg, of blood, and of most of the liquid 
products of the animal organisation ; but it remains indifferent when in contact 
with albumen extracted from the vegetable kingdom, and that which is excreted 
in human urine in certain pathological cases. The azotised substances in the cane 
have not the power of changing the blue liquid to red ; interverted sugar is the 
only body-carjable of producing this transference of colour. 
Y Y 2 
