TESTS FOR COPPER AND STARCH. 103 



Dissolve in water a few grains of sulphate of copper, and mix 

 some of this solution with the starch paste ; place the whole in a 

 bladder, and suspend this in water as in the last experiment. Next 

 day, take out some of the water, and test half of it for copper and 

 half for starch ; the former will be found, the latter not ; because 

 the copper sulphate, being a crystalloid, will have passed through 

 the membrane, whilst the gelatinous starch, being a colloid, will 

 not pass through it ; the liquid inside the bladder, however, will 

 readily show the presence of starch on similarly testing. 



Expt. 95. To Test for Copper. In order to test for the copper 

 sulphate, proceed as follows. To a portion of the watery fluid add a 

 little solution of sulphuretted hydrogen. The same result will ensue 

 as when silver nitrate and sulphuretted hydrogen solutions are mixed 

 (Expt. 12, No. 5), i.e., a black precipitate will be formed by double 

 decomposition, consisting in this case of sulphide of copper. 

 Another test is, to add to a separate portion of the watery fluid a 

 little solution of ferrocyanide of potassium ; this will act on the 

 copper sulphate by double decomposition, just as it does on solution 

 of perchloride of iron (Expt. 57) ; but with this difference, that 

 the precipitate produced with copper is of a peculiar brown-red or 

 mahogany colour, instead of being blue as with iron. 



Expt. 96. To Test for Starch. The test for starch is as 

 follows. Shake up with water a few fragments of iodine for some 

 time ; by and by the water will become slightly yellow in colour, 

 a little iodine becoming dissolved ; if alcohol be used instead of 

 water, the iodine dissolves much more freely to a liquid, which is 

 of a pale or brown sherry colour, according to the strength of the 

 solution. To the liquid supposed to contain starch add a few 

 drops of iodine solution; if starch is present, a blue colour will be 

 developed, owing to the combination of the starch with the iodine. 

 This blue iodide of starch possesses a somewhat remarkable pro- 

 perty, viz., that it becomes colourless on heating. If a little of the 

 blue liquid be boiled in a test-tube, the blue colour will disappear ; 

 but if the test-tube be allowed to cool, the blue colour will again 

 become visible. If the test-tube be quickly cooled by immersing 

 it in a tumbler of cold water, the blue shade will first appear at 

 the bottom of the test-tube, because the hotter fluid tends to 

 ascend to the top * (Chapter XX.). 



* The presence of starch in sweetmeats, &c., may be shown by the aid of 

 this test, thus : Pour cold water on the substances, and allow it to stand 

 (stirring up occasionally if requisite) until the sugar is dissolved ; if starch 

 be present a sediment will be left undissolved. Pour off the liquor and 

 transfer the sediment to a thin glass flask, and add some boiling water ; this 

 will gelatinise any starch present. Allow the whole to cool, and add a few 



