6 RAINKY, ON STARCH GRANULES. 



from which it precipitates a modified form of starch, and, on 

 an alkaline solution of starch, from which it precipitates 

 pure starch. To show the first property — that of a general 

 precipitant — it is necessary to obtain the expressed juice of 

 fresh vegetables, previously bruised or rasped, and filtered 

 through blotting-paper once or twice, so that it may be per- 

 fectly clear. Some of this juice is then to be filtered into a 

 test-tube, into which a small quantity of filtered solution of 

 gum arabic has been introduced, when, after these fluids 

 have remained for a few minutes, the stratum of juice in con- 

 tact with the solution of gum will lose its transparency, be- 

 come turbid, and soon deposit, in greater or less quantity, 

 the vegetable matter which it had held in solution. I have 

 performed this experiment upon the juice of several plants, 

 and always with the same result. The juice of the bruised 

 stems of the potato, as also that of the rasped bulbs, will 

 serve very well for this experiment, and especially the latter, 

 as it can at all times be procured. I may observe, that the 

 filtered juice of some vegetables will, after standing a short 

 time, without the addition of any gum, become turbid and de- 

 posit of itself. But this deposit I have not mistaken for 

 that produced by the gum, the latter beginning to be appa- 

 rent within a minute or two after the contact of the gum 

 with the expressed juice, whilst the former requires several 

 hours, or an indefinite time, for its production. I may also 

 notice, that this property of gum is not, so far as I can dis- 

 cover, attributable to any earthy or metallic salt which it 

 may contain, or to the acid which is generated by it, after 

 being kept for some time in solution, but it appears to be 

 essentially a property of vegetable gum, that is, of a sub- 

 stance which forms with water an adhesive solution, from 

 which it is precipitated by silicate of potash, and thrown 

 down by alcohol in the form of opaque white flakes. As, in 

 order to be assured upon this point, 1 employed in my expe- 

 riments gum from which the salts of lime had been separated 

 by oxalate of ammonia, also gum which had been precipi- 

 tated from its solution in water by alcohol, and alter that 

 dried and redissolvcd in water, also a solution of gum made 

 slightly alkaline, all with essentially the same result as that 

 obtained by the unpurified gum. 1 will now give sonic expe- 

 riments showing the effect of gum upon dextrine, and upon 

 Starch dissolved in a solution of potash. It is well known that 

 dextrine is formed by heating starch, and also by the action 

 of sulphuric acid upon starch ; I therefore obtained a 

 substance known in commerce by the title of soluble gum. 

 It is made by applying heat to starch in a suitable apparatus. 



