and the different amylaceous Substances of Commerce. 363 



rescue them from an error which the microscope alone 

 could disclose to them. I own, that, in the physical sciences, 

 there can only be, on the part of those who cultivate them, 

 conditional or relative belief ; the progress of the human mind, 

 more and more enlightened from time to time, imperiously 

 requires it. But I also feel regret in abandoning an opinion 

 which had become a sort of habit, since it appeared so well 

 confirmed by time and experience : I am also no longer sur- 

 prised, that chemists, very commendable in other respects, 

 should have carried to their graves devotedness to, and con- 

 viction of the doctrine of Stahl, their master. 



Be that as it may, if we are not convinced that the new 

 theory by which M. Raspail explains the chemical phsenomena 

 exhibited by starch, is exact and well founded enough in 

 order to admit it, at least it will have the merit of proving to 

 chemists that the application of the microscope in their habi- 

 tual labours may sometimes make them witnesses of curious 

 phaenomena. 



I shall now examine the principal assertions given by this 

 author in his Memoir : but first I think I should note his 

 most important conclusions. 



1st, Fecula is composed of vegetable organs in the form of 

 globules. 



2dly, Each grain of fecula is formed, 1st, of a smooth inte- 

 gument, which cannot be attacked by water and acids at the 

 ordinary temperature, susceptible of being coloured a long time 

 by iodine ; 2dly, of a soluble substance, which evaporation 

 deprives of the faculty of becoming blue by iodine, and which 

 possesses all the qualities of gum. 



3dly, Consequently, the gums which flow from vegetables 

 are nothing but this soluble substance of the fecula, which has 

 lost from being in the open air the faculty of becoming blue. 



Lastly, The faculty of becoming blue by iodine is owing 

 to a volatile substance. 



I have witnessed, through the kindness of Mr. Edwards, 

 who had a microscope at his command, that the grains of the 

 fecula of potatoes (it is with this substance that the author has 

 always made his experiments) have a globular form, the diame- 

 ter of which varies without end : but nothing proved to me that 

 these grains contained a gummy substance. M. Raspail sub- 

 mitted these grains of fecula to the action of tincture of iodine, 

 upon the object-slider of the microscope : he saw them acquire 

 a blue colour, without losing their form thereby ; and then, 

 having submitted them to the action of potassa and ammo- 

 nia, he saw them lose their colour immediately, without un- 

 dergoing the least alteration in their form and appearance : 



2 Z 2 from 



