M. Martin on the Amylum Grains of the Potatoe. 281 



in the latter case being so much condensed, that in boiling it is 

 unfolded very little or hardly at all. In the proper proportion 

 the grain is seen to increase continually during the boiling, then 

 to split at the kernel ; and at this point the colour becomes 

 lighter in the centre, and the elevated band, which is of a darker 

 colour, continually recedes towards the edge until a completely 

 flat disc lies on the glass. When this disc is pressed between 

 the two glasses, it becomes smooth; and if the pressure be 

 strong, somewhat larger. The experiment seems to succeed still 

 better in a concentrated solution of alum, with as much tincture 

 of iodine as will colour the grains of a steel-blue. The charac- 

 teristic longitudinal wrinkle also appears in some of the grains, 

 but it is easily rendered smooth. 



Although I shall subsequently return to this subject, I cannot 

 avoid asking in this place, Where does the split or tear, at which 

 the grain is considered to burst, actually occur ? 



5. Before developing my views, I have yet to refer to the often- 

 mentioned disc. Its appearance demonstrates that it is perfectly 

 flat, and has a slightly elevated edge, which also becomes flat on 

 pressure. The contour is round, but perfectly sharp. If the 

 two glasses be violently moved from one side to the other whilst 

 pressing the amylum, the disc is torn, and it is distinctly seen, 

 especially in the blue-coloured ones, to consist of two layers, an 

 upper and lower one. Further examination shows that they are 

 collapsed vesicular bodies, consisting of an extremely fine but 

 strong and elastic membrane. Should a disc have a small wrinkle 

 not easily smoothed, then if the lower glass, furnished with a 

 good quantity of water, be moved over the upper one, such a 

 vesicle will be seen, particularly in the alum solution, to turn 

 round on its axis, and the wrinkle to slide over the upper and 

 lower layers of the skin ; over the upper layer in the direction in 

 which the glass is moved, and over the lower layer in an opposite 

 direction. That this vesicle cannot be the grain enlarged in 

 dimensions will be apparent hereafter ; and I now proceed to 

 develope my theory. 



G. The primary form of the amylum grain is, according to my 

 view, a spherical or ovate vesicle. If this be considered as empty, 

 and so contracted that one-half lies in the other half, a watch- 

 glass-shaped basin is formed, which, I may here observe, after 

 boiling and pressure between the two glasses, appears, in conse- 

 quence of the delicacy and elasticity of the membrane, as a flat, 

 round-edged disc. 



Plate VI. fig. 1 represents the edge of the basin-shaped vesicle. 

 At the formation of the grain this edge moves a little inwards, and 

 rolls itself up inwardly, by which a band b (hg. 2) with spiral 

 internal windings, which on the outside .appear elliptical, is pro- 



