22Q 
POTATO STARCH. 
mois. The actual size of the grains varies from l-600th to 
l-30th of a line in diameter. The shape of small (or young- 
est?) grains of potato-starch is circular, or globular; but that 
of the larger ones is elliptical, oblong, ovate, or obtusely 
triangular. Perhaps we may assume that the normal form 
of the fully developed particles is ovate. The structure of 
the grains is the next point deserving of our attention. When 
examined by a polarizing microscope, we observe, by the 
black cross which they present, that they possess a depola- 
rizing, or doubly refracting, structure, which is to be re- 
garded as an indication of their consisting of a structure un- 
equally dense. By the common microscope, we discover, 
on some part of the surface of the grain, one hilum, or, in 
some cases, two hila — one at either end, or two at the same 
end. The hilum is a circular hole, which was formerly 
thought to be a kind of umbilicus, by which, according to 
some writers, the starch-grain was originally attached to the 
parent vesicle in which it was developed. It is now re- 
garded as the circular section of the tube, or passage, by 
which the amylaceous substance is introduced into the inte- 
rior of the starch-grain. On large and old grains of starch 
we observe a number of cracks which commence at the 
hilum. 
<£ On the surface of the grains is a series of curved lines, 
forming a system of either concentric or eccentric rings, or 
zones, whieh surround the hilum. They are similar to the 
curved lines observed on bivalved shells, as the mussel, and 
which indicate the terminations of the successive layers of 
which these shells are composed. The grains >f every 
kind of starch, which I have hitherto examined, present a 
hilum and some traces of rings; but in the smaller grained 
starches, as rice-starch and the Portland arrow-root (starch 
of tflrum macula turn,) they are very imperfectly perceived. 
Tous les mois and potato-starch, probably on account of the 
size of their grains, show these rings in the most distinct 
manner, 
