390 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
of aggregation, they are not soluble in water; but sulphuric, 
nitric, hydrochloric, oxalic, and probably many other acids (from 
which however acetic acid must be excepted), disintegrate their 
constituent particles, in the cold when they are very concentrated, 
or even in a state of very great dilution when the action of these 
solvents is aided by a slight elevation of temperature, or by 
heat and pressure in the autoclave*, This disaggregation is 
gradual; and in proportion as it is more or less advanced, the 
disunited portions remain a longer or shorter time in suspension, 
or dissolved in the water in presence of the acid, or even after 
the latter has been saturated by an alkali, especially if the salt 
formed is not precipitable; and we arrive at last at such a de- 
gree of tenuity that the disaggregated substance may remain 
suspended indefinitely, even in pure water, after all the acid has 
been separated. These fragments of the primitive globule appear, 
according to their degree of attenuation and disorganization, more 
or less complete, also more or less susceptible of being tinted 
by iodine, which colours, as is well known, natural starch blue, 
when held in a state of precipitation in water at temperatures 
below 100°; and the tints they assume vary according to these 
two peculiarities of condition, So that, on breaking up the 
starch globules by mechanical trituration, some fragments may be 
removed from them by cold water, still organized, although so 
minute that they remain suspended in this liquid as in a true 
solution, which, when cold, is coloured blue by iodine, and is 
deprived of colour by an elevation of temperature below 100°, 
and again assumes its colour spontaneously on cooling, if pre- 
cautions have been taken to prevent the whole of the iodine being 
vaporized}, But all these changes are purely physical; for, as 
table of the thicknesses of the laminz of air to the number 21°5, because it is 
there that the change of the tints is most rapid, passing by the least subtractive 
action into red, and by the least additive action into green. This is what [ 
have called, in my experiments on lamellar phenomena, la lame sensible, 
By interposing it, as I have said, in the passage of the rays between the crossed 
prisms, all the sectors which appeared white before its interposition become al- 
ternately red, yellow, blue, or green, according to the direction of the tangent 
plane; and their combination, in the field of vision, presents one of the most 
curious physical spectacles which can be seen, 
* This last experiment is due to M. Jacquelain, assistant at l’Ecole Centrale 
des Travaux Publics; he thus disaggregated the starch, and converted it into 
soluble dextrine, by the action of water to which -4> of oxalic acid had been 
added, by raising the temperature sufficiently in closed vessels. 
+ This curious experiment was described by M. Lassaigne in the Journal de 
Chimie Médicale, vol. ix. p. 651, of the year 1833, I have preserved for some 
years a flask with a ground stopper filled with an aqueous solution of starch 
