CHEiMICAL HISTORY OF STARCH. 101 



solid black pillar, being raised up by the evolution of gaseous 

 matter, which in part becomes entangled in the spongy car- 

 bon. In conclusion, the substance from which Manna derives 

 its flavour was distinguished from sugar. The subject of Sugar 

 having been thus treated, to the great interest of the young au- 

 ditors, evinced by their patient attention during each Lecture, 

 and their pertinent inquiries after it, the history of Starch 

 was next considered. 



This was commenced by a general account of the nature of 

 tlaefecuke of Vegetables, and of the processes adopted for ob- 

 taining those of Wheat and the Potatoe, specimens of both 

 varieties being exhibited, and of the former, in its natural con- 

 dition, as well as in its coloured state as used for stiffening 

 linen. 



The five- or six-sided columnar form of starch, as usually 

 manufactured, was particularly noticed ; and the mode of pro- 

 duction of such columns having become a subject of conside- 

 rable interest in several branches of science, which might here- 

 after engage the pupils' attention, the formation of them in the 

 mud of ponds drying-up by the sun, and their existence in the 

 ore of iron called columnar clay-iron-stone, were also men- 

 tioned, in both which instances their production is referrible 

 to the same cause as that which produces them in starch 

 contraction, from the removal, by evaporation, of the water 

 previously mingled with the particles of the substance *. The 

 chemical properties of starch were described and experiment- 

 ally exhibited ; including the test of its presence afforded by 

 the circumstance of its forming a blue compound with iodinef, 



* Having, in a paper on the remarkable natural excavations in granite, 

 which have been called rock-basins, recently published in the Philosophical 

 Magazine and Annals (vol. viii. p. 331.), alluded to the structure of the 

 columnar iron-stone of the Isle of Arran, as being of igneous origin, it may 

 be proper to mention that the variety referred-to and exhibited in the Lec- 

 ture was that which occurs in lenticular concretions, in the Staffordshire 

 coal-measures, the interior of which presents a columnar structure which 

 must have been produced by contraction, in the manner described in 

 the text. 



It may also be useful to record in this place a fact which I have lately 

 observed, with respect to the production of the prismatic or columnar struc- 

 ture, by the contraction consequent on desiccaporation. The floor of one 

 of the Gymnasia at Bruce Castle having been made of stiff clay, which had 

 not been properly tempered for the purpose, by the admixture of other 

 materials, became divided by deep cracks, all over its surface, into incipient 

 columns oV prisms. But on repeated attentive examination of them, in 

 every part of the floor, I could discover none but irregular trapezoidal and 

 pentagonal columns, no indication whatever of hexagonal divisions being 

 perceptible. 



t See the preceding Syllabus of the Course of Lectures on the Support 

 ers of Combustion, p. 91. 



