116 STARCH ACTION OF HEAT, ETC. [BOOK i. 



even shows a nucleus and layers (kern und schichteri). In 

 this way the whole mass of grains may be uniformly acted 

 upon, and so that no rupture of the outer layer, nor any 

 discharge of the fluid contents is produced. After a longer 

 action of the acid, two different forms of solution are 

 observable in equal abundance, which probably depend on 

 the acid becoming weaker. In weaker acids the grains of 

 starch gradually become transparent and gelatinous, and 

 swell in such a way that they at first appear depressed on one 

 side, and, at last, after becoming cup-shaped, will be entirely 

 dissolved at their edges. In the other form of slow action, 

 which is probably occasioned by the concentrated acid, the 

 so-called kernel at first becomes a visible and recognisable air- 

 vesicle ; this expands and causes one or two ragged slits in 

 the inside of the grain, which gradually swells, becomes 

 gelatinous, the lines disappear as far as the rent extends, and 

 at last the whole grain becomes invisible and dissolves. The 

 next operation of the sulphuric acid seems to be to draw 

 water between the inner layers. 



If some potato starch is just heated on a small plate, so that 

 only a small part immediately on the plate is turned yellow, 

 then many remarkable transitions may be observed to take 

 place gradually, and they give under the microscope the best 

 evidence as to the structure of the grains of starch. The 

 first thing that happens is, of course, a drying, by which the 

 so-called kernel is transformed into an air-bubble ; this always 

 characterises the application of a dry heat, and may always be 

 seen in the farinha of Mandiocca, in Sago, &c. By degrees, 

 as the separation of the layers takes place in consequence of 

 drying, the separating lines become sharper, blacker, broader, 

 and layers of air, of various breadths, become clearly per- 

 ceptible : the layers are firmly attached at some points, less 

 firmly at others, and then they readily form spaces full of air. 

 By degrees the layers peel from one another like hollow cups, 

 whilst an evident change into gum goes on at certain points. 



If the action of water, gradually boiled, is watched, an alter- 

 ation which is at first very like that of sulphuric acid, as last 

 described, is apparent. But in the last stages the action is 

 so far different, that the cleft in the interior becomes a large 



