2i6 THE POPULAR SCIENCE MONTHLY. 



driven off by the heating and stirring which has to reach 240°, in 

 order to effect the changes above described. 



I suspect that the difference between the forms of tapioca and 

 arrowroot has arisen from the necessity of thus driving off the last 

 traces of the poison with which the aboriginal manufacturers were so 

 well acquainted as to combine the industry of poisoning their arrows 

 with that of extracting the starch-food from the same root. No cer- 

 tificate from the public analyst is demanded to establish the absence of 

 the poison from any given sample of tapioca, as the juice of the mani- 

 hot-root, like that of other spurges, is unmistakably acrid and nau- 

 seous. 



Sago, which is a starch obtained from the pith of the stem of the 

 sago-palm and other plants, is prepared in grains like tapioca, with 

 similar results. Both sago and tapioca contain a little gluten, and 

 therefore have more food-value than arrowroot. 



The most familiar of our starch-foods is the potato. I place it 

 among the starch-foods, as, next to water, starch is its prevailing con- 

 stituent, as the following statement of average compositions will show : 

 Water, 75 per cent ; starch, 18*8 ; nitrogenous materials, 2 ; sugar, 

 3 ; fat, 0'2 ; salts, 1. The salts vary considerably with the kind and 

 age of the potato, from 0*8 to 1'3 in full grown. Young potatoes con- 

 tain more. In boiling potatoes, the change effected appears to be sim- 

 ply a breaking up or bursting of the starch-granules, and a conversion 

 of the nitrogenous gluten into a more soluble form, probably by a cer- 

 tain degree of hydration. As we all know, there are great differences 

 among potatoes, some are waxy, others floury ; and these, again, vary 

 according to the manner and degree of cooking. I can not find any 

 published account of the chemistry of these differences, and must, 

 therefore, endeavor to explain them in my own way. 



As an experiment, take two potatoes of the floury kind ; boil or 

 steam them together until they are just softened throughout, or, as we 

 say, " well done." !N'ow leave one of them in the saucepan or 

 steamer, and very much overcook it. Its floury character will have 

 disappeared, it will have become soft and gummy. The reader can 

 explain this by simply remembering what has already been explained 

 concerning the formation of dextrin. It is due to the conversion of 

 some of the starch into dextrin. My explanation of the difference 

 between the waxy and floury potato is that the latter is so consti- 

 tuted that all the starch-granules may be disintegrated by heat in the 

 manner already described, before any considerable proportion of the 

 starch is converted into dextrin, while the starch of the waxy potatoes 

 for some reason, probably a larger supply of diastase, is so much more 

 readily convertible into dextrin that a considerable proportion be- 

 comes gummy before the whole of the granules are broken up — i. e., 

 before the potato is cooked or softened throughout. 



I must here throw myself into the great controversy of jackets or 



