63 



disappears ; those which stand the temperature without bursting, 

 appear then smaller ; structure and navel become indistinct and often 

 unrecognizable. 



The starch granules from the crust of English biscuits are gener- 

 ally somewhat smaller than those from the center, on account of the 

 more direct influence of the heat upon the outside during baking. 

 The same is the case with bread. The straight lines marking the 

 strata appear, after roasting, wavy, caused by the irregular expansion 

 of the layers. The presence of these wavy lines, when well defined, 

 is decisive in the question whether the starch had been roasted or 

 not. Frequently it is easier to decide whether a starch had been 

 roasted at between 100 to 150° C, than to come to a decision re- 

 garding the origin of the starch. 



The magnifying power of the microscope should, for these ex- 

 aminations, be not too small, since often minute details must be ob- 

 served. It is also advisable to make all observations with the same 

 lenses. The size of the granules is thus easier remembered and Rec- 

 ognized, and the diagnosis thereby greatly facilitated. #" 



A magnifying power of from 400 to 500 diameters is sufficient 

 for all purposes. 



Microscopical Examination for 

 Carbohydrates. 



To prepare a sample of milk-food for examination, the fat should 

 first be removed from 0.5 to 1 grm. in the apparatus devised for 

 fat determination, and described on page 26. Some of the sub- 

 stance is then brought on a slide, and mixed with one drop of a 

 mixture of equal parts of glycerine and water. This mixture is 

 preferred, since in pure water the granules swell too rapidly, while 

 with pure glycerine no distinct picture can be obtained. Milk-food 

 is very finely-ground, and it is, therefore, natural that not all the 

 particles in the field of vision will show the required character- 

 istics. These can safely be neglected, and it is, therefore, only 

 necessary to observe such as will present sufficient characteristics 

 for classification. 



A microscopical examination is to be considered only as a qual- 

 itative, and never a quantitative, test in the case of milk-food. 



The presence of starch can also be determined by the iodine 

 reaction. The result thus reached is, however, too general and, 



