MICROSCOPIC OBJECTS. 87 



substances to have a collection of specimens of known 

 purity from different plants mounted and labelled, so that 

 they can readily compare with them any new and un- 

 known kind of starch. 



According' to BerzeHus, Iceland moss contains 44 per 

 cent, of starch, while the tubercles of the potato only 

 contain 20. The rhizomes of plants, as those producing 

 the arrowroot, &c., average from 12 to 26 per cent. 

 Seeds jdeld the greatest quantity ; maize and rice con- 

 taining 80, wheat-flour 70, and peas and beans about 30 

 per cent. 



Tapioca and pearl sago, which during their preparation 

 are submitted to heat, have their grains broken ; hence 

 they swell in cold water, and become blue with iodine, 

 but are readily distinguishable under the microscope from 

 other forms of starch. 



The grains of starch form an excellent test, in the 

 polarising microscope, of the axis of the polarising prisms; 

 they require the line of coUimation to be very true. — The 

 magnifjdng power necessary for the examination of these 

 grains is 150 diameters. — Note, In mounting these objects 

 they must be protected from pressure ; a thin paper should 

 be inserted between the glasses. 



Sections of Wood (recent) . — The usual and common 

 mode of examining microscopically the- structure of 

 wood, has been to cut a thin shce or section from the 

 branch, and then view it as a transparent object. In this 

 manner a variety of beautiful reticulated lacelike objects 

 may be obtained, but httle information is acquired of the 



