296 DIFFERENTIATION AND SPECIFICITY OF STARCHES. 



Our methods of differentiating stereoisomers are almost entirely quantitative, that 

 is, by differences in density, solubility, color, temperature of gelatinization, melting-point, 

 degree of decomposability, digestibility, precipitability imder given conditions, color reac- 

 tions, intensity and rapidity of staining with aniline dyes, etc. By physical methods they 

 may be distinguished by their intensity and direction of rotation of the ray of polarized 

 light, by the peculiarities of the interference figure in polarized light, by the form of crystals, 

 etc. Sometimes seemingly qualitative differences in the reactions may appear which may 

 be more or less illusory, as, for instance, in the behavior of different parts of the starch- 

 grain to different anihne dyes. This is well illustrated in experiments in which two dyes 

 are used that have different affinities for different parts of the grain, as in the experiments 

 of Denniston (page 56) . Thus, when grains are exposed to a solution of gentian violet for 

 5 minutes and then treated with orange G for different periods varying from 1 minute 

 to 3 hours, it was found that with exposure to the orange G for 1 minute the peripheral 

 layer is stained a pale liolet and the inner layers a dark violet. With exposure to the 

 orange G for 5 minutes a peripheral orange layer is plainly differentiated which extends 

 entirely around the violet inner portion of the grain. With exposure to the orange G for 

 60 to 100 minutes the layers inside still show a pale violet and the outer layer orange. 

 This differentiation might be taken to mean a specific quaUtative chfference in the reac- 

 tions of the imier and outer parts in the sense that only the outer part reacts to orange G; 

 but if exposure to the orange G be longer the inner part also becomes orange except a few 

 layers midway between the liilum and the distal end of the grain, which doubtless by longer 

 exposure would also become orange. The inner part of the grain has a greater affinity 

 for the gentian violet than for the orange, while the reverse is the case with the outer parts, 

 the presence of the violet interfering for a time with the combination with the orange G ; but 

 with an increase in the orange G the inner part also becomes orange, showing therefore 

 merely a difference in the intensity of the reactions of these two fundamental parts of the 

 grain. Therefore, in attempts to differentiate stereoisomers by means of physico-chemical 

 and chemical means we are to expect in the present state of our knowledge differences in 

 degree rather than in quality, and that differences that many assumed to be qualitative 

 may be found to be merely quantitative if properly investigated. 



By processes of exclusion and selection a number of methods were finally adopted 

 which for various reasons seemed especially desirable in view of the main objects and the 

 conditions attending the prosecution of the research. Undoubtedly some of these methods 

 might be replaced by others that are better, but an investigation of the relative values 

 of different methods would have of itself proved a laborious inquiry. The following methods 

 were employed : 



HISTOLOGICAL METHOD. 



As has been pointed out in preceding chapters, this method has been found to be of 

 signal usefulness; and, in fact, up to very recent years it has been the sole reliance in 

 attempts to determine the kind of starch. It was, however, perfectly obvious at the very 

 inception of this research, and rendered clear as far back as the investigations of C. Nageli 

 in 1858, that this method, unless associated with others, could not be depended upon, 

 and that it was liable to be absolutely misleading. Moreover, differences in form may 

 not in the least imply differences in the starch-substance, as has been made evident in early 

 chapters. Magnification ranging from 85 to 400, sometimes higher, was used, according 

 to the size of the gi'ains and incidental conditions. A sufficient amount of dried starch was 

 placed on a slide and mounted in a very dilute Lugol's solution, care being taken not to 

 add a larger quantity of iodine than is sufficient to accentuate the lamelke. Since starches 

 of different sources show wide differences in the intensity with which they become colored 

 with iodine it was found convenient to have on hand a number of solutions ranging from 

 1 to 2 per cent down. By the aid of such ordinary microscopic technique there were 



