CRYSTALS OF HEMOGLOBINS EMPLOYED IN THIS RESEARCH. 145 



and the crystallographic test of differences of chemical constitution is 

 recognized as the most delicate test of such differences. For instance, 

 in the case of isomerides, the chemical differences between such substances 

 consist in the differences in the arrangement of their constituent atoms, the 

 position of a replaced hydrogen atom in a group (in which several such 

 replacements are possible) altering the structure. Hence the following is 

 true: "Isomeric substances possess different crystal structure." (Groth, 

 Chemical Crystallography, trans, by Marshall, 1906, 63.) Such differences 

 in crystal structure can generally be readily recognized, but to detect chem- 

 ical differences between isomerides by any centesimal chemical analysis is 

 obviously impossible. Chemical differences between such substances are 

 detected by differences in solubility, in melting-point, in rotatory power, in 

 reactions in which the substance is altered or decomposed; but when large 

 numbers of isomerides are possible, as in the enormous molecules of the 

 proteins, the detection of differences between them by purely chemical pro- 

 cesses has thus far, except in rare instances, been found impossible. 



The crystallographic method is, of course, adapted to detecting differ- 

 ences between substances that show differences in centesimal composition 

 even better than between isomerides, for here the differences in structure 

 may be more profound than in the case of the isomerides; and differences 

 in centesimal composition must of necessity imply differences in structure. 

 Hence the general law may be enunciated : Substances that shoiv differences 

 in crystallographic structure are different chemical substances. 



THE PETROGRAPHICAL MICROSCOPE AND ITS USE. 



The necessity of studying small crystals, especially sections of such 

 crystals as are met with in rock sections, has resulted in the evolution of a 

 form of microscope which is at once a goniometer, a polariscope, and an 

 instrument for measuring optic axial angles in short, for determining the 

 physical crystallographic constants of small crystals. Of necessity, in some 

 of its measurements it is not so exact as other instruments that may be 

 employed for the same purpose, for its parts must be light, and its circles 

 can not be read to the same degree of accuracy as those of the reflecting 

 goniometers and spectrometers. The determination of the angle of a crys- 

 tal by this instrument is, under favorable conditions, not accurate to less 

 than 10' of arc. But when it is remembered that carefully made measure- 

 ments on the reflecting goniometer often vary as much as this in different 

 crystals of the same substance it is seen that data of value may be procured 

 with the aid of such a microscope. 



The polariscope portion of the petrographical microscope enables the 

 observer to determine the position and relative value of the elasticity axes 

 of crystals, to observe the position of the optic axes, and to determine their 

 inclination to each other and to the elasticity axes. From these data the 

 optical character of the crystal is determined. These optical reactions may 

 be studied by this instrument with as much ease, and in general with as 

 much accuracy, as with the larger and better graduated polariscope; and 

 the data thus obtained are quite as accurate in most cases as those obtained 



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