CHEMICAL MICROSCOPY 



At about this time a new means of ob- not the first, as stated by the "Dictionary 



serving crystals microscopically was devel- of National Biography", for Talbot had al- 



oped, that is, the polarizing microscope, ready taken the first photomicrographs of 



Double refraction had been observed by history as early as 1835, and reported on his 



Bartholinus as long ago as 1069, and about work in January 1839. Both men used the 



1678 was partially explained by Huygens, solar microscope, on which Reade made im- 



who later in the century also made further provements. Talbot specifically called atten- 



observations bearing on polarization. The tion to the value of the photography for 



definitive discovery of polarized light is recording the appearance of microscopic, 



credited to Mains, over a hundred years chemical crystallizations, 

 later, in 1808. By 1811 it was completely At least as early as 1838 toxicologists were 



explained in terms of light theory by Fres- concerned with how they might identify al- 



nel and by Arago. The optical crystallog- kaloids, for obviously no drastic treatment 



raphy of minerals was developed rapidly by could be used as in the separation of a min- 



Malus, Biot, and others, especially Sir David eral poison, and then there was the problem 



Brewster from 1811 to 1819 and later. of distinguishing these poisons from harm- 



Brewster seldom referred to magnifying less, possibly prevalent, natural bases. Al- 



aids but used them as convenient; e.g., by ready some suggestion had been made that 



mounting a plano-convex lens on a plate of microscopic distinctions might be possible, 



tourmaline or agate as an analyzer, and even, but in 1838 there was as yet little or no idea 



on occasion, directing polarized light through of using a reagent to produce crystals which 



a compound microscope. Sources of the could be seen microscopically to be different 



polarized light were reflection (Mains' origi- for different substances, 

 nal observation), or oblique transmission This idea, however, gradually appeared 



through a set of plates of glass, as well as during the 1840's and 1850's. It was present, 



light transmitted through tourmaline, which somewhat rudimentally, in some work of 



has extreme dichroism. A black mirror could Thomas Anderson of Edinburgh, who de- 



also be used to "analyze" by reflection; and scribed test-forms of some free alkaloids and 



also, as early as 1819, Brewster had discov- their thiocyanates in 1848. A specific crystal 



ered how to extinguish one of the images of test appeared in 1852-53, with Herapath's 



calcareous spar, nearly perfecting the ana- discovery of the remarkable quinine iodo- 



lyzer, and anticipating, in a way, Nicol's sulfate, and his own recommendation that 



invention. the crystals could be formed in a drop of 



Nicol invented his remarkable prism in reagent as a test for quinine. Both of these 

 1828. In 1834 (W.) H. F. Talbot made the were included in "The Micrographic Die- 

 polarizing microscope a definite instrument, tionary", by J. W. Griffith and Arthur Hen- 

 and immediately applied it to chemical sub- frey, first edition 1856. In 1859 Taylor, in 

 jects. This w^as the same Talbot who became the second edition of his toxicology, "On 

 one of the founders of photography, and of Poisons", gave seven different microcrystal 

 modern archeology. Apparently chemists in tests for strychnine, and even made refer- 

 general were not alert to utilize the new in- ence to the "polarizing properties" as help- 

 strument, and only in recent years have ing to characterize the crystals, as well as 

 they begun to borrow it back from the min- giving some other microcrystal tests, includ- 

 eralogists. ing one for cyanide using crystals given by 



J. B. Reade, another chemist, microsco- HCN vapor in a hanging drop of AgNOs ; 

 pist,and photographic discoverer, took photo- but the microcrystal tests were not system- 

 micrographs in 1839. However, they were atically developed. 



40 



