524 PROCEEDINGS OF THE AMERICAN ACADEMY 



early pai^er Professor Cooke was able to quote thirty years later as 

 the exi3ression of his riper judgment. 



An investigation upon the alloys of zinc and antimony, which 

 followed soon after, in 1854, proved the existence of two definite crys- 

 talline compounds of the two metals. A subsequent study showed 

 that the crystalline form of these bodies remained constant with a 

 tolerably wide variation in percentage composition. This fact led him 

 at the time to suggest that perturbation of the law of definite propor- 

 tion could be effected by mass, and that these perturbations became 

 serious wherever chemical affinity was weak. Six years later he ap- 

 plied the same course of reasoning to the composition of minerals, and 

 came to the conclusion that the geneial formula in this case was but 

 the typical formula toward which the mineral tended, but which per- 

 haps was never realized with any actual specimen. Twenty-five years 

 afterwards, when Butlerow again discussed the possible variability of 

 the law of definite proportion, Professor Cooke, after referring to 

 his own earlier views upon the subject, wrote that he felt the weight 

 of evidence to be in favor of the atomic theory, and that absolute 

 definiteness of combining proportion which this theory involves. 



In 1860 appeared the detailed account of the brilliant researches 

 of Kirchhoff and Bunsen upon spectrum analysis ; and with character- 

 istic enthusiasm Professor Cooke was soon absorbed in the study of 

 this new mode of chemical investigation. Several papers appeared 

 upon the construction of spectroscopes, the projection of the spectra 

 of the metals, and upon the aqueous lines of the solar spectrum. 

 While thus engaged in the study of the spectroscope, he found time to 

 make a crystallographic examination of the acid tartrates of caesium 

 and rubidium and of childrenite, and to investigate the dimorphism of 

 arsenic, antimony, and zinc. The results of these investigations were 

 published from time to time in the American Journal of Science. 



For several successive years he was now engaged in the study of 

 new mineral species. In 1866 he described danalite from Rockport, 

 Massachusetts, and in the following year cryophyllite from the same 

 locality. He also described at this time two analytical processes 

 which had evidently been -^suggested by his work upon these minerals. 

 A few years later appeared a paper upon the vermiculites, in which 

 two new species or varieties, hallite and culsageeite, were described, and 

 this was supplemented in the following year by an account of the in- 

 vestigation, with F. A. Gooch, of two more varieties of the same 

 family. These researches, together with a note upon melanosiderite 

 and a crystallographic study of some American chlorites, were the last 

 of his purely mineralogical contributions. 



