OP ARTS AND SCIENCES. 7 



chemist most frequently relies. As early as ISGO, we ourselves showed 

 that constancy of crystalline form was, under certain conditions, com- 

 patible with quite wide variations of compositions ; * and our conclusions 

 have been most fully confirmed by the results of investigations since 

 made, which reveal the great complexity of crystalline structures. In 

 the compounds t of zinc and antimony which we investigated, the 

 variation was large, because the affinity was weak ; but we gave abun- 

 dant evidence that similar variations, although of less extent, are not 

 uncommon in more stable compounds, and from our subsequent experi- 

 ence we feel confident that in proportion as analytical processes become 

 more accurate tliese variations will become more apparent. At present, 

 they are to a great extent concealed, because the possible errors of our 

 analytical processes are so great; and we very properly refer, at the 

 outset, any seeming variations from exact atomic proportions to errors 

 of this class. But, in the more accurate methods on which we should 

 alone rely for fixing atomic weights, such variations, when they 

 exist, become of great importance. For call by whatever name you 

 please that small portion of either constituent of a compound which 

 may be present in a crystal over and above the atomic ratio, — regard 

 it merely as so much "dirt" entangled by the crystalline structure, — 



* Crystalline Form not Necessarily an Indication of Definite Chemical Com- 

 position. I'hilosophical Magazine, June, 1860; also Poggendorff's Annalen, 

 Band cxii. 90 ; also Memoirs of American Academy of Arts and Sciences, New 

 Series, vol. v. p. 337. 



t Rammelsberg lias expressed the opinion — Pogg. Ann., cxx. 61, 1863 — that 

 all crystalline alloys are isomorphous mixtures of the constituent nietals, and 

 includes under this category the two compounds of zinc and antimony here 

 referred to; but, had this accurate observer repeated our experiments and 

 measurements, he would have seen that the whole order of the phenomena we 

 studied is inconsistent with such an assumption. Although the two metals may 

 be alloyed in all proportions, yet we give in our paper the strongest evidence 

 that the union is attended with definite chemical action, which has two maxima 

 at the points represented by the symbols Sb^Zn., and Sb^Zng; and althougii the 

 measurements of the two sets of crystals may be referred approximately to the 

 same fundamental form, as is frequently the case in the simpler systems, 

 especially if we admit such ratios as 4:5, yet the two types of crystals differ 

 completely in their habit, and the very remarkable circumstances of their for- 

 mation prove that they are essentially distinct. For a full statement of these 

 circumstances we must refer to our original paper above cited. In the very 

 partial abstract from this paper — Pogg. Ann., xcvi. 584 — to which Rammels- 

 berg alone refers, none of these important facts were given ; and, moreover, 

 Rammelsberg does not seem to have noticed tliat our paper published in 1855 

 preceded Schneider's determination of the atomic weight of antimony. 



