CHEMISTRY 



By H. Carrington Bolton, 



Professor of Chemi»trrj, Trinitjj College, Flarlford. 



GENERAL AND I'HYSICAL. 



The Color of Chemical Compounds as a Function of the Atomic Weights 

 of their Constituent Elements. — Thomas Caruelly finds that the color of 

 chemical compounds depends on at least three conditions — (1) the tem- 

 peratnre, (2) tlie quantity of tlie electro-negative element, and (3) the 

 atomic weights of the constituent elements. Of these three conditions 

 the two former have been esi)ecially studied by Mr. Ackroyd, whose re- 

 sults are thus summarized: 



(1) All the chromium comi)ounds change their color in a definite serial 

 order, that of the spectral colors, in such a manner that as the tempera- 

 ture rises the color approximates more and more tothe red end of the 

 spectrum, and ultimately, at a sufficiently high temperature, passes into 

 brown and black. Most frequently the transition of color is direct from 

 white to a pale yellow, while violet, indigo, blue, and green are passed 

 over as transition stages. 



(2) In binary compounds an increase of the quantity of the electro- 

 negative element involves a change of color to the red end of the spec- 

 trum, and ultimately into brown and black. 



(3) Dr. Carnelly expresses his law as follows. Influence of the atomic 

 weight: in some series of compounds, Ax Ry; Bx Ky; Cx Hy; &c., 

 in which E is an element or a group of elements, while A, B, C, &c., 

 signify elements belongiug to the same subordinate group in Men- 

 delejeff's table of tlie natural classification of the elements, the color 

 pas'ses entirely or partially through the following scale : White or 

 colorless, violet, indigo, blue, green, yellow, orange, red, brown, black, 

 with the increasing atomic weight <>f the elements A, B, C, &c. In 

 other words, the higher the atomic weight of the elements A, B, C, rises, 

 the more the color of the compound approaches the red end of the 

 spectrum, and passes in certain cases into brown and black. This rule 

 applies only when the elements A, I>, C, &c., belong to tlie same sub- 

 group. In 420 cases examined the author found only 1-4 exceptions, 4 

 four of which, DiCl3,V20:j, CrO;j, and CdO, do not admit of satisfactory 



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