400 Intelligence and Miscellaneous Articles. 



NaCl and KC1 by 11, KC1 and EbCl by 16-5. This regular differ- 

 ence-increase is certainly suggestive of some law yet to be clearly 

 made out. 



A similar relation to Kopp's hydrogen-volume is also afforded by 

 the two other compounds. Sodium hydride (Na 2 H), discovered by 

 Troost and Hautefeuille*, with a density of 0*959, has a molecular 

 volume of 49-1, or very nearly 5*5x9, Still more interesting is 

 iodine monochloride, so carefully studied by Hannayt. At 0° the 

 solid substance has a specific gravity of 3-263, and a molecular 

 volume of 49*8, thus varying only 0*3 from a multiple of 5*5. At 

 101° the chloride boils, and at 98° its density is 2-958, having a 

 volume of 54*9. Probably an absolutely correct determination at 

 its boiling-point would give a value of 55. So we may say that 

 iodine monochloride, both as a solid at 0°, and as a liquid at its 

 boiling-point, has molecular volumes multiples of that of hydrogen. 



As for the haloid salts of silver, they cannot with certainty be 

 included among the substances connected by this multiple relation. 

 The fluoride agrees fairly, however, having a density of 5-852, Gore, 

 and a molecular volume of 21*7 instead of 22. The chloride and 

 iodide may be forced to agree by selecting out the density-determi- 

 nations of certain investigators, and rejecting other decidedly dis- 

 cordant data. The bromide does not agree at all. A determination 

 of my own for precipitated AgBr gives a density of 6-215, 17°, and 

 a corresponding volume of 30*25. Other determinations are even 

 more discordant than this. Silver salts generally have molecular 

 volumes equal or nearly equal to the corresponding sodium com- 

 pound, that of sodium bromide being 33*0. Silver fluoride, it will 

 be seen, diverges also from the sodium salts. For thallium our 

 data are insufficient. Its monochloride has a molecular volume 

 approximating to a multiple of 5*5, but not closely enough to be 

 satisfactory ; the sesquichloride does not even approximate. At 

 some future time I hope to be able to revise and extend our specific- 

 gravity determinations for this class of thallium salts. 



Now to sum up. Including the silver and thallium salts we know 

 the densities of twenty-five substances containing only univalent 

 elements. Of these, twenty have molecular volumes multiples of 

 that of hydrogen, three are doubtful, two apparently disagree. We 

 may therefore safely assert the following general law, subject to 

 possible exceptions : — Every compound containing only elements of the 

 hydrogen group has a molecular volume an even multiple of that of 

 hydrogen. This is probably but a hint of some more general regu- 

 larity connecting other elements and other groups. 



Postscript. — Since the above pages were written, there has been 

 published by Johnson a density-determination for potassium tri- 

 iodide, KI 3 (Chem. News, vol. xxxiv. p. 256). This determination, 

 3*498, corresponds to a molecular volume of 120*1. 121 is an exact 

 multiple of 5*5, and gives a theoretical density of 3*472. The mul- 

 tiple relation now holds good in twenty cases out of twenty-five. — 

 Silliman's American Journal, April 1877. 



* Comptes Sendus, vol. lxxviii. 970. t Journ. Chem. Soc. II. xi. 818. 



