294 F. W. Clarke-Note on Molecular Volumes. 



we compare the five chlorides given in the table we shall see 

 that upon arranging them in the order of their molecular 

 weights the differences between successive members of the series 

 increase as we ascend. Thus LiCl and NaCl differ by 5-5, 

 I KC1 by 11, KC1 and RbCJ by 16-5. This regular 

 difference-increase is certainly suggestive of some law yet to be 

 clearly made out 



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

 forded by the two other compounds. Sodium hydride, Na 3 H, 

 discovered by Troost and Hautefeuille * with a density of 0-959, 

 has a molecular volume of 491, or very nearly 5'5x9. Still 

 more interesting is iodine monochloride, so carefully studied by 

 Hannay.f At 0° the solid substance has a specific gravity of 

 3-263, and a molecular volume i ing 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 molec- 

 ular 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 o-8o2, Gore, and a molecular volume of 217 instead of 22. 

 I he chloride and iodide may be forced to agree by selecting 

 out the density determinations of certain investigators, and 

 rejecting other decidedly discordant 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 compounds ; 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 sesquicbloride does not even approxi- 

 mate. 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 appar- 

 ently disagree. We may, therefore, safely assert the following 

 general law, subject to possible exceptions. Every compound 

 * Compt Rend., vol. Ixxviii, 970. t Jour . C hem. Soc, H, xi, 818. 



