398 



NATURE 



\March i6, 1876 



of the atomic weights of the constituent bodies. In fact the 

 order of the divergences would seem to render it probable that 

 the specific volumes of the several members of a family of ele- 

 ments increase with their atomic weights. 



In a former communication to the Royal Society the author 

 has given the results of a series of observations on the specific 

 gravities, boiling points, and rate of expani^ion of certain liquid 

 chlorides of phosphorus. Since Roscoe has shown that vanadium 

 is a member of the phosphorus group of elements, it appeared 

 to him that a comparison of the specific volumes of the analo- 

 gously constituted phosphoryl and vanadyl trichlorides might 

 serve to throw additional light on this question of the relation of 

 the specific volumes of the members of a family of elements to 

 their atomic weights. 



Three determinations of specific gravity of phosphoryl tri- 

 chloride made with different bottles gave a mean number of 

 171 185 at 0° compared with water at same temperature ; com- 

 pared with water at 4° the specific gravity is 1 71163. 



The rate of expansion of phosphoryl trichloride from 0° to its 

 boiling-point may be accurately represented by the expression — 

 V = I + 0001 064 309/ + o-ooo 001 12666/'* 

 + o'coo 000 cx)5 299/'. 



Its specific gravity at 107 "23 is i '50987; hence its specific 



volume = ^^^ ^„ = 101-58. 

 1-50987 



The results obtained with the dilatometer in the case of vanadyl 



tiichloride may be represented by the formula 



V = I + Q-ooo 965 236 if + o-ooo 000 898 26/' 



+ o-ooo 000 003 191 63 1"^. 



The mean of three experiments gives the specific gravity of 



vanadyl trichloride at 0°, compared with water at 4°, as 1-86527. 



The specific gravity of vanadyl trichloride at I27°-I9 is 1-63667 ; 



hence its specific volume is — '^^ '} - = 106 •';4. 

 ^ 1-63067 ^^ 



It is thus evident that the specific volumes of vanadyl and 

 phosphoryl trichlorides are not equal ; the compound with the 

 higher molecular weight has the greater specific volume. 



In the communication on the chlorides of phosphorus already 

 referred to, it is shown that if it be assumed, as appears in the 

 highest degree probable, that there is a relation between the 

 manner in which the oxygen atoms in a compound are held in 

 union and their specific volume, it follows that the oxygen atom 

 in POCI3 possesses the smaller of the two values 12-2 and 7-8 

 assigned by Kopp to oxygen, and accordingly that this atom is 

 attached to the phosphorus by only one combining unit. 



Thus— 



CI 



I 



P— O— CI 



I 



CI, 



showing that the phosphorus atom in phosphoryl trichloride 

 possesses the same atomic value as in phosphorus trichloride. 



As the difference between the two values for the volume of 

 oxygen, viz., 12-2 - 7-8 = 4-4, is but little less than between 

 the specific volumes of VOCI3 and POCI3, viz., 106-54-101-58 = 

 4-96, it is possible that the difference in the specific volumes of 

 the two liquids may be due to the different manner in which 

 the oxygen atoms are united to the vanadium and phosphorus 

 atoms ; for, if V be regarded as a pentad, VOCI3 must be 

 written — 



CI 



I 

 CI— v=o 



I 



CI, 



the oxygen atom having the value 12-2. Assuming Kopp's 

 value lor CI, viz. 228, this would leave for P and V nearly the 

 same specific volume, viz. : — 



P 25-4 



V 25-9 



From the uncertainty respecting the particular volume to be 

 assigned to the oxygen atom in vanadyl trichloride, our know- 

 ledge of the specific volumes of VOCI3 and POCI3 gives us little 

 aid towards solving the question whether the several members 

 of a family of elements have identical specific volumes. 



With a view to obtain further evidence, the author has rede- 

 termined flrith special care the boiling-points, specific gravities, 



and rates of expansion of the tetrachlorides of silicon, titanium, 

 and tin. 



The atomic weights of Si and Ti and of P and V show about 

 the same gradational difference ; — 



Si 28-10 P 31-00 



Ti 5o'oo V 51-35 



And, since the tetrachlorides are free from oxygen, the un- 

 certainty arising from the specific volume of that element is eli- 

 minated. He has also compared the specific volumes of the 

 trichlorides of phosphorus, arsenic, and antimony, making us2 

 of Kopp's determination in the case of the last-named com- 

 pound. Material is thus obtained for the discussion of the 

 question from analogous derivatives of two well-defined groups 

 of elements, viz. : — 



Si 28-10 P 31-00 



Ti 50"00 V 51-35 



As 55-15 



Sn 118-10 Sn i22'3o 



The results of the determinations of the specific volumes of the 

 tetrachloride of silicon, titanium, and tin, liquids of analogous 

 constitution and all derivatives of the tetrad group of elements, 

 serve to establish the conclusion indicated by the difference in 

 the specific volumes of phosphoryl and vanadyl trichlorides, that 

 the specific volumes of the several members of a family of ele- 

 ments are not identical, but that the specific volume increases 

 with the atomic weight of the member. 



Molecular Specific 



weight. volume. ' 



SiCl^ 169-94 ••• 121-13 



TiCl4 191-84 ... 12603 



SnCli 259-94 ... I3i'4i 



It is also noteworthy that the difference between the specific 

 volumes of tin and titanium tetrachlorides is almost the same as 

 the difference between the specific volumes of vanadyl and phos- 

 phoryl trichlorides ; the difference between the atomic weights of 

 vanadium and phosphorus is nearly the same as that between the 

 atomic weights of titanium and silicon. 



Mol. weight. D if. Spec. vol. Diff. 



POCI3... 153-38 ... ) 2o-7< J'°»*58 •••L.Q6 

 V0C]3... 17373 -S ^°35 I 10654 ... h96 

 SiCl4 ... 169-94 ■■■\ ,,.0 1121-13 



21-9 



4-90 



TiCU ... 191-84 ... J "' y I 126-03 

 It would seem from this that the constitution of vanadyl tri- 

 chloride is similar to that of the phosphoryl compound, and 

 must therefore be expressed by the formula — 

 CI 



I 

 V-O— CI 



I 



CI 

 in which V appears as a triad, the oxygen atom having the same 

 specific volume as in phosphoryl trichloride. On the other 

 hand, the order of the divergences shown by P, As, and Sb 

 {vide infra), would appear to indicate that V may be pentad ia 

 this compound, whence O would have the volume 12-2. 



The numbers representing the specific volumes of the trichlo- 

 rides of phosphorus, arsenic, and antimony exhibit a gradational 

 order similar to that shown by the volumes of the tetrachlorides 

 of silicon, titanium, and tin, and also by the specific volumes of 

 phosphoryl and vanadyl trichlorides : — 



Mol. weight. Spec. vol. 



PCI, 137-38 93-54 



ASCI3 181-53 94-64 



SbClj 228-68 9782 



although the differences are^ much less than in the cases of the 

 two latter groups. 



III. Oti the Specific Volumes of Bromine and Iodine Mono- 

 chloride ; and of Ethene Bromide and Ethene Chlor iodide. 

 The molecular weight of bromine is, as is well known, nearly 

 equal to the arithmetic mean of the molecular weights of chlorine 

 and iodine : hence the molecular weights of bromine and of 

 iodine monochloride (ICl) are nearly identical. These sub- 

 stances closely resemble each other in physical properties. Both 

 are dark-red liquids about three times heavier than water. 

 Bromine boils at about 59°-5, and solidifies at -24°-5 ; iodine 

 monochloride melts at -f 24° -5, and boils at lol" : the interval 

 between the boiling- and melting-points is therefore approxi- 

 mately equal. 



