388 MM. Deville and Cahours on Vapour- densities. 



of-mercury vapour is not decomposed but only dissociated. The 

 phenomenon is at the same time an instructive lecture expe- 

 riment. 



The above observations of Deville led Cahours* to undertake 

 new determinations of the vapour-density of PCI 5 . This body 

 boils between 160° and 165°; and the density observed at 170° 

 to 172°, though considerably greater than that at 182° to 185°t, 

 was considerably below that calculated for a condensation to 4 

 volumes. The author could not observe a decomposition of 

 pentachloride vapour at this temperature. Hence he assumes 

 that the normal vapour of PCI 5 occupies 8 volumes, correspond- 

 ing to a union of PCI 3 and CI 2 without condensation. 



The vapour-density of acetic acid, determined at 350°, corre- 

 sponded exactly to 4 volumes; but at 440° a considerably 

 smaller number was obtained. It was found, however, that at 

 this temperature a small quantity of marsh-gas and carbonic 

 acid had been formed. Hence the author assumes that only 

 condensations to 4 or 8 volumes occur. In the cases of appa- 

 rent condensation to 3 or 6 volumes, it is only necessary to make 

 the determination at a higher or lower temperature to obtain at 

 once an alteration in the numbers. 



Deville J adduces a surprising instance of an abnormal vapour- 

 density. The double salt NH 4 I, 2HgI has, according to his 

 and Troost's experiments, at 350° the vapour-density 6*49, and 

 at 440° 6*88, corresponding to a condensation to 8 volumes. 

 Assuming in the experiment a dissociation into NH 4 Iand Hgl, 

 a condensation to 12 volumes should result. 



Chizinski has made, under the direction of Dr. Lothar Meyer, 

 an investigation on the influence of masses in chemical affinity §. 

 He took solutions of chloride of calcium and of double chloride of 

 magnesium and ammonium, and, having added to them relatively 

 smaller quantities of ordinary tribasic phosphate of soda, deter- 

 mined the amounts of lime and magnesia precipitated on the addi- 

 tion of ammonia. 



He found that the composition of the precipitate which con- 

 tains lime and magnesia is not materially influenced by the pre- 

 sence of large quantities of water (that is, by the degree of concen- 

 tration), nor by the quantity of ammonia added. Its composition 

 depended essentially on the quantities of chloride of calcium and 

 chloride of magnesium which were present. If in a series of 

 experiments with one and the same quantity of phosphoric acid 

 always the same quantity of chloride of magnesium, but variable 



* Comptes Rendus, vol. lxiii. p. 16. 

 t Liebig's Jahresbericht, 184/-48, p. 363. 

 X Comptes Rendus, vol. lxiii. p. 16. 



§ Liebig's Annalen (Supplement), vol. iv. part 2. Zeitschift fur Chemie, 

 April 1866. 



