_The vapour density_ of phosphorus trichloride and oxychloride corresponds with their formulæ (Cahours, Würtz)--namely, is equal to half the molecular weight referred to hydrogen. But it is not so with phosphorus pentachloride. Cahours showed that the vapour density of phosphorus pentachloride referred to air = 3·65, to hydrogen = 52·6, whilst according to the formula PCl_{5} it should be = 104·2. Hence this formula corresponds with four, and not with two, molecules. This shows that the vapour of phosphoric chloride contains two and not one molecule, that in a state of vapour it splits up, like sal-ammoniac, sulphuric acid, &c. The products of disruption must here be phosphorous chloride, PCl_{3}, and chlorine, Cl_{2}, bodies which easily re-form phosphoric chloride, PCl_{5}, at a lower temperature. This decomposition of phosphoric chloride in its conversion into vapour is confirmed by the fact that the vapour of this almost colourless substance shows the greenish-yellow colour proper to chlorine. This dissociation of phosphoric chloride has been considered by some chemists as a sign that phosphorus, like nitrogen, does not give volatile compounds of the type PX_{5}, and that such substances are only obtained as unstable molecular compounds which break up when distilled; for example, PH_{3},HI, PCl_{3},Cl_{2}, NH_{3},HCl, &c. To prove that the molecule PCl_{5} actually exists, Würtz in 1870 observed that when mixed with the vapour of phosphorous chloride the vapour of phosphoric chloride distils over (from 160° to 190°) perfectly colourless, and has a density which is really near to the formula--namely, to 104--and the same density was determined for the pentachloride in an atmosphere of chlorine. Hence at low temperatures and in admixture with one of the products of dissociation, there is no longer that decomposition which occurs at higher temperatures--that is, we have here a case of dissociation proceeding at moderate temperatures. An important proof in favour of the type PX_{5} is exhibited by phosphorus pentafluoride PF_{5}, obtained by Thorpe as a colourless gas which only corrodes glass after the lapse of time; it may be kept over mercury, and has a normal density. It is formed when liquid arsenic trifluoride, AsF_{3}, is added to phosphoric chloride surrounded by a freezing mixture: 3PCl_{5} + 5AsF_{3} = 3PF_{5} + 5AsCl_{3}.