In those days Gmelin’s “Handbuch” was the chief repository of chemical knowledge—or the want of it—and many suggestions as to possible profitable fields of inquiry were to be gleaned from its pages. One such subject was perchloric acid and its compounds, concerning which but little was then known beyond the composition of potassium perchlorate, established by Stadion as far back as 1816. Roscoe, with Schorlemmer’s assistance, made a fairly complete investigation of perchloric acid and its hydrates, and a number of its salts.[14]
He narrowly escaped a serious accident when working with ethyl perchlorate, first prepared in 1840 by the American chemists Hare and Boye, and known to be extremely unstable. He was engaged in filtering a few cubic centimetres of the liquid into a test-tube, when the compound exploded with great violence, and a deep hole was bored into the base of the filter-stand, and many hundreds of fragments of glass were driven into his hand. That filter-stand was long an object of interest to visitors in the private laboratory of the old Owens College.[15]
The writer subsequently prepared thallium perchlorate for him in a pure state, determined its composition, and established its isomorphism with the alkaline perchlorates, the crystallographic characters of which had been previously ascertained by Kopp.[16]
But Roscoe’s most important contribution to inorganic chemistry was unquestionably his research upon vanadium and its compounds, which occupied him for the greater part of five years. About 1865 his attention was drawn to the occurrence of vanadium in some of the copper-bearing beds of the Lower Keuper Sandstone of the Trias which were then being worked at Alderley Edge and Mottram St. Andrews in Cheshire. He obtained possession of a large quantity of a lime precipitate, which was found to contain about 2 per cent. of vanadic acid. It was a most unpromising material, but eventually a method was worked out by which the vanadium was extracted as an ammonium vanadate: this on heating yielded vanadic acid. Great difficulty was met with in freeing the vanadic acid from accompanying phosphoric acid. Even small quantities of phosphoric acid cause the vanadic acid after fusion to solidify as a pitch-like amorphous mass. It was the writer’s privilege to assist in the early stages of this investigation, and it fell to his lot to carry out the various experiments which eventually served to establish the composition of the oxides of vanadium, the true nature of its volatile chloride, the existence of hitherto unknown oxychlorides, and of the mononitride which Berzelius had regarded as the metal, and lastly to fix its real atomic weight and to show that it was approximately 16 below that assumed by Berzelius. It was only very gradually that the true chemical relationships of vanadium revealed themselves. For a time the indications were contradictory and perplexing. The first clue was given by Rammelsberg’s observation that vanadinite is isomorphous with pyromorphite and mimetesite—analogously constituted minerals containing phosphorus and arsenic. The next significant fact to be discovered was that by the action of a reducing agent it was possible to obtain a solution of a vanadium oxide which on reoxidation to vanadic acid appeared to require as much oxygen as Berzelius’s vanadium, regarded as metal, would have needed. When it was discovered that the volatile chloride which Berzelius had considered was a trichloride and free from oxygen, in reality contained oxygen, and was analogous in constitution to phosphoryl chloride, the whole matter was rapidly cleared up, and the chemical affinities of vanadium to phosphorus, arsenic, and the other members of the trivalent group were established. This, of course, necessitated altering the formulæ of the vanadium compounds hitherto described.
At the close of the College Session 1866-1867, Roscoe took away with him the laboratory journals containing the results of the inquiry as far as it had progressed, and worked at them at Roddam Hall, near Alnwick, which he had taken for the Long Vacation.
The following letters have reference to this matter:
Roddam, near Alnwick,
August 26, 1867.
I write a line to say that I hope you are getting on well and that I shall soon hear from you.… I want you very much to stay with me till April to settle the vanadium and light matters and help me in London with my lectures.… I have at last found out about vanadium. The acid is V₂O₅ like P₂O₅. The chloride VOCl₃ like POCl₃ and the solid chlorides VOCl₂, VOCl, etc. This explains the isomorphism of the vanadate of lead and the corresponding phosphate and lots of other points. It becomes very interesting now.
Pray write a line and say whether you will stay till April, and when you will be back.
The first paragraph in the next letter alludes to the circumstance that the present writer had just returned from Lisbon, where he had carried out the photometric measurements already referred to.