Roddam Hall, near Alnwick,
September 12, 1867.
I was very glad to hear by your note received to-day of your safe arrival and the success of your observations.… You did quite right in returning home rather than wait indefinitely for the Jerome. The working out of your Pará and of the Lisbon direct and diffuse-light experiments will take some time and labour, but I believe the results will repay the trouble.…
I have been up to the Dundee [British Association] meeting for a few days, but I now, in all probability, shall stop here, so that I can at once answer your letter.
Please ask Joseph [Heywood] to send me per book-post Pogg. Ann., vol. 98, in which volume is Rammelsberg’s paper on the isomorphism of vanadates and phosphates. There is no doubt in my mind that vanadic acid is V₂O₅, and it will be exceedingly interesting to work out the vanadates which must all be explained as phosphates. The ordinary white NH₃ salt is NH₄VO₃ (like NaPO₃) and is a meta-vanadate. The bi-vanadates can also be explained, but all need re-preparation and analysis. Did I tell you that we have now got
V₂O₅, V₂O₄, V₂O₃, V₂O₂ (I wish we had V also!)
V₂O₂Cl₆, V₂O₂Cl₄, V₂O₂Cl₂, and V₂O₂Cl₆, or VOCl₃, VOCl₂, VOCl
At St. Andrews I saw Professor Heddle; he has a crystal half apatite and half vanadinite, and he threw out the suggestion long ago that vanadic acid is V₂O₅.… I hope you will write soon and let me hear what you have to say about my plan. I will then write what I think of your ideas.
Roddam,
Tuesday, September 17, 1867.
I have now edited the atomic weight determinations by oxidation, and also the various oxides of vanadium. I have now to do the chlorides. Many points still remain requiring clearing up.
(1) As regards the slow oxidation of the V₂O₃ (Berzelius’s suboxide); a sample made on November 13, 1866, weighing 0·7507, weighed on June 12, 1867, 0·8733. This corresponds to an oxide higher than V₂O₄ (V = 51·4). Now I want this oxide and tube drying under the air-pump and weighing carefully and keeping for further examination (the weight of tube and oxide was 3·6066 on June 12th last).