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. 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.