Your carbonic acid observations are _very_ interesting: you seem to have settled Lewy completely, and I hope you will get some more experiments made on your return voyage. Could you not manage to make several series of _daily_ observations (photochemical) on your return voyage? It would not much matter if the surface was not perfectly horizontal always, and you could use your pendulum concern to steady the exposed paper to a certain extent. Perhaps your ship does not give you a sufficiently free horizon. However, you will do what is possible. Your meeting with Agassiz was very fortunate, and I was glad to hear that the other friends whom you found were likely to prove agreeable. We have not much news to send you. The book [“Lessons in Elementary Chemistry”] is not yet out. I have this day, however, corrected the last proofs of the Index, and I fully expect that it will be ready (and I hope a great number sold!) before you arrive here. My new assistant [Mr. Francis Jones] is a very careful and accurate worker. He has with great care determined the atomic weight of vanadium by the loss of weight of VO₃ in hydrogen, and curiously enough gets _exactly_ Berzelius’s number of 68·5! This in two experiments on large quantities. We are now preparing pure chloride to try again whether we get 67·4 (your number), and we have got hold of some very queer reactions, which I can only understand either by the presence of another metal having the same (or nearly) atomic weight as Va, or else by the existence of an isomeric (solid) modification of the chloride. However, time, I hope, will show. My lecture on June 1st at the Royal Institution will, of course, be shorn of some of its interest as I cannot tell them how much chemical light there is in the tropics, but I hope to have enough to make an interesting hour, and have got some nice experiments to illustrate the opalescence of the atmosphere. My wife tells me to remind you to be so good as to bring her something tropical—some birds’ skins would do or anything you see or fancy. Only no monkeys, if you please, for me! I hate the animals. Your father will doubtless send you all the news of the times, as well as local information, and so I will only add that the Laboratory [rowing] crew were again beaten last week. The races were very good—better as regards equality than last year. The umpire, poor R⸺, got upset and nearly suffocated in the Irwell mud! Hoping to see you safe back at the end of July or beginning of August. The day after this letter was written Roscoe received some of the preliminary results of the photometric observations, so that he was enabled to give his Royal Institution audience some idea of the amount of chemical light in the tropics. He made this matter one of the chief features of his lecture.