I must not omit to mention Mr. Lea's collection of fresh-water shells,--a series of the magnificent Unios of the rivers and lakes of America, comprising four hundred species, represented by some thirty specimens of each. Mr. Lea has promised me specimens of all the species. Had I not been bound by an engagement at Washington, and could I have remained three or four days longer in order to label and pack them, I might have taken at once these valuable objects, which will be of great importance in verifying and rectifying the synonyms of European conchologists. After having seen the astonishing variations undergone by these shells in their growth, I am satisfied that all which European naturalists have written on this subject must be revised. Only with the help of a very full series of individuals can one fully understand these animals, and we have only single specimens in our collections. If I had time and means to have drawings made of all these forms, the collection of Mr. Lea would be at my command for the purpose, and the work would be a very useful one for science. There are several other private and public collections at Philadelphia, which I have only seen cursorily; that of the Medical School, for instance, and that of the older Peale, who discovered the first mastodon found in the United States, now mounted in his museum. Beside these, there is the collection of Dr. Griffith, rich in skulls from the Gulf of Mexico; that of Mr. Ord, and others. During my stay in Philadelphia, there was also an exhibition of industrial products at the Franklin Institute, where I especially remarked the chemical department. There are no less than three professors of chemistry in Philadelphia,--Mr. Hare, Mr. Booth, and Mr. Frazer. The first is, I think, the best known in Europe. How a nearer view changes the aspect of things! I thought myself tolerably familiar with all that is doing in science in the United States, but I was far from anticipating so much that is interesting and important. What is wanting to all these men is neither zeal nor knowledge. In both, they seem to compete with us, and in ardor and activity they even surpass most of our savans. What they need is leisure. I have never felt more forcibly what I owe to the king for enabling me to live for science alone, undisturbed by anxieties and distractions. Here, I do not lose a moment, and when I receive invitations outside the circle of men whom I care particularly to know, I decline, on the ground that I am not free to dispose for my pleasure of time which does not belong to me. For this no one can quarrel with me, and so far as I myself am concerned, it is much better.