The next immediate thing was the actual crystallisation of penicillin itself, which made it unnecessary to try and work out the detailed structure of the separate...

[Q] Exercise?

And the, this was, the sodium salts were crystallised in the Squibb Institute, and I rang up Edward Abraham and said, make me some sodium salt quickly, and he said, oh, you don't need to make it, we've got buckets of it. We keep it in a desiccator. So we took the sodium salt, their sodium salt, out of the desiccator, and put a lump of it on a slide which I was taking up to the look at under the microscope. And it immediately took up water from the air and crystallised on the slide with the water of crystallisation. Well, that was, in fact, two-pentenyl penicillin, and it had a slightly complicated molecular molecule. The weight suggested there were two molecules in each cell and a certain amount of water. And I think, from observations since, made by our people in other labs, that, in fact, if it had been crystallised from acetone, it might have been as simple as benzylpenicillin. But at least we knew that the benzylpenicillin was a new molecule, equals two molecules that we could use, and obviously much better for us. So I asked Henry Dale whether he'd get... I think he offered to get some for me from, from America and he did in fact get about 10 mg from Merck, and this... Kathleen Lonsdale brought over from London by hand, and I remember, sort of immediately, it came in a little tube. I took some out of it to crystallise, to try to recrystallise, because the crystals were quite small. And I had a telegramme from them in America saying, ‘Dissolve in minimum - emphasise the minimum...’ and I think, I forget what it was actually in, but it's in the book, and then add dropwise until crystals start to grow. So I carried out this process of dissolving it in two drops of ethyl acetate and having the precipitant. And Kathleen and I were just walking at the bottom of the little... talking at the bottom of the little ladder in the X-ray room, while I had this up top on the shelf above, so we left this there while we were talking, and then we saw that the crystals grew quite nicely in layers. And after a time they didn't grow any more, so I then took out the largest that I could see, and put it on the X-ray tube, and we took the first photographs. And they looked to us perfectly good enough to work on. So, in fact, I worked on those crystals, and particularly on the one I had first picked out. I just took the whole set of Wiessenberg photographs of it for the three-dimensional data, and that... the whole of the structure practically came out of that one crystal.