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Let's get started. Settle down. The weekend has not quite started. Let's look at what happened on Wednesday. There are the scores. Class average about 67, standard deviation about 17, so that means it is a bell curve sitting right on the 50. A lot of people improved. There were a lot of rags to riches stories, but there are still a fair number of people down here. And so my suggestion to you is that if you are down here and you want to -- Too much talking, people. If you want to start your weekend soon, I will help you by inviting you to leave. You don't have to stay here if you don't want to, but if you do choose to stay, please sit in silence. This is 3. 91. If you are down here, get in and see us. Please talk to your recitation instructor, come on in and see me, and we will talk about strategies for studying, strategies for success. I want you to pass this class, but just doing the same thing over -- This is, I think, Einstein's definition of insani...

Settle down, settle down. This is not some other class, this is 3.091. So, I have an important announcement. There won't be a lecture next Wednesday. Instead, there will be another celebration of learning. You're going to come, and you're going to show us what you've learned. It's going to be so festive. And, I want to make sure we are all celebrating the same thing. I meant to patch in. It didn't get it patched in. I apologize. So, let me tell you what it is. We're going to look at is the lectures of starting with lecture 17, which was on x-rays. And, halfway through that lecture was the break point for the last test. So, we will start with the material beginning with Bragg's law. We didn't talk about diffraction on the last test. The coverage talked about generation of x-rays, but not their use in diffraction. So, starting with x-ray diffraction up through the end of last day's lecture. And, the reason I'm doing this is that t...

OK, let's get started. Settle down. A couple of announcements to make: it is with great sadness that I announced that there will be no lecture on Monday. Yeah, I know. With all the Kleenex consumed, Kimberly-Clark went up two points on the New York Stock Exchange yesterday, which is incredible. And, in view of the timing of where we are on the lectures versus the homework, and no opportunity to interact with the recitation instructors on Monday, what we are doing just this one time, we're going to slide the weekly quiz over to Thursday. And, that gives more time for people to contact their recitation instructors. Well, let's get on with the lesson. Perhaps some of you haven't figured out that the lecture has started. And I still hear talking. And I will invite people to leave if they insist on talking. So, last day, we talked about secondary bonding, and we looked at various forms of secondary bonding. Here's the cartoons, dipole-dipole interactions, d...