3.
Sherlock withdrew a spatula from the plastic bottle he was holding and was just about to put the fine grey dust into a beaker when the laboratory door opened and John walked in, accompanied by a small child.
"Sherlock..." he began, but Sherlock had already glanced at the boy and interrupted John.
"Molly's nephew. Why are you babysitting Molly's nephew?" he asked accusingly. The boy took one look at Sherlock and plodded over, eagerly examining the array of equipment on the desk.
"What? How did you... oh, well, Sherlock, this is Joseph. Molly asked me to keep an eye on him while she did a post-mortem. Apparently her sister is ill and couldn't get a babysitter, so she asked Molly to look after him for the day. This little guy has been very excited about his tour of the hospital." As if to prove John's point, Joseph took that moment to grab an empty test tube and jump up and down. "Don't do that, Joey," John said, glancing at Sherlock.
"No, let him. The hospital owns plenty of test tubes, the loss of one isn't going to be a problem. Well, it won't be my problem," Sherlock said, coldly, before returning to his bowl of reddish-brown dust he had retrieved from a crime scene the night before. He measured out eighty grams of the stuff and added it to the thirty grams of the silvery powder already in the beaker.
"Mister Scientist, what are you doing?" the boy piped up as John took a seat next to Sherlock. Sherlock grinned.
"I'm mixing aluminium powder and iron oxide powder to make thermite." Sherlock replied.
"Why?" was all the boy said in response. Sherlock was delighted, it wasn't every day he was invited to talk about a particularly interesting subject. He didn't even notice that John had spotted the gleam in his eye and was giving his "here we go again" look, he just turned to the child and clapped his hands together.
"Well, I got this iron oxide from a crime scene yesterday and I need to check that it really is iron oxide, so I'm proving it is by putting it into a reaction which looks quite distinctive in order to demonstrate exactly what it is so that we might solve a crime," Sherlock said, and he watched Joseph's eyes gleam in the way his own were.
"Sherlock, he doesn't need to know about that..." John began, but again he may as well have been trying to stop a parade of tanks with a feather for all it did to persuade Sherlock to shut up.
"Iron oxide itself is incredibly interesting, even disregarding the fact that it can be used in such a spectacular reaction," Sherlock continued. "This particular kind (if it actually proves to be common Iron III oxide, or Fe2O3) is also found in a lot of places, it's known as rust and formed when various iron objects are exposed to water for a long period of time. That's what the brown stuff coating iron objects is." John rolled his eyes, but Sherlock didn't notice.
"Aluminium is also enthralling," Sherlock said quickly, clapping his hands together again in excitement and starting to bounce up and down himself. "The metal is the third most abundant element and the most abundant metal in the Earth's crust. For such a strong material it is very light and therefore extremely useful in transport, in addition to being a very good electrical and thermal conductor. Of course, the Americans will insist on calling the element "aluminum," but it is preferable to include the second "I", after all the International Union of Pure and Applied Chemistry prefers it themselves and what they say goes." Sherlock smiled at Molly, who had apparently appeared out of nowhere and was now standing in front of Sherlock. "Hello, Molly, I was just explaining to your delightful nephew why the components of thermite are so endlessly fascinating. Anyway, to start the extremely exothermic reaction one must use magnesium which itself produces a lot of thermal and light energy when it is burned..."
"It's all right, Molly," John said over Sherlock's lecture. "You can take Joseph, Sherlock won't even notice that his audience has gone." Molly smiled gratefully at him and proceeded to lead the boy out of the room.
"That's lovely, Sherlock, why don't you tell me about what happens when you burn magnesium?" John said, knowing that although he was bored stupid by Sherlock's rambling and repetitive sermon on the history of chemistry the best thing to do was just to listen, smile, nod and ask the right questions.
Sherlock allowed John to put the thermite mixture into a separate container and pack it away, leading him back out of the laboratory to go home to test the stuff as he began enumerating the discovery of the transition metals. Sherlock could certainly talk for hours about the subject (it helped him to consolidate his knowledge and keep his mind palace organised, after all) and he was glad that John understood: he was the first person Sherlock had found who was willing to endure his infodumps instead of unfairly triggering one and then ordering him to shut up, as the rest of the world did.
Later, as the pair were sitting in the flat and Sherlock had finally shut up about a load of elements John had never heard of, a thought occurred to John.
"Sherlock, isn't iron magnetic?"
"Yes, John."
"Okay, then, is that rust magnetic? I can't think of any reason why the extra oxygen would stop it from being magnetic..."
Sherlock sighed. "That's because it doesn't. Iron oxide is ferromagnetic." He noticed John scowling.
"You made up that whole experiment just because melting a hole in the kitchen table is far more exciting than holding a magnet to the powder, didn't you?"
Sherlock's face instantaneously contorted into a huge smirk. John himself descended into giggles, and Sherlock knew it was because John had twigged early on that the best strategy with him was sometimes (or always) just to smile and nod.