For what willis575 posted about SI and H. SI is a french acronym, made in 1960 for the universal measurements accepted by the global science community. SI units include grams, liters, meters, etc. I think H refers to hydrogen. Im not sure exactly why though. It could be for the infamous bomb or what is a component in the nerve gases. I have not actually read the document. Im just going on what willis575 posted. Look up wikipedia for more info on SI measurements i you want to learn more.