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The representative elements are those identified in Mendelev's original tables 169 to 1871 as fitting into the 'a' groups, in each column. The orginal tables did not look anything like our modern version so here is a modernised version of Mendelev showing the classification scheme. This was to use Roman numerals to show originally six columns, later increase to eight and then nine. Each column was further divided into two sub columns, labelled a and b. I have ringed the a in my attachment. Elements in the a column were called 'representative elements' (my translation has 'the typical elements'). Mendelev classified by chemical compounds and reactions. It was not until 1913 that the modern atomic number listing was established by Mosely's square root law. As exchemist says, the classification is obsolete today.

Jeff's, erm, rocket...

No ,I just had chapter II in mind https://www.coursehero.com/file/p7hr8lu/If-for-instance-a-cloud-is-hovering-over-Trafalgar-Square-then-we-can-determine/ (although I did ,ambitiously have the moving rods of chapter XII as my next preoccupation) But I think @swansontmay have disabused me of the notion that there is some kind of a "proto unit" of spatial distance that I would need to give meaning/reference to the 1 metre rod. https://www.scienceforums.net/topic/125530-einsteins-rods/?do=findComment&comment=1182529 I feel that I see now that distance measurements are relative and so different lengths can be compared to each other without the need to refer to some absolute unit of spatial distance. For the purposes of creating a rod of unit spatial distance any convenient length will do . @joigus I think I was hung up over the mistaken idea that there was some kind of fundamental unit of spatial distance to which all rods or rulers had to be compared. There seemed to be a process involving time in that but I now feel it is unnecessary (and wrong)

Do what, exactly? To what end, then? Can we not proceed with improving the horrid response that’s wedged people against vaccines, masks, and social distancing based purely on political affiliation or “news” source AND strive for continuous improvement of lab safety protocols? Summarized: You seem to me to be focused on the wrong things. Bright shiny object syndrome. Perhaps instead consider focusing on how to stop active disinformation campaigns from splitting us apart as a society and causing millions to act in ways deleterious to our collective health as if this is just another edge issue AND explore procedures for further minimizing lab leaks / breakage of safety rules and how best to enforce those rules and protocols just in case the origin wasn’t natural. We can, and should, do both IMO. You seem to be singularly focused on one narrow piece of this puzzle and I believe it’s distracting you from other more important pieces that make up the larger whole. And TBH, I don’t really care what you think I like to do a lot. These are my thoughts, being added authentically to a discussion in a discussion forum consistent with the forum rules to which we each agreed when joining. You’re free to ignore them, but not free to stop me from sharing them. And I’d prefer that the phrase “avoid it like the plague” still actually meant something and hadn’t been cheapened by tens of millions of idiots who distrust experts and refuse to take obvious and simple steps to mitigate spread and needless death (not speaking of you, of course).