[tex-eplain] \vadjust gyrating, not migrating

[Michael Talbot-Wilson]

Dear all,

Dunno whether this is an eplain thing or a tex thing, but hope you can
tell me what I'm doing wrong.  Using texlive 2012.

A new paragraph starts at the end of the page (A4, but...).  So, there
is the end of the first paragraph, a parskip, then the first line of
the next para and a page break.

I want to insert of page scan, slighty reduced, between the two pages
so that they look just the same, the first page looks just the same,
with the floating first line of the next para at the end of it, but
with an image (produced by pnmtops) alone on the second page.

In the test file below, the page break occurs after the first line of
the new para IF the "%" comment marks are left in place.

If they are removed the page break happens where the para break is.
Then, the first line of the next para is at the top of the second page
and the image occupies the remainer of the page.

Shouldn't \vadjust simply cause its body to migrate out of the line,
without moving the line?  Without changing the place where the page
break occurs?  Or doesn't it work that way?

Hope you can solve my puzzle.

Regards to all.

\input eplain
\input epsf
\newbox\imbox

\parindent0em\parskip6pt
\vsize 240mm
\topskip 2pc

\footline{\hfil}

\headline{\hbox{\vbox{\nointerlineskip{
\hbox to\hsize{Lavoisier\hfil\folio}
\vskip -6pt
\hbox to \hsize{\hrulefill}
}}}}

\vglue 7.25in

A third phenomenon, which requires particular consideration, is that
none of the metals produce effervescence by solution in oxygenated
muriatic acid.  During this proces the metal, in the first place,
carries off the excess of oxygen from the oxygenated muriatic acid, by
which it becomes oxidated, and reduces the acid to the state of
ordinary muriatic acid.  In this case there is no production of gas,
not that the muriatic acid does not tend to exist in the gaseous state
in the common temperature, which it does equally with the acids
formerly mentioned, but because this acid, which otherwise would
expand into gas, finds more water combined with the oxygenated
muriatic acid than is necessary to retain it in liquid form; hence it
does not disengage like the sulphurous acide, but remains and quietly
dissolves and combines with the metallic oxide previously formed from
its superabundant oxgen.

{\it The fourth phenomenon}\/ is that metals\vadjust{
%\def\epsfsize#1#2{0.7#1}
%\setbox\imbox=\hbox{\epsffile{/home/lavoisier/pic1.ps}}
\box\imbox
} are absolutely insoluble in such acids as have their bases joined to
oxygen by a stronger affinity than these metals are capable of
exerting upon that acidifying principle.  Hence silver, mercury, and
lead, in their metallic states, are insoluble in muriatic acid, but,
when previously oxidated, they become readily soluble without
effervescence.

\bye