I have demonstrated "Leidenfrost" many times to student groups and never 
suffered a burn. Its possible though, if you move your finger very rapidly, 
then the gas envelope cannot form and heat transfer rate in any medium is 
greatly improved by speed.
Cheers
Jim Darley
ProSciTech                 Microscopy PLUS
PO Box 111, Thuringowa  QLD  4817  Australia
Ph +61 7 4774 0370  Fax:+61 7 4789 2313  service@proscitech.com
Great microscopy catalogue, 500 Links, MSDS, User Notes
ABN: 99 724 136 560                      www.proscitech.com

On Wednesday, October 04, 2000 11:20 PM, Macke, Gail 
[SMTP:gmacke@shrinenet.org] wrote:
> Jim,
>     I disagree with you on how long it takes to freeze with liquid nitrogen.
> Please don't put your hands in or on liquid nitrogen!!!!!!  It took less
> than a second for me to get second degree burns on my left thumb and index
> finger in December of 1997.  It was a very painful event and one I
> personally never let happen to me again.  Liquid nitrogen will freeze warm
> objects faster than blinking your eye.
>     Gail Macke, HTL
>      Shriners Hospital for Children: Shriners BURNS Hospital--Cincinnati,
> Ohio
>  P.S. Yes, I did get a lecture on chemical burns, believe I did.
>
> > -----Original Message-----
> > From:	Jim at ProSciTech [SMTP:jim@proscitech.com]
> > Sent:	Wednesday, October 04, 2000 7:16 AM
> > To:	'Greer, Bonnie'; 'Vinnie Della Speranza';
> > histonet@pathology.swmed.edu
> > Subject:	RE: Freezing tissue
> >
> > Sorry Vinnie, better ask for a refund on your tuition- or was that a slip
> > of
> > the pen? The gas forming around a warm object inserted into liq N2 forms
> > an
> > insulating envelope (Leidenfrost). Freezing speed in liq N2 is slower than
> > in
> > near freezing point Iso-pentane.
> > You can stick a finger into liq N2 for a couple of seconds and have no
> > harm,
> > but don't try that in cold iso-pentane.
> > Faster freezing results in smaller (ultimately no) ice crystals. It's
> > those
> > crystals that destroy tissues.
> > For the light microscopist the goal is to have those crystals small enough
> > to
> > not cause tissue damage that is visible under the light microscope. There
> > are
> > numerous methods to attain that goal.
> > Cheers
> > Jim Darley
> > ProSciTech                 Microscopy PLUS
> > PO Box 111, Thuringowa  QLD  4817  Australia
> > Ph +61 7 4774 0370  Fax:+61 7 4789 2313  service@proscitech.com
> > Great microscopy catalogue, 500 Links, MSDS, User Notes
> > ABN: 99 724 136 560                      www.proscitech.com
> >
> > On Wednesday, October 04, 2000 7:59 AM, Greer, Bonnie
> > [SMTP:Bonnie.Greer@stjude.org] wrote:
> > > I did muscle bx`s for a long time I used a dewar filled with liquid
> > nitrogen
> > > and I used another container filled with the isopentane.... suspended
> > inside
> > > until frozen. You thaw  just enough to freeze your tissue for 20
> > seconds. I
> > > was taught that the isopentane insulates the tissue causing lee artifact
> > > .Email me if I can help.
> > >
> > > -----Original Message-----
> > > From: Vinnie Della Speranza [mailto:dellav@musc.edu]
> > > Sent: Monday, October 02, 2000 9:51 AM
> > > To: histonet@pathology.swmed.edu
> > > Subject: Freezing tissue
> > >
> > >
> > > I would welcome your opinions and feedback.
> > >
> > > What is the optimal method of freezing tissues with liquid nitrogen?
> > with or
> > > without isopentane and why?
> > >
> > > thanks,
> > > Vinnie
> > >
> > >
> > >
> > > Vinnie Della Speranza
> > > Manager for Anatomic Pathology Services
> > > Medical University of South Carolina
> > > 165 Ashley Avenue
> > > Suite 309
> > > Charleston, SC  29425
> > > ph:  (843) 792-6353
> > > fax: (843) 792-8974
> > > email: Dellav@musc.edu
> > >
> >
> >