Priestley's Burning Lens and Experiments Generating oxygen
Some early replications of Priestley's chemical work including
comments on solar heating and decomposition of mercuric oxide.

Jim & Rhoda Morris  781 245 2897

In the beginning God said let there be light.
Priestley took God up on this gift used sun light  (the violet  visible and near infra red) captured by a lens  that focused an image of the sun on a sample of mercury oxide which was contained in a glass vessel. This  was a form of pristine clean heat that could bring a  sample to temperatures greater than 500 C. decomposing HgO it into  pure oxygen and mercury vapor. The mercury vapor was condensed to  room temperature pressure by passing through a water bath. Prtiestley then used the oxygen for a number of experiments which  included the very important study of  the affect of oxygen and carbon dioxide on  plants and animal life He found that each fed on the waste products of the other. A discovery of immense importance to our future energy balance

To rerun the animation hit refresh button. F5 on explorer.

This photo shows a likely replica stand that Priestley would have used. It has  a temporary  lens rim holder, etc. It had not  received its final finish at the time of the photo This lens is 10 inches in diameter and has a 24 inches focal length.

Some  a quick tests were made with the sample  held in a 2 cm dia test tube. Note the image of the sun has overloaded the camera and the sample appears white. The sample is really dark brown to black. to the eye.  The  only change is  when it reaches 500C - it simply disappears.   The correct exposure for the sun's image is not the optimum exposure for the sample next to it. Its intensity can be compensated to get both but it takes care.  This set up was used to test the color change by heating the sample container with a torch instead of the sun.  it did as is described in the right hand column starts off. red  turns black simply  disappears. However;

The whole affect of the red powder scattered about the inside of the glass, the   silver-ish mercury droplets clinging to the glass surface giving it cloud like sheen,  the bright image of the sun if it is moved about when held by the hand as Priestley probably did, can and does make a dramatic surrealist hellish Mars type scene see below. and all of this is real with practice.


A note of caution about mercury:
Mercury vapor  is not good for your health.. Depending upon room temperature and poor ventilation there can be  low but dangerous amounts of mercury inhaled which if  integrated over long exposure times is not good

A very rough guide:     exposure time * amount = trouble

Most of the mercury vapor is condensed in the sample vessel and the residual is trapped in the water bath.  So as long as the gas tube to the trough is intact when the vessel is at high temperatures, there should be no exposure problem.

Notes of caution about the sun's image from the burning lens: Suitable eye protection is needed
One must be careful staring at the image of the sun on the sample.  It is very bright. When you look away your center vision will be somewhat impaired..  This will be for a given length of time, depending on your age. It can be from seconds to minutes. It might take several minutes for you to be able to regain your normal vision.

Caution -
It can take a minute or  more to adjust the sun's image on the sample and if care is not taken one might accidentally get the image of the sun cast into the eyes.. Proper glasses have to be provided to protect the operator from the sun's high intensity infrared radiation which can burn the retina without the person feeling any pain.  This is the same warning for protection given to viewers for solar eclipses.
 The plano-convex lens used in these Priestley experiments  has a nasty feature in that it forms a reflected image of the sun in front of the lens. That is on the suns side of the lens.  It's only four percent-ish of  the real image but should be treated with care to protect people's eyes. It's sitting out there and you can not see it!

 The other danger can result from improper storage of the lens.  It can cause a fire inside or outside if stored such that it images the sun on a flammable substrate.     

Jim & Rhoda's sketchy notes/ principally for themselves  Warning!: This document may not be comfortable for those who cherish rules of grammar, spelling, punctuation, organization and style more than content. but others can browse with leisure.

  Priestley project  on HgO - Not for publication.

He generated  oxygen a number of different ways. The principal process was by  heating a  sample using a lens to concentrate an image of the sun on mercuric oxide (or what ever)

.Priestley had to learn more than a bit about optics  that was not published in a text book to effectively learn how to optimize his lens location etc for the most effective heating by the lens method. One does not just stick up a lens and go at it to replicate his experiments. The following are some of the pit falls one can fall into.

What happens to the sample of mercuric oxide as it is heated and what do  observers see?  What can a  camera record?
As the sample is heated it changes from  an orange red at room temperature to near black  in the neighborhood of 400 C. Then, within a few degrees of 500 degrees C it simply disappears by decomposing.  The decomposition products are mercury vapor and oxygen-both colorless gases. Most of the mercury vapor soon condenses into very tiny droplets on the cooler parts of the inner walls of the container. The vapor can be harmful if inhaled when it is not removed by cooling it properly.  The mercury condensed on the walls of the glass vessel, of course, obscures the sample from the sun's rays and the camera viewing the sample.

Remember to pre-heat the container wall through which the camera views the sample.. Purpose; to reduce the amount of mercury condensing on that part of the container wall.

It will be useful to note that the ratio of the brightness of the  sun's image  on the sample is several orders of magnitude greater than the surroundings. This makes it taxing for most cameras or displays to accurately portray how the sample is decomposing in the sun's image. The suns image is just white.

In general its a challenge to use a burning lens quantitatively.

Basics of solar radiation.
1, About half of the energy radiated by the sun is beyond the wavelengths we see the in the visible image of the sun. Therefore the  visible image is not the best indicator of the total heat available. There will be days when the sun and its image will appear to be  bright but a good share of the infrared radiation can be missing through scattering and  transmission losses of the intervening  air.
The  absorptance of the surface of a sample is a critical factor of how much radiation and will be absorbed. It depends in the 1st order of the absorptance of the sample.   ??

2, The condition of any optical surfaces  between the lens and sample may adversely affect the amount radiant heat getting to the sample.

3, If the lens is not perpendicular to the optical axis this will cause image distortion  and an unfavorable distribution of the thermal radiation getting to the sample.  On the other hand it makes for some weird interesting images during focusing.

Surprise Last but    not least because there can be a significant problem with chromatic aberration with the lens the location of  infrared image of the sun will not be where the location of the visible image is. The different between  these two images can be as  much as  1 to 2 inches  from the lens. The result you may have a sharp image of the sun in the visible and not be focused for the infra red image where much of the heat is. located..

The photos below were taken while subjecting the HgO samples to either one or both  sources of heat;  an external one such as a small electric or gas torch, the other an  internal source from radiation absorbed from  the sun through a burning lenses.

We also include here photos of samples exposed to the burning lens and  various  views of the operator in and around the experimental apparatus to expand the opportunities for visual affects.

( click on the photo to enlarge the interesting detail)
Testing out decomposition of the HgO with the  camera  above  and shooting thru another large lens. Note Rhoda is cooling most the residual mercury vapor in the  a water bath  keep most of  it out of harms way. No, this is not the flask intended for the shoot.

(Click on the photo to enlarge the interesting detail that can be observed if the camera can move in on the scene close enough ) Above is the sample in a 500 cc test  flask ,similar to the retort to be used during the filming, being heated by  sun light through the burning lens . As the sample of HgO heats up, it turns black-ish, and disappears as the  mercury and oxygen separate.   The  inside of the wall of the flask, which is cooler, as we noted before, clouds up  with condensed mercury vapor building up more and more with time until it is nearly opaque. It can be removed by external heating. which could be done with a  gas or electric torch. This so called window area can be  kept clear of condensate by pre-heating this area before the application of the sun.  The bright spot on the right is the sun's reflection off the glass and on the left off the mercuric oxide powder.  The sample spot appears white because of over exposure but it  is really blackish  if properly exposed.  In some ways these tiny droplets of  Hg on the surface  of the vessel seem to add to the drama in the scene.

Getting a repeatable visual affect of this mercuric oxide decomposition process is very touchy but do-able. The visual affect can depend on the history of the local environment, i.e. previous conditions such as, where and  how  the application of heat was previously applied, where the sample was located in the vessel etc. Producing and collecting oxygen is easy compared to documenting the process
Collecting the oxygen
Priestley directed the oxygen he  generated into a water displacement trough like shown below. The oxygen  was fed through a flexible tube into an invert jar filled with water that had been place on a shelf in the trough.. The gas bubbled out of the other  end of the tube into an inverted jar displacing the water inside it with oxygen .   See the lower right hand picture below.  Priestley had any number of these troughs made for him.. Our replica  is the size of one of them.



 Below a collection of photos of  opportunity
This is not a staged scene . Rhoda is surrounded by the equipment clutter and most times it's worse.  It was probably the same for Priestley's lab table except for the age of some of the articles. Experiments are usually messy - a mixtures of many trial and errors of more than one test.. Yes these are not period glasses. Priestley probably used a smoked piece of  glass to view the bright sun images.  These could easily be made by holding a piece of glass over a candle flame.

Looking through the lens at the experiment some distance from the lens..

The oxygen generated is collected by water displacement in a trough but the gas includes some non-condensed  mercury vapor which is filtered by  the water, an added safety feature.  The tube leading from the retort to the oxygen collection glass should not be removed from the water until the flask is cooled to room temperature and a well ventilated room should be used for the experiment..

 Just another view showing the diversity of uses of the lens.

As the sample is being heated by the image of the sun it can be seen reflected off the surface of the lens superimposed on the operator who is  adjusting the image of the sun onto the sample. It is an interesting view and  Rhoda is in a safe position. 

But some caution should be taken for the one taking the photo.

Again repeat of Caution (as given above)- it can take a minute or  more to adjust the lens to cast the  sun's image on the sample and if care is not taken one might get in the wrong place and  accidentally get the image of the sun cast into the eyes.

To repeat: The plano-convex lens  has a nasty feature that if forms a reflected image of the sun in front of the lens. It's only four percent-ish of   the real image but should be treated with care. Storing the lens in a safe location so that it is not imaging the sun or near flammables, is also important.

 Proper glasses should  be provided to protect the operator from the sun's high intensity infrared radiation which can burn the retina without the person feeling any pain.  This is the same warning for protection given to viewers for solar eclipses.   

Another photo showing the reflected image of the sky etc from the surrounds

Lens 10 inches in diameter 24 inch focal length. Samples were held in 500 cc.  flask.

One of our original setups used an 8 inch diameter lens cut to a  square format. It has a 12 inch focal length. Samples were held in a test tube rather than a flask. We were able to use a  2  inch diameter lens with a 4 inch focal length to decompose the mercury oxide in even earlier experiments. The lens size made little difference; it just effected the size of the sample that could be .decomposed.  Just a simple demonstration that the "F number" is the all important  specification of the lens used in Priestley's work, not the size.  Confusing? Yep.  Important?  Yep! only to the techs though.

The pictures below show how the shape of the sun's image is altered as a lens is rotated away from the optical axis. This could be an interesting feature that can be used while the operator is setting up the lens for the experiment. It also shows that the heating effect of the sample can be reduced by improper adjustment of the lens.

Image of sun lens properly adjusted perpendicular to optical axis.

Lens rotated 10 from axis.


 Lens rotated 60 from axis.


lens rotated 80  from the axis.     




A note of detail & interest---  I would wonder about  even the existence of the so called big lens that Lavoisier was supposed to have used? There should be reasonable doubt that anyone could have poured  such a large piece of glass without cracking it during the cool down process which would have taken months. (Remember the  trouble Priestly had heating and cooling little pieces of glass.  Hale had big trouble, with the 200 inch telescope, even when Pyrex was used  it took  more than a year to cool it without cracking it -  let alone grinding  and polish it to any useable figure!!)


Side Note
Too bad that their is not enough time to open the scene with a full screen image of one of NASA motion mixtures of the sun in Ca or Na light. Better to include the movies with the turbulence images.

There are those practicing  scientist including a Nobel prize winners that suggest strongly that at least 5 scientist contributed to the discovery of oxygen over at least 5 years. Rhoda and jim have no stand on this subject nor a Noble prize.

Random Note
This all may seem disconnected but it not it is how science works bit by bit by bit. most time it takes along time to get these connections.

The following web page dealing with the long scale is not directly a Priestley topic and can be skipped. It for those of  who are apposed to global climate change. and have just enough knowledge of the subject to have badly miss read the counter physics of it

Admittedly,  when the reader gets to it, the  scale below is long, but it's to demonstrate that it is actually quite short, compared to full output of the sun. It also contains the  transmission of  the earth's atmosphere. At the dips the atmosphere absorb the sun's energy and heats up just as the mercury oxide does in its container. If we  were  to draw the whole thing (for all wavelengths) it would extend from  near  zero a very large number, without calculating it perhaps several feet.

This is the sort of data
of interest to global climate change issues. The wave length is in microns. Note how tiny the visible spectrum take up out of the more or less total spectrum. It has been  a long time between Priestley and his companions used the power of the sun for their work and the generation of the scale below but he and his gang help start us down the path. They are like prospector looking for gold some only finding pieces here and there but  missing the seam.