The Horace de Saussure Hot Box

Written by Jerry L Krause

First I must credit Joseph Postma for alerting me to the existence of Horace’s (easier to write than de Saussure) hot box.  For May 31, 2016 he posted an article—The Radiative Greenhouse Effect & Ontological Mathematics—on his website—Climate of Sophistry.  Horace’s hot box was the focus of this article.  horace

So having no knowledge of Horace’s hot box I went to the internet and found: “He [Horace] had constructed the first known Western solar oven in 1767, trying several designs before determining that a well-insulated box with three layers of glass to trap outgoing thermal radiation created the … highest temperature—230 °F.” (Wikipedia)   

At http://solarcooking.org/saussure.htm I read:  “the increased use of glass during the eighteenth century made many people aware of its ability to trap solar heat. as Horace de Saussure, one of Europe’s foremost naturalists of the period, observed: “it is a known fact, and a fact that has probably been known for a long time, that a room, a carriage, or any other place is hotter when the rays of the sun pass through glass.”

This French-Swiss scientist was quite surprised that such a common phenomenon had not led to any empirical research on the maximum temperature attainable in a glass solar heat trap. when experimenting with solar energy, his contemporaries preferred to work with burning mirrors, which could perform such amazing feats as burning objects at a distance or melting the hardest metals within seconds. in 1767, de Saussure set out to determine how effectively glass heat traps could collect the energy of the sun.” 

What followed on this technological website is a great illustration of good experimental science so I strongly suggest that one should go to the referenced site to read about this.  However the focus of this article is to understand the functioning of Horace’s hot box using the necessary knowledge, which Horace could not have known about at that time and which seems not commonly known today.  But there is one bit of information, which Carl Allen shared with Joe and which I have not found written elsewhere, that I consider is critically important to this understanding of the functioning of Horace’s hot box. 

Carl had written to Joe: “Another thing that your thought experiment overlooks is the fact that glass is not 100% transparent to sunlight since sunlight carries a lot of infrared energy. Set a piece of glass up against a south-facing wall that 1) is protected from the wind and 2) receives direct sunlight at noon. By 1:00PM the glass will be too hot to touch due to the sunlight that it has absorbed. Then touch the wall behind the glass and you will discover that this clear plate of glass has been “shading” the wall, which will be significantly cooler than the glass.”

This remark caused me to search for something I had been taught in a theoretical physics course.  The textbook for the course wasIntroduction to Theoretical Physics—Classical Mechanics and Electrodynamics and its author was Roald K. Wangsness.  And it is very true that I understood little of the mathematical reasoning taught in this book. 

However, I can read and in Section 29 (Reflection and refraction of plane waves) I read (pp 309):  “In other words, waves which are most strongly absorbed are very strongly reflected.  A good example is afforded by the optical properties of thin sheets of gold.  They appear yellowish by reflection; this means that, in the originally white light transmitted through the sheets, the yellow is practically all absorbed.  As a result, the transmitted light appears greenish or bluish.”  So this stated consequence of the theoretical considerations I could understand.  However, I also recognized that gold as a metal is a good conductor and not a very poor conductor like glass or water.

Hence, I looked for more conformation that “a good absorber is a good reflector”.  And I found it in The Feynman Lectures on Physics Vol II pp 33-11.  “Metals do not reflect 100 percent, but many do reflect visible light very well.  In other words, the imaginary part to their indexes is very large.  But we have seen that a large imaginary part of the index means a strong absorption. 

So there is a general rule that if any material gets to be a very good absorber at any frequency, the waves are strongly reflected at the surface and very little gets inside to be absorbed.  You can see this effect with strong dyes.  Pure crystals of the strongest dyes have a “metallic” shine.  Probably you have noticed that at the edge of a bottle of purple ink the dried dye will give a golden metallic reflection, or that dried red ink will sometimes give a greenish metallic reflection.  Red ink absorbs out the greens of transmitted light, so if the ink is very concentrated, it will exhibit a strong surface reflection for the frequencies of green light.

 “You can easily show this effect by coating a glass plate with red ink and letting if dry.  If you direct a beam of white light at the back of the plate, as shown, there will be a transmitted beam of red light and a reflected beam of green light.”

And even though Feynman taught that “very little gets inside to be absorbed” we know from Carl’s report that the glass facing the sun got too hot to touch even though the wall behind the glass did not.  Hence, we know that Horace’s hot box was strongly heated at two locations:  by the IR portion of the solar radiation at the top surface of the top glass and by the visible portion of the solar radiation on the ‘black’ surface beneath the bottom glass, where the absorbed visible radiation heated the surface to the reported maximum temperature of 230oF. 

The obvious key to obtaining this high temperature was to reduce the transfer of energy, via conduction or radiation from the hot box’s interior.  To reduce conduct through the walls and bottom of the box required that they be ‘insulated’ as well as possible.  And because glass, because it is a poor conductor, served this function for the top.  And the panes of glass were separated from each other because air has a low thermal conductivity as long as there is no convection.  Which later research by modern technologists found that a spacing of about 0.7 cm prevented vertical convection. 

It is possible that Horace experimented with the spacing as I read that he had started with five panes of glass.  But that he discovered his hot box achieved its greatest temperature by using only three.  Now, it should be acknowledged that Horace in his experimenting was not being a scientist as chemists, at least, define a scientist.  He was being a technologist because he goal was to achieve the greatest temperature and not to understand how (why) he achieved this greatest temperature.

What Horace probably did not know was that glass was a very good absorber of the radiation being emitted from the bottom of his box which was absorbing the visible radiation which was transmitted through the glass.  Hence, the glass effectively eliminated the loss of energy via radiation from the interior of the box because a good absorber is a good reflector. 

So the major loss of energy from the interior to the exterior out of the top of the box was thermal conductivity and the rate of thermal conductivity is now commonly known to be proportional to the temperature gradient.  And here is the critical importance of Carl’s observation.  The fact that the top surface of the top absorbed a portion of solar radiation’s invisible IR radiation to heat that surface so it was too hot to touch, greatly reduced the temperature gradient driving the heat flow via conduction from the interior.

So given our presently known scientific knowledge, it is easy (simple) to understand (explain) how Horace’s hot box achieved such a great temperature.  

Comments (10)

  • Avatar

    Geoff Wood

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    Hi Jerry. There are multiple issues in this post sir, I hope I can provide some sensible input.

    I will address these in the order written.

    Firstly,
    “. By 1:00PM the glass will be too hot to touch due to the sunlight that it has absorbed.”

    I try, in my endeavours to reason through life, to quantify in order to accept.

    My reasoning leads me to this;

    Relatively simple optical accountancy puts limits upon glass absorption from the solar flux. Having looked up the transmission spectrum for ordinary glass I can see that the transmission is 92% plus from UVA to 2.5μm. That limits the total of absorption and reflection (as physically separate processes) to 8% of the solar spectrum. Typically for light incident near the normal reflection for glass is around 4%. From this the remainder, of again 4% of the ground insolation is the maximum absorption up to 2.5μm. Typical high Sun through blue sky at the surface is a maximum of around 1200W/m-2.

    4% of this being 48W/m-2

    The absorptivity of glass rises significantly for wavelengths longer than 2.5μm, but, and it is a big one, very little energy resides there in the solar flux. A black body calculator was used on a 5880K model to tell me that the band radiance from 2.5 to 20μm was 3.2% of the total spectral radiance. So even if the glass absorption here is total the limit is 38.4W/m-2.

    This gives an absolute maximum absorption flux for glass of 86.4W/m-2 to heat the glass in full perpendicular Sun.

    Compare with sand at 0.5As = 600W/m-2

    Or a black surface at 0.9As= 1080W/m-2.

    Given that the long wave emissivity of glass is around 0.9 plus and ‘too hot to touch’ is around 325K then the glass’ in vacuo radiative potential is around 560W/m-2. Given atmospheric radiative potential of around 300W/m-2 gives losses by radiation of an excess of 260W/m-2, from glass in the Sun under normal sky. How can radiative losses of 250W/m-2 be maintained by an absorption of 85W/m-2??

    Although I am aware that a single repeatable experiment can prove anything incorrect, something is very wrong here!

    Moving on,

    “Hence, I looked for more conformation that “a good absorber is a good reflector”.  And I found it in The Feynman Lectures on Physics Vol II pp 33-11.  “Metals do not reflect 100 percent, but many do reflect visible light very well.  In other words, the imaginary part to their indexes is very large.  But we have seen that a large imaginary part of the index means a strong absorption.”

    Replace the word ‘interaction’ with ‘absorption’ and that makes sense. Otherwise it doesn’t. Reflection and scattering are temporary excited states that do not include necessarily the radiationless transition to make electronic transitions thermal (i.e. protonic).

    Simple optical accountancy prevails. Radiative energy through matter is either transmitted, reflected or scattered (by temporary absorptive interaction) or thermalised by irreversible process, this latter being absorption, the only radiative physical interaction that leads to equilibrium with the supportive fluxes.

    You have said,

    “You can easily show this effect by coating a glass plate with red ink and letting if dry.  If you direct a beam of white light at the back of the plate, as shown, there will be a transmitted beam of red light and a reflected beam of green light.”

    The green is the decay of some of the excited transmissions back to the initial state. Energy not thermalised, This re-emission can be in any direction. Different from a true reflection where the valence band electrons form a uniform absorption wavefront.

    And you have said,

    “Hence, we know that Horace’s hot box was strongly heated at two locations:  by the IR portion of the solar radiation at the top surface of the top glass and by the visible portion of the solar radiation on the ‘black’ surface beneath the bottom glass”

    No significant heating of the top layer by IR unless the properties of glass are incorrect, or matter is heated by a discrete portion of the solar flux, or radiative losses by long wave radiation in a coupled thermal system are truly false!

    And you have said,

    “What Horace probably did not know was that glass was a very good absorber of the radiation being emitted from the bottom of his box which was absorbing the visible radiation which was transmitted through the glass.  Hence, the glass effectively eliminated the loss of energy via radiation from the interior of the box because a good absorber is a good reflector. ”

    Again some confusion here. Did the sky outside the box annihilate the surface radiative losses or the glass. Which was there first to reduce the efficacy of long wave energy transmission calculated?

    And again,

    “So the major loss of energy from the interior to the exterior out of the top of the box was thermal conductivity and the rate of thermal conductivity is now commonly known to be proportional to the temperature gradient.  And here is the critical importance of Carl’s observation.  The fact that the top surface of the top absorbed a portion of solar radiation’s invisible IR radiation to heat that surface so it was too hot to touch, greatly reduced the temperature gradient driving the heat flow via conduction from the interior.”

    Jerry, please take a moment to think about what you have said here and place this carefully within your experience.

    Low thermal conductivity is enough.

    A heated exterior is not required with low thermal conductivity.

    A heated exterior cannot be maintained with high input transmission and low thermal conductivity. The warmest of the heated houses is the one with the snow on the roof!

    It has the lowest losses.

    • Avatar

      Jerry L Krause

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      Hi Geoff,

      Thank you for your evaluation of what I wrote and what I only quoted.

      I will accept the blame for not placing a quotation before–So there is a general rule … . It should have been “So there is a general rule … . For this paragraph was a continuation of what was written in The Feynman Lectures as referenced. So, relative to the issue that a good absorber is good reflector, all that you wrote to me is actually written to this theoretical physicist who has significant achievements. I do not believe everything that is written but have to accept the physics which professors of physics write in their textbooks. In Feynman’s Blunder, he was teaching about chemistry, my field, for the atomic theory of matter was not established by physicists; it was established by chemists, alchemist even.

      But you wrote: “I try, in my endeavours to reason through life, to quantify in order to accept.” I thought you were a scientist. But maybe you write off Feynman entirely because he made a mistake. But Vol 1, pp 1-1 he wrote: “The principle of science, the definition, almost, is the following: The test of all knowledge is experiment. Experiment is the sole judge of scientific “Truth.” ”

      Now, too hot to touch, is a qualitative term because people have different sensitivities to hot or cold. I tested Carl’s observation and it did not get two hot for me to touch but I could classify it as hot instead of warm. So I did not doubt Carl’s observation which I believe was made a lower latitude and nearer to the summer solstice, was a month of more past when I read his remark and first learned about the de Saussure device and the temperatures he (de Saussure) observed. What is important is that de Saussure did something beyond reasoning.

      “What Horace probably did not know was that glass was a very good absorber of the radiation being emitted from the bottom of his box which was absorbing the visible radiation which was transmitted through the glass.” I also made an error here; it should be: … the bottom of his box which was absorbing the visible and the IR radiation which was transmitted through the glass.”

      Your wrote: “Replace the word ‘interaction’ with ‘absorption’ and that makes sense. Otherwise it doesn’t. Reflection and scattering are temporary excited states that do not include necessarily the radiationless transition to make electronic transitions thermal (i.e. protonic).” I have searched and searched for the word “interaction” in my article. And it is not part of my article. But you later use–interaction–a couple more times in your comments.

      Horace, made a device and made an experiment. I tried to explain the results using the ‘knowledge’ of other scientists. I previously admitted that I had made a mistake in this article which I was addressing in article yet to be published.

      And I see that PSI has drawn to your attention to a second published article which also considers the de Saussure device. So, since I have made so many errors, perhaps you could consider it and explain the possible paradox of the device’s experimental result.

      Have a good day, Jerry

      • Avatar

        Geoff Wood

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        Hi Jerry,

        I have written no one off sir. I am pointing out here that the argument you made or R. Feynman made,

        “A good absorber is a good reflector”

        is logically flawed if we take the conventional meaning for absorption. In determining equilibrium temperature in a radiative model absorption is 1-(reflection(albedo)). How can reflection be confused with absorption??

        I invite you to counter with your own appraisal; to question things I might say that might not make sense to you. If I am incorrect I will struggle to defend and explain my beliefs.

        You seem unhappy with my use of the word ‘interaction’. Take the dictionary meaning. ‘Inter’, meaning between, and ‘action’, as in something happens, unspecified. So ‘something happens between’ radiation and matter if there is an interaction. Think of it as a ‘physical process’.
        Absorption however in optical accountancy is very specific and physically different to reflection.

        For a given radiative flux, the transmission T, is

        T= 1-(total opacity)

        Where opacity is the sum of independent physical interactions between radiation and matter,

        Total opacity is the sum of absorption a, reflection r, and scattering s

        So transmission T = 1-(r+a+s)

        From that, by energy conservation, we can see that in the absence of any available physical process (interaction), (r+a+s =0) all of the radiant energy is transmitted.

        If any component of opacity has a coefficient of 1, ie total reflection is 1, or absorption is 1, or scattering is 1, then the matter is opaque and the transmission is zero.

        You can see from this, which is simple conservation of energy, to have, simultaneously, a high coefficient of absorption and a high coefficient of reflection is impossible. The maximum for both is 0.5. If absorption is high then reflection has to be low. For glass the transmission is high, ie virtually all of the energy passes through, so this places limits upon the availability of other processes that constitute opacity.

        I have indicated to you that the transmission spectrum for ordinary glass to solar irradiance is high; above 92%.

        This limits, by conservation/optical accountancy the total of all constituents of opacity to 8% of the total energy to 2.5μm which is where around 95% of the total radiant energy lies in the solar flux.

        Unless you can ‘show’ otherwise, you cannot say to me,

        “So we know that a good absorber is a good reflector”

        My eyes tell me that a matt black surface does not look or measure like a mirror!

        The statement does not conserve energy if the coefficients are large even at an individual spectral line.

        I can prove that once band limited, glass does not reflect radiant thermal energy as you have casually concluded. Glass absorbs radiant thermal energy. Kirchhoff’s Law is the proof of this, and the ‘measures’ taken by glass suppliers to produce glass that reflects thermal energy. To qualify the latter firstly, energy saving low-emissivity glass has a wavelength tuned ‘sputtered’ thin metallic film that does the reflection. The first layer absorbs or transmits the IR and the ‘metal’ film on the second reflects it back into the room, not the glass. Why would they bother with a metal film if glass already reflected thermal IR???

        Also, and perhaps you will have to think a little more about this one. Everyday experience (experiment if you pay attention:-)) tells us that once band limited nothing can raise its temperature by its own physical properties. In a room without heat source everything achieves the same temperature. For a temperature to be stable we know that there can be no net flux. That is, for ANY object, once band limited the efficacy of absorption to emission is unweighted; the absorptivity to emissivity ratio is one. So from direct experience of our world we know that once band limited the coefficient of absorptivity is numerically the same as emissivity for any material. The emissivity of glass is catalogued, again in engineering data books as around 0.9. This by logical inference sets its absorptivity to 0.9. This by conservation limits its reflectivity to a maximum of 0.1. If this were not true then the glass would be warmer or cooler than its surroundings, which experience tells us does not happen.

        I apologise if you are uncomfortable with what I say to you but I am plainly inviting you to counter with criticism of logic. Does it make sense? If not which part doesn’t?

        Best regards.

        • Avatar

          Jerry L Krause

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          Hi Geoff,

          You wrote: “I have written no one off sir. I am pointing out here that the argument you made or R. Feynman made,” If you are uncertain who it is that has concluded that a good absorber is a good reflector, Feynman or me, it seems you have a definite reading problem.

          For I had replied: ” For this paragraph was a continuation of what was written in The Feynman Lectures as referenced. So, relative to the issue that a good absorber is good reflector, all that you wrote to me is actually written to this theoretical physicist who has significant achievements.”

          So while your seem to consider you are qualified to debate Feynman, I do not consider I am.

          I do not debate. I review information of which I find others may not be aware and/or observations (reproducible experimental results with which others may not be aware. You, as a reader, can do whatever you wish with what I review. And relative to logic; my philosophy is that science is not logical. I consider that the Greek philosophers who were found to get certain fundamental ideas about the physical world totally wrong is adequate evidence that logic does not work in science. As Feynman wrote: ” “The principle of science, the definition, almost, is the following: The test of all knowledge is experiment. Experiment is the sole judge of scientific “Truth.” ” I ask you: Is it logical that the atom of which solid matter is composed is itself mainly empty space occupied by a very tiny nucleus and from one to more very tiny electrons?

          You wrote: “Unless you can ‘show’ otherwise, you cannot say to me,

          “So we know that a good absorber is a good reflector” ”

          Nor did I ever write that. But as you cleverly wrote in such a manner that suggested that I had. But if I replied that this was a lie, you could point out that I (Geoff) never said you had. I (Geoff) could state I merely wrote: “Unless you can ‘show’ otherwise, you cannot say to me,”. I am familiar with such clever word games and I will not play.

          Have a good day, Jerry

          • Avatar

            Geoff Wood

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            I am not playing word games with you Jerry. I can assure you of that. Just defend the statements you are upholding. Physics is logical.

          • Avatar

            Geoff Wood

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            Direct quotes from your post,

            “In other words, waves which are most strongly absorbed are very strongly reflected”

            “Hence, I looked for more conformation that “a good absorber is a good reflector”. ”

            “Metals do not reflect 100 percent, but many do reflect visible light very well.  In other words, the imaginary part to their indexes is very large.  But we have seen that a large imaginary part of the index means a strong absorption.”

            “So there is a general rule that if any material gets to be a very good absorber at any frequency, the waves are strongly reflected at the surface and very little gets inside to be absorbed. ”

             “Hence, the glass effectively eliminated the loss of energy via radiation from the interior of the box because a good absorber is a good reflector. “

          • Avatar

            Geoff Wood

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            But the notion that ‘a good absorber is a good reflector’ is ‘nonsense’ for glass because we have its emissivity in engineering textbooks that answer to commerce.

          • Avatar

            Geoff Wood

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            And, of course our eyes that tell us that a black surface (good absorber) does not resemble a mirror (good reflector) except in their, what was it, ‘imaginary indexes’?

          • Avatar

            Jerry L Krause

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            Hi Geoff,

            Thought I made it clear. Relative to ‘a good absorber is a good reflector’ is not my idea, it is the idea taught by two physicists who wrote about it in their textbooks. I clearly stated: I try to write about information that I consider might not be widely known. And I clearly stated it was up to you with what you do with this information. I do accept what these physicists wrote (taught); but I do not need to convince you about it.

            I assume you are an adult and therefore responsible to what you think (believe) and it should not matter to you what Feynman, Wangsness, and I believe. I have been wrong before and yet I suffer under the illusion that I still am alive. So, I have discovered that being wrong isn’t always fatal.

            You have warned me of my problem so you duty to me has been fullfilled.

            Have a good day, Jerry

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