Dalton's atomic theory, Avogadro, and caloric theory

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Summary:

I was reading about Dalton's atomic theory and Avogadro's hypothesis. Back then, caloric theory played an important role. I need your help to visualize how Dalton and Avogadro were thinking of gas molecules in context of caloric theory during that period.

Main Question or Discussion Point

Hi,

The following is an excerpt from Avogadro's biography.

"Obviously, Avogadro could not accept the rule of simplicity adopted by Dalton. And thus he challenged the composition attributed by Dalton to ammonia, water, and the oxides of sulfur and phosphorus. As for the weights of ammonia and water, Avogadro pointed out that the apparent consistency of his figures and Dalton's were only the result of compensating errors. However, as the first section of the 'Essai d'une manière' indicates, the difference in the interpretation that the two men had of the structure of matter in the gaseous state was more substantial. Both thought of gases as formed by particles of roughly globular form, whose size was represented by a hard center surrounded by an atmosphere of caloric. A repulsive force, inversely proportional to the particles' affinity for caloric, balanced their mutual attraction; a lessening in the mutual repulsion of the particles would then correspond to an increase in affinity for caloric. For Dalton such a decreased repulsion meant a condensation of volume of the atmospheres of caloric and a contraction in the size of the particles, while no variation occurred in the actual amount of heat surrounding them. Since different gases had different affinities for caloric, Dalton argued, their particles had to have different sizes and, therefore, they must be in different numbers in a given volume.

Avogadro, aware of Dalton's views on the subject only through Thomson, discussed them in the introductory part of his 'Essai d'une manière'. He contended that by assuming them as correct, it would be impossible to explain the very simple ratios found in the combinations of different gases reported by Gay-Lussac. For him – while he conceded that the 'Laws' concerning the amount of caloric surrounding the molecules are still 'unknown' - the more likely and logical answer lay in supposing that in gaseous bodies the intermolecular distances are so large that no mutual action between such molecules could take place; under these conditions, a change in the attraction for the caloric displayed by each molecule might affect the amount of caloric condensing around it, but not the volume. The distances between molecules of different gases were not influenced by their uneven attraction for the caloric. Thus, it was reasonable to assume that, under equal volumes (or under equal temperature and pressure) there was always the same number of molecules."
Source: Amedeo Avogadro: A Scientific Biography By M. Morselli, pages 88-89


I was further reading the English translation of Avogadro's original work from 1811. I though that it could provide more context about that period of history.

"M. Gay-Lussac has shown in an interesting Memoir (Mémoires de la Société d'Arcueil, Tome II.) that gases always unite in a very simple proportion by volume, and that when the result of the union is a gas, its volume also is very simply related to those of its components. But the quantitative proportions of substances in compounds seem only to depend on the relative number of molecules which combine, and on the number of composite molecules which result. It must then be admitted that very simple relations also exist between the volumes of gaseous substances and the numbers of simple or compound molecules which form them. The first hypothesis to present itself in this connection, and apparently even the only admissible one, is the supposition that the number of integral molecules in any gases [sic] is always the same for equal volumes, or always proportional to the volumes. Indeed, if we were to suppose that the number of molecules contained in a given volume were different for different gases, it would scarcely be possible to conceive that the law regulating the distance of molecules could give in all cases relations as simple as those which the facts just detailed compel us to acknowledge between the volume and the number of molecules. On the other hand, it is very well conceivable that the molecules of gases being at such a distance that their mutual attraction cannot be exercised, their varying attraction for caloric may be limited to condensing the atmosphere formed by this fluid having any greater extent in the one case than in the other, and, consequently, without the distance between the molecules varying; or, in other words, without the number of molecules contained in a given volume being different. Dalton, it is true, has proposed a hypothesis directly opposed to this, namely that the quantity of caloric is always the same for the molecules of all bodies whatsoever in the gaseous state, and that the greater or less attraction for caloric only results in producing a greater or less condensation of this quantity around the molecules, and thus varying the distance between the molecules themselves. But in our present ignorance of the manner in which this attraction of the molecules for caloric is exerted, there is nothing to decide us à priori in favour of the one of these hypotheses rather than the other; and we should rather be inclined to adopt a neutral hypothesis, which would make the distance between the molecules and the quantities of caloric vary according to unknown laws, were it not that the hypothesis we have just proposed is based on that simplicity of relation between the volumes of gases on combination, which would appear to be otherwise inexplicable." Source: http://web.lemoyne.edu/~giunta/avogadro.html


My questions are basically about the part in bold from the first reference by Morselli.

Question 1:
Both references talk about "mutual attraction". I had thought that it was only concluded by van der Waals around 1860 that the molecules of real gases also exert attractive forces on each other so how could Dalton or Avogadro talk about 'mutual attraction' back in 1811? I have also read that Dalton had assumed that the atoms of same element always repel each other. What am I missing?

Question 2:
Caloric was thought to be weightless, invisible fluid composed of particles that repel each other and are attracted to the atoms of ordinary matter. Then, how come the repulsive force between gas particles was inversely proportional to the gas particles' affinity for caloric? As the gas particles are given more caloric, they should repel each other more strongly; in other words, more affinity for caloric means more repulsion.

Question 3:
I would really appreciate if you could elaborate on the part in bold from first reference. I couldn't make much sense of it.

Thanks a lot!
 

Answers and Replies

  • #2
Andrew Mason
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I am not sure why anyone would want to resurrect the caloric theory, but, here is my take for what it is worth:

1. Since gases would condense to a liquid or solid at certain temperatures, both of which were much more dense than the gas, there had to be some basic mutual attraction between the molecules. It was only by the addition of sufficient "caloric" or "heat flow" or "heat" and the repulsive forces that the caloric brought to each molecule that this attraction was overcome so the substance could become a gas.

2. They were aware that different gases had different caloric capacities (the amount of caloric required to raise a gram of the gas one degree). So if the gas A had less affinity for caloric than gas B this meant that a gram of gas A would take up less caloric for a given pressure/temperature increase than gas B. So, at constant volume, the same pressure or temperature increase would be obtained with less caloric for gas A than for gas B. Or a greater temperature/pressure increase in gas A would result per unit of caloric than with gas B. Pressure increase was thought to be the result of inter-molecular repulsion. Since the pressure increase/repulsion increase for a given amount of caloric added, was greater for gas A with the lower caloric affinity than gas B, this repulsion must be inversely proportional to the affinity for caloric of the gas.

3. I'll have to think about it a bit more. It is difficult to study an obviously antiquated and wrong theory.

AM
 
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I am not sure why anyone would want to resurrect the caloric theory
It is difficult to study an obviously antiquated and wrong theory.
Thanks a lot!

I think that many a time it helps a lot understand to know about antiquated and wrong theories. I genuinely appreciate your help.
 
  • #4
256bits
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Interesting piece of history there.
 
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"For Dalton such a decreased repulsion meant a condensation of volume of the atmospheres of caloric and a contraction in the size of the particles, while no variation occurred in the actual amount of heat surrounding them. Since different gases had different affinities for caloric, Dalton argued, their particles had to have different sizes and, therefore, they must be in different numbers in a given volume."
Source: Amedeo Avogadro: A Scientific Biography By M. Morselli, pages 88-89

"Dalton, it is true, has proposed a hypothesis directly opposed to this, namely that the quantity of caloric is always the same for the molecules of all bodies whatsoever in the gaseous state, and that the greater or less attraction for caloric only results in producing a greater or less condensation of this quantity around the molecules, and thus varying the distance between the molecules themselves."
Source: http://web.lemoyne.edu/~giunta/avogadro.html


To me, the parts in red suggest that Dalton thought that amount of caloric surrounding each atom of all the gases was same. For example, if it takes 10 joules of energy to raise temperature for gas A by one degree and for gas B it takes 5 joules, it simply means that atoms of gas A have more affinity for caloric and hence can condense or pack more amount of caloric around them (i.e. more caloric in smaller volume and therefore smaller size of atoms) compared to the atoms of gas B. But the atoms of both gases have equal amount of caloric surrounding them and the difference lies in the volume/size of atom and the caloric surrounding them. In other words, gas A needed more energy compared to gas B because gas A contained more atoms in the same volume under same temperature and pressure.

Do I make any sense? Thanks.
 

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