A.M. Amorim da Costa (Portugal). Professor catedrático da Universidade de Coimbra.
Departamento de Química. acosta@ci.uc.pt

A.M. AMORIM DA COSTA
Vicente Coelho Seabra (1764-1804) on fire, heat and light

1 - Introduction 

Vicente Coelho Seabra, Professor of chemistry in the University of Coimbra since 1787 till 1804, was born in 1764, in Minas Gerais, one of the most developed states of the Portuguese colony of Brazil. In 1783, he came to Portugal to study Natural Philosophy, Mathematics and Medicine at the newly reformed University of Coimbra. In 1787 he had completed a degree in Philosophy; in 1791, he was granted a degree in Medicine. In 1787, he was appointed demonstrator to the chair of Chemistry and Metallurgy, and became a member of the Royal Academy of  Sciences of Lisbon. Other appointments followed: two years later, in 1793, he became Lecturer of Chemistry and Metallurgy; in 1795, substitute-Lecturer to the chairs of Botany and Zoology; and finally, in 1801, substitute-Professor of Chemistry. He died in 1804 (1).

Seabra had published on chemistry since his student days. He was concerned not only with theoretical topics, such as the interpretation of fermentation or the role of heat in chemical reactions, but also with topics of chemistry applied to agriculture and hygiene and with the natural sciences.

In 1787, the year of his graduation in Natural Philosophy, he published a Memoir on Fermentation considered both on general and on its different species (2), and in 1788, he published a Mémoir on Heat (3) and the first volume of a Handbook on chemistry entitled Elements of Chemistry (4;5) followed by a second volume, published two years latter, in 1790.

Seabra never travelled abroad, but in all these publications, he showed a deep awareness of the latest scientific advances, and immediately grasped the importance of Fourcroy’s and Lavoisier’s work, in particular. 

In this year of 2015, the International Year of Light, here, I will pay a special attention on Seabra´s ideas on fire, heat and light, following the content of  his Dissertation on Heat and his Elementos de Chimica on the same subject. 

2 – The nature and action of heat in Vicente Seabra´s chemical writings  

In his chemical works, Vicente Seabra subscribes Lavoisier’s pneumatic theory, claiming specially to be inspired by Fourcroy´s Mémoirs and the first edition (1786) of the Elémens d’Histoire Naturelle et de Chimie to ‘‘whose – he says - I owe most of my chemical knowledge’’.

Following the University Statutes syllabus for the programme of the lectures on chemistry, Seabra starts his considerations on the Chemistry matters with a survey of the history of chemistry, tracing its origins back to antiquity, then focusing on alchemy, and finally giving an updated historical description of 17th and 18th century chemical knowledge. In this survey, he presents a qualitative discussion of chemical affinity and its various kinds, a discussion of the general principles of bodies (including phlogiston, fire, light, heat, air and water), and the main chemical operations (combustion, calcination, fusion, vitrification, distillation, dissolution, precipitation, etc.).

It is on the discussion of the general principles of bodies that Seabra considers in detail phlogiston, fire, light, heat, air and water, by considering Stahl, Sage and Demeste´s theories on combustion, Macquer´s theory on fire, light and heat and what he calls a first and a second Lavoisier’s theories, on heat, air, combustion and calcination, demonstrating that a portion of pure air combines with bodies,givingbirth to the so-called “Pneumatic Theory’’. Acknowledging all these theories, Seabra defends what he called an intermediate theory, an attempt at a synthesis between Stahlian and Lavoisier´s theories, in a reconciliation proposed by Macquer and supported by Fourcroy: it is the conclusion of his historical survey; “in this century we have three main theories about chemical phenomena, the Stahlian, the Pneumatic and the Intermediate (...) The great Macquer, however, argued that there is a combined light instead of Stahl’s phlogiston and recognised that there was a combination with air, instead of a liberation of phlogiston. In this way he established an intermediate theory (...) The indefatigable Lavoisier, after formulating his theory of heat in a Mémoir delivered at the Parisian Academy of Sciences, accommodated Macquer’s theory, but giving his own re-interpretation. He claimed that phlogiston or matter of heat did not separate from bodies, as Macquer claimed, but resulted from the decomposition of the air, which combines with them. However, we will adopt a theory slightly different from Lavoisier, and we will show that Macquer’s theory together with that of  Lavoisier constitute the true one, and the one we should follow (6).

In the development of his own theory of heat in his Memoir on Heat of 1778³,Vicente de Seabra refers to Stahl’s phlogiston as the pure inflammable principle of the combustible bodies as a principle rejected by the new Pneumatic Chemistry on the basis that it had been impossible to isolate it by analysing its properties and it had also been impossible to show that the colours of the bodies do not depend directly on such a principle and to show the ways it leaves the bodies. With Macquer, if one wants to admit phlogiston as a sui generis inflammable principle, it should be the light in itself, the true and immediate cause of colours. Phlogiston and Light are the same thing, when the combustible bodies are burnt. Like to what happens in the burning of sulphur or phosphorous, a luminous fluid is released, the flame, and with the flame, heat, the elementary heat of bodies, the elementary fire, transformed into one of the principles of combustible bodies. In fact, light is a sui generis fluid which enters in the composition of all the bodies, pervading the whole world, the reason why it is believed to be the true Anima of the Universe. Thus, Phlogiston, Fire, Light and Heat are the same thing, as clearly was admitted by Macquer in his Dictionaire de Chymie (7).

Light and Fire, Fire and Heat, not to mentioned anymore Phlogiston, the combined light, being the same in nature, it is in the way they leave the bodies and the way they affect our senses, that they appear either as light or as heat, or as light and heat simultaneously.

In particular, heat is the feeling of the amount of the elementary fire of the bodies through our sense of touch associated to an higher or lower temperature, caused by differences in the intensity of friction and compression phenomena to which  the bodies are subjected, which lead to different modifications and mouvements in the parts where those phenomena occur.  It is also due to such differences that the elementary fire and light lay in the bodies under different forms. Being the same in nature, in the bodies, light and fire can lay in different ways with different properties. All the light existing in the bodies obeys to the same and general Newton´s attraction law ruled by the different bodies´ affinities. It is in the difference of affinity interaction between light and bodies that we sense different kinds of light and heat. There is the general fluid of light and of heat existing everywhere in the Universe, the absolute heat. But there is a specific amount of heat and of light existing in each body, different from one to another, the specific heat.

With J. H. Magellan (1722-1790) (8), Vicente Seabra defends that every body has its own specific heat, that portion of the absolute heat which has combined with the other parts of the body to form it, becoming a part of its essence. This heat is insensible to us due to the perfect neutrality resulting from the combination of all the parts.   Being so, it is possible to determine by chemical analysis the specific heat of each body. Vicente Seabra goes ahead with a Table of specific heats for a great number of substances and solutions taken from Kirwan´s data expressed as decimal fractions with the specific heat of water taken as unity (9).

Due to its perfect neutrality with all the components parts of a body, the specific heat is not sensible. Adding or subtracting grains of absolute heat to a body the total amount of heat, the so-called mixed heat, named after Lavoisier, it becomes sensible. It is the Sensible Heat. If the mixed heat of a body is greater than its specific heat, we feel the body as warm; if the mixed heat is less than its specific heat, we will feel it as cold. On absorving absolute heat to states of higher amounts than the amount of their specific heat, the bodies become more fluid and turn to an aeriform state at high content of mixed heat. On the opposite, when the mixed heat content of any body is lowered, in a colding process,the body becomes less fluid and acquires progressively more and more the characteristics of a solid. In the case of sufficient increasing of the sensible heat of a body, this can become luminous, enough or not to emit a flame. Thus, heat has the property of transforming solid and liquid bodies into more elastic and aeriform bodies. Under enough amounts of heat, ice turns water, metals melt, etc. By absorving heat, when the bodies become more fluids produce cold; and when the fluids become solids produce heat and warm the surroundings (10). 

3. Vicente Seabra´s ideas on the nature of heat in Lavoisier´s context  

During the eighteenth century, there were two primary explanations for heat, a vibration mechanical theory and a fluid theory. For F. Bacon (1561-1626), R. Boyle (1627-1691), and I.Newton (1748-1826) heat was due to the vibrations of small particles. For E.Stahl and the French pedagogues at the Jardin du Roi heat was caused by an imponderable fluid, the phlogiston, that when added to a substance would increase its total heat. The fluid theory was particularly useful for explaining evaporation, fluidity, expansion, heat capacity; while the vibration theory was thought to be better for explaining ignition.

In the Mémoire sur la Chaleur (11), in 1780, Lavoisier and Laplace, looking for a new method of measuring the heat, just start by considering this question: is heat a subtle elastic fluid present everywhere in bodies interposed among the particles and scattered all over the whole universe, or is it the result of insensible but continuous oscillatory motions of the particles that enter the composition of matter? Without being able to decide for one of these possible causes against the other, their opinion was that in some cases one of the two possibe causeswill be more probable than the other, but in another cases the opposite seemed to be the contrary, whereas in another ones a mixed combination of the mechanical and fluid explanations will appear to be the most adequate.

Lavoisier´s interest and discussions on a heat theory date from before 1777.  His concept of  heat forms an integral part of his chemical system (12).  But,the extensive treatment given to the subject in his Mémoire sur la Chaleur  in collaboration with Laplace marks all the Lavoisier´s theory of heat and  became the milestone of his caloric theory (13). It has been the main source of Vicente Seabra´s wiritings on heat.

In this Mémoire, Lavoisier and Laplace write on the concepts, terminology and techniques for dealing with the different kinds of heat: the free heat, the combined and mixed heat, the specific heat of bodies and the released heat. The first and general statement regards the existence of a fixed substance which allows for changes of state, namely, condensation, exhalation, or combustion. That substance is identified as a subtle matter, a subtle fluid, the same fluid of lihgt and of fire. Lavoisier was indifferent to the problems of differentiating between heat and light. Although the J. H. Magellan´s articles seem to be the main source of information of Lavoisier and Laplace´s “Mémoire sur la Chleur”, they are virtually ignored and never cited. The terminology they use is not also the same (14). For instance,  they do not refer such a fixed substance as the latent heat existing in the bodies, a term that J. H. Magellan had rejected on the grounds that the effects of heat in this form are sensible not latent (15).  And they do not use the terminology specific heat in the same sense as the used by J.H.Magellan.

Later, treating change of state as a physical rather than a chemical transformation, with fire acting as a mechanical agent to separate the particles, Lavoisier believed that specific heats or heat capacities of substances should  be related to the volume of empty space within them (16), consisting in the fixed quantity of heat which produces change of state without a change of temperature. All changes of state are caused by the combination and release of fire; aeriform fluids are vapours and vaporisation is a chemical process caused by the combination of the matter of  fire influenced by external pressure of the atmosphere,  The particles of substances are continually acted upon by two opposing forces, the igneous fluid tending to separate them, and an opposing attraction or affinity tending to draw them together. This igneous fluid, the matter of heat, fire and light is the caloric. It can exist as either free, adherent, or combined with the elementar constituents of other substances. In fact, strictly speaking, it can never be absolutely free because of the mutual adherence between it and the particles of other substances. This matter of fire has no sensible weight and therefore chemists cannot use it to account for any weight change observed in chemical reactions (17). However, as a form of matter, caloric must obey the laws of common matter in general, namely, the universal gravitation, although its weight is too small to be detected. The combustion and calcination processes explained by Stahl as due to the action of  phlogiston existing in the bodies, are due to the air which exists  in the matter of  heat being released during  the process. Instead of placing the admitted phlogiston in the combustible, Lavoisier explains the process by placing there the matter of heat. Fire is not fixed in combustible substances; it is rather contained in caloric. As an elastic fluid, it is an igneous compound in which the matter of fire or light forms what he called the dissolvent and another substance forms the base; if the base unites with a substance for which it has a greater affinity than it has for fire matter, the dissolvent (fire) would be set free. This is what happens during  combustion and calcination (18).

From these observations, we can say that, as a whole, Vicente Seabra´s ideas on heat easily incorporate in Lavoisier´s theory, with some differences in terminology, mainly in the names of the different kinds of heat, with Vicente Seabra using and acknowledging Magellan´s terminology rather than Lavoisier. Like Lavoisier, Vicente Seabra discusses change of  state, namely in combustion and calcination,  in terms  of the principle of fluidity and elasticity of heat, behaving as chemical constituent in its chemical combination with substances. However, whereas Lavoisier strongly believed that the cause of sensible heat was a material substance, the caloric, Vicente Seabra never has been very much enthusiastic with this explanation, not favouring its existence, avoiding  a clear commitment to the concept of  heat as a material substance, the caloric. Actually, in the Vicente Seabra´s wiritings the term caloric is ignored except for a single reference to say that it is the same as heat and  disapproving the use of new words  to refer hypothetical situations neither demonstrated nor yet confirmed by experiments (19). He continued to use the term phlogiston, following Macquer, defending that Phlogiston, Fire, Light and Heat are the same thing, as we already have referred. The phlogiston as a substance sui generis, differing from light, from the elementary fire and from heat, would be something whose existence has never been demonstrated, a vague idea, an idea sine subjectu, no well-grounded, appearing only as a fictious and sacred-anchor in the explanation of a few phenomena.

But it is not possible to consider Vicente Seabra´s ideas on fire, heat and light without to consider his ideas on combustion and calcination. Notwithstanding his criticism on caloric as a fluid substance, Vicente Seabra recognised that Lavoisier´s caloric theory became the foundation stone of the pneumatic chemistry, having its starting point in the understanding of these phenomena by opposing Stahl´s phlogiston theory, with the entire refusal of phlogiston.

Following Lavoisier´s ideas, Vicente Seabra explains combustion due to action of air in the integrant parts of the combustible bodies when heated. It never can occur without the intervention of air. After having suffered such an action, the combustible bodies become incombustibles, with a different nature and an increase in weight. In his criticism on the Lavoiser´s caloric theory, Vicente Seabra compares the explanation of combustion presented by Lavoisier and the explanation presented for the same process by Macquer, and concludes that both are correct, but thaught that the explanation presented by Lavoisier that the combustion was the combination of oxygen with the combustible body was not sufficiently convincing, mainly in the cases occuring  with the development of a flame. Consequently, for him the theory presented by Lavorsier was not enough completed because it did not explain all kinds of combustion.For him, the expalanation presented by Macquer and accepted by Fourcroy, the so-called the pneumatic-sthalian or intermediate theory, would be a better theory, admitting that all the combustion processes, including those occuring with a sparkling discharge, as combination processes of the oxygen existing in the air with the combustible body and the specific heat of  the air, alone or together, with the specific heat of the oxygen entering in the reaction, according to the nature of the incombustible which is formed.

Concerning the process of the animal respiration in its relation with heat, Vicente Seabra totally agrees with Macquer, Fourcroy, Lavoisier and Laplace that it is a combustion process, where one must consider the role of air with its content in oxygen and the animal heat.