The agency of the air around us, water, and heat and cold, have often been the objects of chemical inquiry, from their known great influence upon the animal economy. The changes effected by the phenomena of animal life upon these agents have been accurately examined, and partly reduced to a mathematical precision of calculation.
Spallanzani and others have viewed the subject as it regards physiology, but with such results as left the field open to subsequent investigation. Dr. Edwards seems to have seized upon the deficiencies of his predecessors, and, by going over their ground, and extending his own inquiries, he has arrived at most interesting and important results. These he has divided into four parts, as they relate to the different orders of the animal creation. The first part includes some of the lower animals, particularly tenacious of life, and of cold blood, such as frogs, toads, and salamanders. The second part is devoted to other animals of cold blood, and of the vertebrated order, as fish, and those reptiles which include lizards, snakes, and turtles. The third part refers to warm-blooded animals; and the fourth part of the work is dedicated to the influence of the physical agents upon the human race and vertebrated animals. To these the author has added the discoveries of modern times, relative to electricity on the animal economy, in an Appendix. A collection of tables is appended to the work, exhibiting the principal series of his experiments, as they regard the relative influence of physical agents on the duration of life, and the phenomena resulting from their mutual action.
The great importance of the four grand divisions of the work forbids our hastily reviewing them, and we will endeavour to condense so much of the information they contain as may forward the objects of our analysis. Dr. Edwards thus announces the arrangement of his work:—
“Ces recherches auront donc rapport à l’air dans les conditions de quantité, de mouvement et de repos, de densité et de raréfaction; à l’eau liquide et à la vapeur aqueuse; à la température, dans ses modifications de degré et de durée; à la lumière et à l’électricité. Ces causes agissent à la fois sur l’économie animale, ordinairement d’une manière sourde et imperceptible; et toujours [p140] l’impression qu’on reçoit est le résultat de toutes ces actions combinées.”
“Lors même que, par l’intensité de l’une d’elles, il nous arrive de distinguer la cause qui nous affecte, l’observation de l’effet se borne le plus souvent à la sensation, et les autres changemens qui l’accompagnent nous échappent. On conçoit par la que l’observation la plus attentive des phénomènes tels que la nature nous les présente, ne saurait démêler dans cette combinaison d’actions l’effet propre à chaque cause, ni reconnaître des effets qui ne seraient pas révélés par la sensation.
“Il est une méthode qui règle les conditions extérieures, qui fait varier celle dont on veut apprécier l’action, et qui fait juger, par la correspondance entre ce changement et celui qui survient dans l’économie, du rapport de cause et d’effet: c’est la méthode expérimentale; c’est celle que j’ai suivie. Pour en tirer parti il fallait, d’une part, déterminer l’intensité de la cause, d’autre part celle de l’effet. La physique nous fournit ordinairement les moyens de remplir la première indication.”
In the true spirit of philosophical investigation, Dr. Edwards, in the first place, proceeds to examine the action of physical agents upon the simplest forms, and least elaborately developed organised beings, extending his inquiries upwards, in the scale of the animal world, to man, the most perfect creature, and the ultimate object of all physiological researches.
The peculiarity of constitution belonging to cold-blooded reptiles, among which there is so little mutual dependance of organs, renders these the best tests of the relative and proportionate influence of the different agents, the intense action of which is liable to destroy the more perfect animals; and the great development of the nervous system in the higher orders gives them a wider and more acute range of sensibility. It is difficult, at all times, and often impossible, to insulate corporeal functions among the warm-blooded classes, so as to ascertain the amount and limits of physical agency. The four classes of vertebrated animals, or such as are furnished with true spines, afford ample means of comparative illustrations; and these departments have engaged the author’s attention, in order to display the result of the action of the same agent exercising a uniform influence upon constitutions very differently constructed. The air, for example, exercises its influence uniformly upon he four mentioned classes of vertebratæ, and their different families are similarly exposed to the action of the atmosphere by respiration.
Curious and interesting as is this subject, it is singular [p141] that, while it was among the first to be noticed, it has been the latest in producing satisfactory results. Among the opposing causes of the advancement of knowledge in this department, the ignorance of our ancestors in chemical science seems to be the principal. Without chemical aid it is perfectly useless to attempt the investigation. The composition of the air respired must be well understood; the different gases must be carefully examined, or the physiological inquiry will be darkened and obscured.
Dr. Priestley laid the foundation of our chemical knowledge of gases in their relation to respiration; but some time elapsed before it was understood in what manner the air was connected with animal organisation. Oxygen gas, one of the known constituents of atmospheric air, was Priestley’s discovery, in its effect upon the blood, of converting this fluid from a dark purple to a bright crimson. Lavoisier founded a chemical theory upon this discovery of the agency of air, which was subsequently applied by Goodwin to physiology. The latter author demonstrated, by a series of excellent and correct experiments, that the exclusion of atmospheric air produces death in animals, in consequence of the dark-coloured blood usually circulating in the veins being prevented from becoming crimsoned. The state in which any animal may be thus placed, is known by the term ASPHYXY, and by which is to be understood a deficient or suspended aërification of the blood, from whatever cause it may proceed that the atmospheric air is prevented from access to the blood as it circulates through the lungs.