“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.