Lastly, he gives an experiment on the blood (which shows how he was working in every direction), in aid of the opinion that the blood derives its scarlet colour from the action of oxygen. "I took the coagulum of venous blood left in a basin after bleeding, and, turning it bottom upwards, waited till its surface had become of a scarlet colour. I then took slices of this surface, and similar slices of the interior part of the coagulum, which had a very dark appearance, and exposed them repeatedly to azotic and nitrous gases. The scarlet colour gradually faded upon such exposure; and the azotic gas being afterwards examined, was found to contain oxygen, while nitrous gas was much diminished, doubtless by combining with the same principle. The gases to which the dark-coloured blood was exposed underwent no change in this experiment. That blood takes oxygen from the air, when it becomes florid, will not, I suppose, be denied, and the experiment I have related shows that it will again part with it, though slowly, without _any alteration in its temperature_." The principal interest, as we think, of this paper on "Irritability," is the evidence it affords of his determination to keep his mind free from preconceived notions on a subject which was at that time calculated to mislead him; especially as he then participated in the general impression that the Oxygen was "the great source of animal heat;" a view which he afterwards, and as we think for excellent reasons, mistrusted. This view has been revived, but, as far as we know, in no very philosophical spirit. Whilst we would respect the opinions of men, we can only reason on the paramount authority of nature; and we see increasing ground to believe that he who would leave out of physiological inquiries so large a portion of the necessary induction as the phenomena of disease, no matter what be his authority, will only add to the number of those who have shown that, the moment we neglect the most comprehensive search for facts of which our knowledge admits, we fall into error. Mr. Hunter has recorded his opinion of the impossibility of obtaining a knowledge of functions without considering the phenomena of disease; and all experience hitherto has tended to give this observation the validity of an axiom. CHAPTER XII. OF EXPERIMENTS ON ANIMALS. "Know, Nature's children all divide her care; The fur that warms a monarch warmed a bear." POPE. In the foregoing experiments, the reader will have observed the significant words, "having killed a frog"—Abernethy not approving of experiments on living animals. When we reflect for a moment on the thousands of dreadful experiments which have been made on living animals, and the utter inconclusiveness of them for any useful purpose, there are, amongst the numerous errors by which so many philosophical inquiries have been delayed or defeated, few that are more lamentable.