The power of the Chameleon's changing colour long exercised the ingenuity of the old naturalists. Hasselquist thought that the changes of colour depended on a kind of disease, more especially a sort of jaundice, to which the animal was subject, particularly when it was put in a rage. M. D'Obsonville thought that he had discovered the secret in the blood, and that the change of colour depended upon a mixture of blue and yellow, whence the different shades of green were derived; and these colours he obtains from the blood and the blood-vessels. Thus he says that the blood is of a violet hue, and will retain its colour on linen or paper for some minutes if previously steeped in a solution of alum, and that the coats of the vessels are yellow; consequently, he argues, that the mixture of the two will produce green. He further traces the change of colour to the passions of the animal. Thus, when a healthy Chameleon is provoked, the circulation is accelerated, the vessels that are spread over the skin are distended, and a superficial blue-green colour is produced. When, on the contrary, the animal is imprisoned, impoverished, and deprived of free air, the circulation becomes languid, the vessels are not filled, the colour of their coats prevails, and the Chameleon changes to a yellow-green, which lasts during its confinement. Barrow, in his "Travels in Africa," declares that previously to the Chameleon's assuming a change of colour, it makes a long inspiration, the body swelling out to twice its usual size; and as the inflation subsides, the change of colour gradually takes place, the only permanent marks being two small dark lines passing along the sides. Mr. Wood conceives from this account that the animal is principally indebted for these varied tints to the influence of oxygen. Mr. Spittal also regards these changes as connected with the state of the lungs; and Mr. Houston considers this phenomenon as dependent on the turgescency of the skin. Dr. Weissenborn thinks it not unlikely that the nervous currents may directly co-operate in effecting the changes of colour in the Chameleon.