[Illustration: FIG. 87.--CHLOROFORMING A COBRA IN ORDER TO COLLECT VENOM, AT THE FRENCH SETTLEMENT OF PONDICHERRY, IN INDIA (STAGE II.).] Except when a snake is _moulting_, the venom can be extracted from its glands about every fortnight; and it is better that the extraction be not performed concurrently with artificial feeding, since, owing to the fact that the venom serves the animal as digestive juice, the reptile will soon perish if deprived of the means of digesting the food that it is obliged to receive. It is best, therefore, to select one day of the week for artificial feeding, and the corresponding day of the following week for the extraction of the venom. When the venom has been collected, it must immediately be in a desiccator over calcium chloride or sulphuric acid, in order to dry it rapidly. In hot countries, and where no laboratory specially equipped for the purpose exists, it will suffice to dry the venom in a current of air, or even in the sun. It then concretes in scales of a citrin colour, more or less dark, according to the concentration of the liquid. In this dry condition, placed in well-corked bottles, protected from damp air, it may be kept almost indefinitely without losing anything of its original toxic power. On the contrary, if the desiccation be imperfect it undergoes a somewhat rapid change, and assumes a disagreeable odour of meat peptone. I have kept samples of various venoms, dried as described, for _fifteen_ years without any sensible diminution of their activity. [Illustration: FIG. 88.--COLLECTING COBRA VENOM AT PONDICHERRY (STAGE III.).] CHAPTER V. _THE CHEMICAL STUDY OF SNAKE-VENOMS._ In the condition in which they are received on issuing from the glands, venoms always present the appearance of a thick saliva, of an oily consistency and more or less tinged with yellow, according to the species of snake by which the poison has been produced. They are entirely soluble in water, the addition of which renders them opalescent. Tested with litmus they exhibit a slightly acid reaction; this acidity, which is due to the presence of a very small quantity of an indeterminate volatile acid, disappears on desiccation, so that solutions of dried venom are neutral. The taste of venoms is very bitter. Their density, which is slightly greater than that of water, varies from 1030 to 1050. Venoms are composed of a mixture, in variable proportions, of proteid substances, mucus and epithelial _débris_, fatty matters and salts (chlorides and phosphates of lime, ammonia and magnesia), with from 65 to 80 per cent. of water. The elementary analysis of Cobra-venom made by H. Armstrong[7] gave the following results:-- Carbon 43·04 per cent. Hydrogen 7·00 “ Nitrogen 12·45 “ Sulphur 2·50 “ Residue Small quantities.