All these elaborate researches as to the character of serpent venom were essential to enable the next step to be taken in the elaboration of the antidote. Before this great achievement could be accomplished it was necessary to first succeed in artificially immunising animals against the effects of this powerful toxin, so that the serum of such animals could be applied for the protection and cure of other animals from the effects of snakebites. It may be readily conceived that the task of artificially rendering animals immune from snake poison was not an easy one, for the process depends upon training the animal to gradually withstand larger and larger doses of the venom; and considering the intensely toxic character of the substance which had to be handled, the danger was ever present of the animal succumbing to venom poison before its serum had acquired the requisite pitch of protective power to render it of service as an anti-toxin. Dr. Calmette tells us that he carried out a very large number of experiments before he met with success. But it is not necessary here to discuss his various efforts; suffice it to say that at length his labours were rewarded, and the following extract from one of his memoirs describes the methods which he adopted for this purpose:-- "The best method of procedure for the purpose of vaccinating large animals destined to produce anti-venomous serum consists in injecting them from the outset with gradually increasing quantities of the venom of the cobra mixed with diminishing quantities of a one-in-sixty solution of hypochlorite of lime.[8] The condition and the variations in the weights of the animals are carefully followed, in order that the injections may be made less frequently if the animals do not thrive well. Quantities of stronger and stronger venom are in turn injected, first considerably diluted, and then more concentrated; and when the animals have already acquired a sufficiently perfect immunity, the venoms derived from as large a number of different species of snakes as possible are injected. The duration of the treatment is of considerable length--at least fifteen months--before the serum is sufficiently active to be used for the purposes of treatment." [8] More recently the snake venom employed by Dr. Calmette for the immunisation of his horses consists of a mixture of colubrine and viperine poisons, the former making up about 80 per cent. of the mixture. A solution of this mixture is heated at about 73° C. for half an hour and then filtered, and injected into horses.