The serum of the animal is tested from time to time against a known amount of toxin, i.e.
By growing this bacillus in broth a toxin is formed which remains in solution and can be separated from the bacilli themselves by filtration.
Then we have the property of adaptation, in which the negative reaction may be changed into a positive; a given toxin may at first repel the cell, but by a gradual process the cell becomes accustomed to such a toxin and will move towards it.
Of such are tetanus and diphtheria, now known to be due to the establishment from without of a local microbic infection, from which focus a toxin is diffused to the nervous matter.
Though an enormous of amount of work has been done on the subject, no important bacterial toxin has as yet been obtained in a pure condition, and, though many of them are probably of proteid nature, even this cannot be asserted with absolute certainty.